Metabolites, water and mineral exchanges across the rumen wall: mechanisms and regulation D Rémond, F Meschy, R Boivin

To cite this version:

D Rémond, F Meschy, R Boivin. Metabolites, water and mineral exchanges across the rumen wall: mechanisms and regulation. Annales de zootechnie, INRA/EDP Sciences, 1996, 45 (2), pp.97-119. ￿hal-00889546￿

HAL Id: hal-00889546 https://hal.archives-ouvertes.fr/hal-00889546 Submitted on 1 Jan 1996

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Review article

Metabolites, water and mineral exchanges across the rumen wall: mechanisms and regulation*

D Rémond F Meschy R Boivin

1 Station de recherche sur la nutrition des herbivores, Centre Inra de Theix, 63122 Saint-Genès-Champanelle; 2 Laboratoire de nutrition et alimentation, Inra Ina-PG, 78352 Jouy-en-Josas; 3 Service de physiologie, ENV de Lyon, 69280 Marcy-l’Étoile, France

(Received 14 February 1995; accepted 16 August 1995)

Summary — In ruminants, the forestomachs, and especially the reticulorumen, have walls with anatomical and histological properties that permit the exchanges of various metabolites, water and min- erals between the rumen contents and the blood. The development of papillae on the walls and the local blood circulation favour these exchanges. They depend to varying degrees on the food supply and the concentrations of volatile fatty acids (VFA) produced by the microbial catabolism of polysaccharides. The absorption of VFA and ammonia occurs essentially by a process of passive diffusion of their non- ionised form through the epithelial cell membranes. For each of these substances, the existence of a transport system for the ionised forms is also suspected, but its relative importance is unknown. Short- term modifications in the absorption of these two substances are thus primarily determined by varia- tions in their intraruminal concentrations and pH. Other factors may also be implicated, and it is known in particular that the absorption of ammonia is enhanced when the intraruminal concentration of VFA or the carbon dioxide (C02) level increases. The movement of urea through the wall occurs from the blood towards the rumen content according to the concentration gradient. The main factors liable to influ- ence the transepithelial flux of urea seem to be the blood urea levels and factors that act on the con- tact surface between the blood compartment and the epithelium (C02, VFA). Ruminal ammonia con- centration also affects the net urea transfer across the rumen wall but the mechanisms involved in this regulation are not clearly understood. The absorption of water through the rumen wall results from an osmotic pressure gradient between the rumen and the plasma. This is modified not only by factors that modify the blood flow rates at the wall, but also by electrolyte concentrations. The absorption of min- erals from the rumen has also been demonstrated (Mg, Ca, Na, Cl, K, sulphur and certain metals). This occurs by mechanisms of varying complexity according to the element involved. rumen wall I volatile fatty acid I ammonia I urea I water I minerals

* Report presented in the 9th Conference on Nutrition and Feeding of Herbivores, Clermont- Ferrand, France, 16-17 March 1994. Résumé — Échanges de métabolites, d’eau et de minéraux à travers la paroi du rumen : méca- nismes et régulations. Chez les ruminants, les préestomacs, et plus particulièrement le réticulo- rumen, ont une paroi dont les caractéristiques anatomo-histologiques permettent les échanges de différents métabolites, d’eau et de minéraux entre le contenu ruminai et le sang. Le développement des papilles de la paroi et l’importance de l’irrigation sanguine favorisent cette absorption et sont plus ou moins dépendants des apports alimentaires et de la concentration en acides gras volatils produits par le catabolisme microbien des polysaccharides. L’absorption des acides gras volatils et de l’ammo- niaque s’effectue essentiellement selon un processus de diffusion passive de leur forme non ionisée à travers les membranes des cellules épithéliales. Pour chacune de ces substances, l’existence d’un système de transport pour les formes ionisées est également suspectée, mais on ne connaît pas l’importance relative de cette voie de passage transmembranaire. Les modifications à court terme de l’absorption de ces deux molécules sont donc en premier lieu déterminées par des variations de concentration intraruminales et des variations de pH. D’autres facteurs peuvent également intervenir et on sait notamment que l’absorption d’ammoniaque est accrue lorsque la concentration intraruminale en acides gras volatils ou la teneur de C02 augmente. Le passage d’urée à travers la paroi s’effectue du sang vers le contenu ruminai en fonction du gradient de concentration. L’urée étant rapidement et totalement hydrolysée dans le rumen, les principaux facteurs susceptibles de faire varier ce flux tran- sépithélial d’urée semblent être l’urémie et les facteurs agissant sur la surface de contact entre le compartiment sanguin et l’épithélium (CO2, acides gras volatils). La concentration ruminale en ammo- niaque joue également un rôle dans la régulation du transfert d’urée ; son mode d’action n’est cepen- dant pas clairement déterminé. L’absorption d’eau à travers la paroi du rumen résulte d’un gradient de pression osmotique entre le rumen et le plasma ; elle est modifiée, non seulement, par les facteurs qui modifient le débit sanguin au niveau de la paroi, mais également par les concentrations en électrolytes. Les minéraux peuvent également être absorbés à travers la paroi du rumen (Mg, Ca, Na, CI, K, soufre et certains métaux) ; cette absorption s’effectue par des mécanismes plus ou moins complexes et son intensité est variable selon l’élément considéré. paroi ruminale / acide gras volatil / ammoniaque / urée / eau / minéraux

INTRODUCTION de to absorption in the first stomach com- partments&dquo;. Although it is now well estab- lished that the mucosa of the reticulorumen The development of voluminous forestom- an function, it is achs, and in particular the development of possesses absorptive only in recent years that significant progress has the reticulorumen, gives ruminants a highly been made in the evaluation of characteristic digestive physiology. Certain quantitative the absorption processes, through the use of aspects of this digestive physiology are well dilution that allow nitro- known. Numerous studies have been isotopic techniques flow rates to be estimated and devoted to the motricity of the reticuloru- gen (Nolan Leng, 1972), and the development of meth- men, its regulation and its effects on diges- ods for ruminal blood flow measurement tive transit. Likewise, the biochemical and ruminal vein catheterisation et aspects of microbial digestion and its con- (Remond which make it to mea- sequences for nutrition and metabolism in al, 1993c), possible sure net fluxes of metabolites across the ruminants have been thoroughly studied. Inn contrast, the extent to which the reticuloru- ruminal wall. The parallel use of in vitro tech- et al, has men is able to absorb and recycle materials niques (Martens 1978) generated essential information for the through its wall is less well known. It was understanding mechanisms involved in these long assumed, as stated by Colin in 1853, transepithe- lial that &dquo;the epithelium of the rumen is less per- transport processes. meable than that of the mouth and oesoph- For certain substances derived from agus, and is without doubt the main obsta- microbial metabolism or present in the feed, the rumen can represent the most impor- muscle separating the mucosa from the tant absorption site in the digestive tract underlying tissues. (VFA, ammonia, magnesium, etc). The The epithelium is formed of four cell lay- extent of quantitative this absorption ers: the stratum basale, the stratum depends primarily on the absorption capac- spinosum, the stratum granulosum and the of the which is determined the ity rumen, by stratum corneum (Steven and Marshall, structure of the epithelium and the amount 1970). The cells of the stratum basale of surfaces between the exchange diges- appear to be the most metabolically active tive contents and the blood. It also depends since they contain numerous mitochondria on the mode of transport specific to each and free ribosomes. A large proportion of substance. the assimilation and metabolism of the sub- stances absorbed from the rumen may thus occur in this cell layer. It is here that the cell MORPHOLOGICAL AND FUNCTIONAL division takes place. Thereafter, the cells CHARACTERISTICS OF THE RUMEN differentiate as they migrate towards the WALL stratum corneum. According to Steven and Marshall (1970), the fusion of the cell membranes Structure of the ruminal mucosa (tight junc- tions) in the stratum granulosum leads to an obliteration of the intercellular spaces The reticulorumen is a highly voluminous and thus causes this cell layer to act as a compartment (80 to 100 L in the cow, 10 to barrier controlling the movement of materi- 15 L in the sheep). It is subdivided into sev- als between the blood and the rumen. The eral compartments by various folds of its presence of this barrier in the stratum gran- wall, which has the effect of increasing the ulosum was not, however, observed by Hen- amount of surface area contacting the rikson (1970), and Henrikson and Stacy digesta. The surface area of the rumen (1971) later observed that the barrier to dif- mucosa is further increased by the pres- fusion through the epithelium of different ence, over practically the entire inner wall, of markers was located in the inner layers of numerous conical or tongue-shaped papil- the epithelium corneum. According to these lae. The distribution, size and number of authors, the selective permeability of the these papillae depend on the species and epithelium may be explained by the coat- also on diet. In cattle, 80 to 85% of the inner ing of mucopolysaccharide on the kera- surface are covered with papillae; in sheep, tinised cells, and by the succession of dif- the papillae are irregularly distributed, and ferent types of membrane junctions the ruminal pillars and the dorsal sac are (desmosomes, tight junctions) beneath the usually devoid of papillae, or bear papillae of cornified layer, rather than by an actual bar- small size. The differentiation of papillae rier in the stratum granulosum. observed in different of the rumen parts The stratum corneum is made up of ker- appears to be linked to the layering of the atinised cells (the cell nuclei have vanished). contents in the rumen. digestive The keratinisation of the surface layer pro- The papillae, as well as the entire inner tects the mucosa against the abrasive action rumen wall, are covered with a mucous of the rumen contents and the penetration of membrane composed of a keratinised multi- microorganisms. Although intercellular layer epithelium, a richly vascularised con- spaces are once more visible in this layer, junctive tissue, crossed by nerve fibres and they are sufficiently narrow to prevent bac- lymph ducts, and a fine layer of smooth teria from passing through (Steven and Mar- shall, 1970). The biosynthesis of keratin in centrate also causes a rapid rise in the the epithelial cells during their movement mitotic index of the ruminal epithelium from the stratum granulosum to the stratum (Goodlad, 1981 Diets that produce an corneum is accompanied by a decrease in abundant production of VFA with a high pro- cell permeability (Fell and Weekes, 1975). portion of butyric acid are considered, there- The keratin formed softens, however, to fore, to be promotors of cell proliferation in some extent, on contact with the rumen con- the rumen. tents, which should make it more perme- The structure of the epithelium is also able. The stratum corneum of the epithe- affected by the rate of cell shedding from lium is thus a complex system containing the surface layers of the epithelium. Accord- both strongly keratinised cells that offer a ing to McGavin and Morrill (1976), the grat- resistance to and ker- high abrasion, partly ing effect of rough forage is necessary to atinised cells with mucus-producing absorp- prevent an excessive accumulation of ker- tive capacity (Sydney and Lyford, 1988). atinised cells at the surface of the ruminal The efficiency of nutrient transport across mucosa. The abrasive action of the rumen the epithelium thus depends to a large contents thus plays an important role in the extent on the integrity and degree of kera- rate at which cells are shed from the stratum tinisation of the stratum corneum. This layer corneum. The contribution of adhering bac- can display various anomalies: parakerato- teria colonising the surface cells of the sis, which leads to an incomplete keratini- epithelium should not, however, be under- sation of the cells of the stratum corneum estimated. Their action can accelerate the and a disappearance of the stratum gran- breakdown of keratinised cells, and thereby ulosum, or hyperkeratosis, characterised their shedding (Cheng and Costerton, 1980). by excessive thickening of the stratum The distribution of these bacteria at the sur- corneum, sometimes associated with a face of the ruminal epithelium indicates that thickening of the stratum granulosum. they are also affected by food abrasion The thickness of the ruminal epithelium (McCowan et al, 1980). The breakdown of tissue bacteria thus be depends on the rate of cell division in the epithelial by may basal layer of the epithelium, the speed of more efficient at locations where the rumen migration of the cells from the deep layers to wall is less affected by physical erosion. the surface layers, and the rate of cell shed- The lumen surface area of the epithe- ding from the stratum corneum. Many stud- lium can vary widely according to the num- ies have been conducted on the regulation ber, size and shape of the papillae that of cell proliferation in the epithelium (Galfi develop on the inner surface of the rumen. et al, 1991 It is known that the mitotic activ- When the diet is changed, the morphologi- ity of the epithelium can be depressed by cal adaptations of the ruminal epithelium fasting, and restored by the subsequent re- are rapid (2 to 3 weeks), and are particularly feeding (Tamate et al, 1974). In addition, it evident in the atrium. In adult ruminants, the is very strongly stimulated by intermittent mass of the mucosa increases linearly with feeding (Sakata and Tamate, 1974). Injec- the amount of food ingested daily (Fell and tion of acetate, butyrate and propionate into Weekes, 1975). This weight increase is the rumen stimulates the proliferation of accompanied by an increase in the length of epithelial cells in fasting sheep (Sakata and the papillae. The diet can also modify the Tamate, 1978, 1979). Butyric acid has a shape and number of these papillae. A diet stronger stimulating effect than either pro- rich in concentrate, generally associated pionic or acetic acid. The replacement of a with high concentrations of VFA in the forage-based diet by one based on con- rumen, results in a stronger development of rumen papillae than is observed with a absorptive capacity of the epithelium (Syd- diet based on forage (Wiegand et ai, 1975). ney and Lyford, 1988). Thorlacius and Gabel et al (1987) observed that the outer Lodge (1973) even observed an increase surface of the ruminal papillae can be in the VFA absorption through the rumen increased by 200 to 400% by a change from wall despite an apparent hyperkeratinisa- a hay-based diet to one containing 64 to tion. 90% concentrate. The inner surface area of the epithelium also varies according to the development Irrigation of the rumen wall of the mucosa (Tamate and Sakata, 1979). For a ruminal mucosa with well developed The ruminal mucosa is richly vascularised papillae, the basal membrane of the epithe- and presents a complex network of anas- lium is not flat and parallel to the outer sur- tomosed vessels in the subepithelial con- face of the epithelium, but displays irregu- junctive tissue (Cheetham and Steven, larly-sized folds. These increase the surface 1966). The basal layer of the epithelium is in area of the epithelium-conjunctive tissue contact with a dense network of capillaries, interface and reduce the effective thickness and loops of capillaries also run through the of the epithelium. This type of interface is folds at the interface of the epithelium and associated with well developed papillae the conjunctive tissue of the ruminal papillae. known to be very important sites of absorp- The presence of vascularisation in these tion (Tamate and Sakata, 1979). folds can double the exchange surface In summary, the ruminal epithelium between the capillaries and the interstitial responds to an increase in ingested food tissue relative to the lumen surface of the quantities, an increase in levels of highly epithelium (Cheetham and Steven, 1966). fermentable substrates in the rumen, and The rate of rumen blood flow can vary to any other factor that induces an increase widely according to the ruminant’s feeding in VFA production (especially butyrate), by pattern and diet. Throughout a feeding cycle a change in the number and shape of the it may account for 15 to 40% of the blood papillae present on the walls, accompanied flow in the portal vein (Barnes et ai, 1983). by an increased proliferation of basal layer For sheep fed one meal per day, the blood causes an in sur- cells, which increase the flow rate could multiply by three or four face area of the epithelium-conjunctive tis- between the beginning and the end of the sue interface. These structural modifica- meal (Barnes et at, 1983; Rémond, 1992). tions of the epithelium result in an increase This increase in flow rate during ingestion of the luminal surface area of the rumen depends on the amount of dry matter an in sur- wall, and increase the exchange ingested; when the same amount of feed is face between the capillaries and the epithe- split up into 16 daily meals, these flow rate lium, thereby giving the rumen a higher variations fall to about 35% (R6mond, 1992). absorption capacity. The rumen blood flow also increases dur- Diets rich in concentrate often result in ing rumination, but far less markedly hyperkeratosis of the epithelium. The thick- (Remond, 1992; Meot and Boivin, 1994). It ening of the stratum corneum may result in also depends on the type of diet, and prob- lowered nutrient absorption rates (Nocek et ably too, on the feeding level. By supple- al, 1980) and the increase in the absorption menting a hay-based diet with starch, surface area observed with this type of diet Rémond et al (unpublished data) observed would then act as a corrective to the hyper- an increase in daily ruminal blood flow of keratinisation in order to maintain the about 50%. The injection of labelled microspheres most particularly to the rate of VFA produc- has shown that the blood flow (per unit tion. These adaptations of the epithelium weight of tissue) in the ruminal mucosa is 8 have a marked impact on the quantitative to 17 times greater than in the parietal extent of transepithelial exchange. The smooth muscles (Engelhardt and Hales, intensity of these exchanges, particularly 1977; Barnes et al, 1983). During ingestion, for those substances that diffuse passively the flow rate rises rapidly in the parietal mus- across the epithelium, is also affected by cle layers, concurrent with the increase in the subepithelial blood flow and the rumen the contractile activity of the rumen. The motricity, which respectively ensure renewal subepithelial blood flow rises more gradu- of the blood and rumen contents in contact ally, and peaks only towards the end of food with the epithelium, thereby maintaining a intake (Barnes et al, 1983). Apparently then, high transepithelial gradient. the blood flow in these two parietal areas is controlled by distinct mechanisms. While the blood flow in the muscle layers seems to be MECHANISMS AND REGULATION OF regulated via parasympathetic innervation, THE TRANSEPITHELIAL MOVEMENTS the subepithelial flow appears to be con- OF SOME METABOLITES trolled locally by the end products of intraru- minal fermentation, independently of inner- vation (Barnes et al, 1986). VFA (especially Volatile fatty acids butyric acid) and C02 are known to bring about a marked rise in the subepithelial blood The microbial fermentation of polysaccha- flow et (Dobson al, 1971; Dobson, 1979). rides (cellulose, hemicellulose, starch, etc) Increased blood flow can raise the rate of in the rumen produces mainly VFA. Depend- absorption of substances through the rumen ing on the food composition, this produc- epithelium by reducing their concentrations tion represents 50 to 75% of the ingested in the interstitial space (washout effect), metabolisable energy (Siciliano-Jones and thereby increasing the concentration gradi- Murphy, 1989; Bergman, 1990). The absorp- ent between the lumen and blood. This tion of VFA, therefore, makes a very large effect of blood flow on absorption is minor for contribution to meeting the ruminant’s substances that require active transport pro- energy needs. VFA absorption through the cesses for absorption (their movement rumen wall, first demonstrated by Barcroft et through the epithelium is relatively inde- al in 1944, represents 65 to 85% of the pendent of the concentration gradient), and intraruminal production (Weston and Hogan, for substances whose absorption is limited 1968; Peters et al, 1990, 1992). The vari- by their low transmembrane diffusability. Inn ability of this proportion is chiefly accounted contrast, for substances that diffuse very for by wide-ranging renewal rates for the rapidly through the epithelium, the absorp- liquid phase of the rumen contents. tion rate is on the blood closely dependent The main VFA present in the rumen are flow rate (Mailman, 1982). acetic, propionic and butyric acids. Their In conclusion to this first section, it is evi- molar proportions range from 70:20:10 with dent that the permeability of the epithelium a hay-based diet to 50:35:15 with a con- (degree of keratinisation and thickness of centrate diet. The VFA are weak acids with the cell layers) and the area of the exchange pKa values of 4.75, 4.87 and 4.81 for acetic, surface between the digestive compartment, propionic and butyric acids, respectively. the epithelium and the blood compartment, Calculations, using the Henderson-Hassel- can vary markedly according to diet, and bach relation, indicate that for the pH val- ues generally observed in the rumen (pH of VFA across the ruminal epithelium seems 6-7), more than 95% of the VFA will occur to occur, therefore, by a combination of pas- in their ionised form. Lowering the intraru- sive diffusion of the non-ionised form and minal pH, and thereby increasing the pro- of facilitated diffusion of the ionised form portion of non-ionised VFA, increases the (fig 1 The relative extents of these two VFA absorption rate (Dijkstra et al, 1993). As transmembrane transport processes are not non-ionised VFA diffuse much more readily yet known. According to Titus and Ahearn across the double lipid layer of the cell mem- (1992), for organs such as the rumen, in branes than their ionised forms, it is gener- which VFA concentrations are high, the VFA ally considered that VFA absorption takes anion transport system would only make a place mainly by simple diffusion of the non- small contribution to the total VFA absorp- ionised form across the epithelium of the tion, acting more as an intracellular bicar- rumen (Bergman, 1990). The absorption of bonate excreting system. VFA in their ionised form cannot be ruled Making the assumption that VFA are out, however. Using the Ussing chamber mainly absorbed by diffusion of the non- technique, Kramer et al (1994) showed that ionised form, their absorption rate will a proportion of the VFA could be absorbed depend on the concentration gradient of this in their ionised form, and that this absorption form between the digestive contents and interacted with that of chloride, which is the epithelium, and between the epithelium absorbed in exchange for intracellular bicar- and the blood (the three compartments bonate. A system of electroneutral trans- involved in the absorption process). These port in which the VFA ions are exchanged gradients are determined by the total VFA with intracellular bicarbonate has already concentration in each compartment, and by been observed in a teleostean herbivore the pH, which governs the equilibrium (Titus and Ahearn, 1992). The absorption between the ionised and non-ionised forms. As the concentration of non-ionised VFA isomers (Oshio and Tahata, 1984). These in the rumen contents is low, conversion of differences in absorption rates may be due the ionised to the non-ionised form close to to solubility differences in the lipid layers of the rumen wall will favour absorption. A ’pH the cell membranes, differences in VFA microclimate’ has been observed near the absorption mechanisms (different degrees of epithelium of the distal colon of the guinea carrier involvement) and differences in the pig and rat (Rechkemer et al, 1979; McNeil extent of VFA metabolism in the epithelial and Ling, 1980). This local pH is stable and cell (cf review Remond et al, 1995). independent of the pH in the lumen bulk phase, but is close to neutral (pH 6.4-6.9). Even if such a ‘pH microclimate’ exists near Ammonia the rumen wall, the fraction of non-ionised VFA will still be tiny. Most of the models pos- The ammonia in the rumen derives mainly tulated for VFA across the rumen absorption from deamination of amino acids released wall involve protonation of VFA before they during the breakdown of food, microbial and cross the apical pole of the epithelial cells. endogenous proteins, together with the The protons may derive either from the of urea and of from microbial fermenta- hydrolysis endogenous any pre- hydration C02 sent in food. Loss of ammonia from the tion, yielding HC03 and H+, or from the rumen can occur by incorporation into micro- Na+/H exchanger present at the apical pole bial proteins or absorption through the of the cells (Stevens et al, 1986). A link rumen wall (35 to 65% of the ammonia loss between the absorption of weak acids and from the rumen), or by export from the the Na+/H antiport has been observed by rumen with the digestive contents (10% of Gabel and Martens (1991). the loss) (Nolan and Stachiw, 1979; Sid- Inside the epithelial cells, a new equilib- dons et al, 1985; Obara et ai, 1991 Loss of rium between the ionised and non-ionised ammonia by absorption through the rumen forms of the VFA is set in the local up pH wall can thus be quantitatively very high. conditions. This tends to increase the non- This absorption was first clearly demon- ionised VFA gradient across the apical strated by McDonald in 1948. Since then, membrane of the cells. This concentration numerous studies have been carried out to gradient is accentuated by the intense intra- try to determine the mechanisms govern- cellular catabolism of the VFA. ing this absorption. VFA from the of the output basal pole Ammonia is a weak base with a pKa of 9 epithelial cells may take place by the same (Leng and Nolan, 1984). The Henderson- process as the input at the apical pole, that Hasselbach equation shows that at pH val- diffusion of the non-ionised form is, passive ues between 6 and 7, practically all the and assisted diffusion of the ionised form. ammonia will be in its ionised form (99.9 Studies in isolated rumens have shown and 98.7%, respectively), that is, in the form that at acid pH (4.5-5.5), the VFA absorption that will diffuse poorly across the lipid layers rate increases with increasing chain length, of the cell membranes. At pH values near or acetic < propionique < butyric acid, neutral, the ammonia absorption increases whereas at a pH close to neutral, the with the intraruminal ammonia concentra- absorption rates of these three acids are tion (Hogan, 1961; B6deker et ai, 1990; very similar (Thorlacius and Lodge, 1973; Remond et al, 1993b). When the intrarumi- Dijkstra et al, 1993). The absorption rates nal pH is lowered (at constant ammonia of VFA with branched chains also seem to concentration), the ammonia flux across the be lower than those of their straight-chain rumen wall is depressed (Hogan, 1961; Chalmers et al, 1971; Bodeker et al, 1990). tion (B6deker et al, 1992b, Rémond et al, At an acid pH, the ammonia absorption 1993b). Several suggestions have been remains stable despite the increase in made concerning the action of these two intraruminal ammonia concentration (Hogan, variables. For B6deker et al (1992a, b; fig 2), 1961 These findings are generally taken the interaction between the VFA and the as evidence that ammonia absorption absorption of ammonia may occur after the across the epithelium of the rumen occurs apical membrane of the epithelial cells. Since by simple diffusion of the non-ionised form the pH inside these cells is close to 7, the (NH3). The possibility of ammonium ion VFA absorbed in their non-ionised form will (NH4+) absorption from the digestive con- dissociate and release protons which can tents has also been considered (Hogan, be used to form NH4+ from the absorbed 1961; Siddons et al, 1985) but as this form NH3. This process would result in a decrease is weakly lipid-soluble, its movement across in the intracellular NH3 concentration and the membranes of the epithelial cells would thereby favour its absorption. Likewise, the require the assistance of carriers. According intracellular release of HC03 and H+ from to B6deker (1994), a system able to trans- C02 and H20 by the action of carbonic anhy- port NH4+ is present in the rumen epithe- drase could serve as a proton source for lial. The quantitative importance of this NH4+ formation (Bbdeker et al, 1992a). At absorption route remains to be assessed. each stage in its diffusion across the epithe- the thus with In the short term, the absorption of ammo- lium, NH3 equilibrates NH4+ to the nia thus depends mainly on the concentra- according prevailing pH, right through to the blood tion of NH3 near the epithelium. The VFA compartment. and the C02 in the rumen can modulate this The effects of VFA and C02 on ammonia dependency and favour ammonia absorp- absorption may also be explained by their action on the subepithelial blood flow (Dob- urea can diffuse across the rumen epithe- son, 1979) and irrigated capillary surface lium. This process is nutritionally beneficial area (Thorlacius, 1972). Remond et al for the ruminant, since the bacteria present (1993b) observed that when butyric acid in the rumen are able to use the urea nitro- was supplied to the rumen contents or when gen to synthesise proteins, the amino acids C02 was blown in, the increase in ammonia of which will subsequently be available for flux across the rumen wall was always lower postruminal absorption. The quantities of than the increase in ruminal blood flow pro- nitrogen recycled in this way vary widely duced by either of the two treatments. The (1 to 9 gN/day in sheep), and may account increase in blood flow rate favours the for 5 to 25% of the nitrogen ingested (table I). absorption of substances by reducing their The work of Houpt and Houpt (1968) concentration in the interstitial fluid steep- showed that urea transfer across the rumen the It is therefore ing capillaries. highly likely wall was linearly related to the rumen-blood that the effect of VFA and on ammonia C02 concentration gradient, and it has been gen- involves both biochemical absorption reg- erally accepted that urea crosses the rumi- ulation linked to the NH3-NH4+ equilibrium nal epithelium by simple diffusion. Intraru- within the different cell in the epithe- layers minal hydrolysis of urea by bacterial ureases and linked to ammonia elim- lium, regulation therefore facilitates the movement of urea ination in the interstitial fluid via capillary cir- through the rumen wall by maintaining a culation. concentration gradient favourable to diffu- The daily absorption of ammonia from sion. Hence, it has been shown that the inhi- the rumen has been recorded in sheep bition of urease activity in the rumen causes under various feeding conditions (table I). a decrease in the transepithelial flux of urea The quantitative data available in the litera- (Houpt and Houpt, 1968; Rémond et al, ture are, however, too sparse to accurately 1993b). In addition, the urea flux is depen- predict the daily fluxes from quantities of dent on the permeability of the epithelium. ingested nitrogen or intraruminal ammonia Since damage to the structure of the stratum concentrations. In contrast to what the corneum results in a marked increase in results of Siddons et al (1985) might sug- urea transport, urea diffusion seems to be gest, it is difficult to establish a linear rela- strongly limited by the low permeability of tionship between ammonia absorption and this epithelial layer (Houpt and Houpt, 1968). NH3 concentrations in the rumen. Given the Modifications to the exchange surfaces extent of the modifications to the absorp- between the blood and rumen contents may tion of the rumen area x capacity (surface also affect the quantitative extent of urea permeability of epithelium) according to diet, transport. particularly when this is rich in rapidly fer- When the feed is supplemented with a mentable energy sources, it is not surprising fermentable source that under these conditions, the variations in rapidly energy (sucrose, extruded wheat the flux absorption cannot be wholly explained by barley, starch), daily of urea across the rumen wall can be dou- variations in the concentration gradient bled and the of the epithe- between the rumen and the blood, and (table I), capacity lium to eliminate blood urea hence the variations in intraruminal NH3- (the transep- ithelial urea flux relative to arterial urea flux) can be multiplied by four (Remond et al, Urea unpublished data). As the blood urea level decreases when these energy supplements Since the work of Simmonet et al (1957), are added, the increase in urea flux must numerous studies have shown that blood be mainly due to modified epithelial surface area and permeability, in response to intraru- and venous capillaries, and the opening of minal VFA levels. The intraruminal urease precapillary sphincter. This suggestion of activity increases when the feed is supple- a regulation of urea flux by the amount of mented with rapidly fermentable carbohy- exchange surface is supported by the drates (Cook, 1976); it could thus also be results of Thorlacius (1972), according to implicated in the long-term regulation of urea which C02 and VFA considerably increase flux. Kennedy et al (1981) observed an the volume of blood present in the ruminal increase in the number of facultative anaer- papillae. obic bacteria to the rumen wall adhering Increasing intraruminal ammonia con- when the diet was supplemented with glu- centration decreases the urea flux across cose. a effect of the However, regulating the rumen wall (Engelhardt et al, 1978; epithelial urease activity on urea transfer Remond et al, 1993b). According to Remond still has not been demonstrated. In firmly et al (1993b), the ammonia absorption may the intraruminal urease addition, activity be responsible for reducing urea flux. The seems to be in excess relative to always effect of ammonia on urease activity is long the influx of urea in the rumen (Norton et term (Cheng and Wallace, 1979) rather than al, 1982a; Whitelaw et al, 1991). Since the short term, and the mechanism by which urease activity associated with the epithe- ammonia regulates the transepithelial flux lium is generally greater than that of the of urea during short-term variations is not rumen fluid (Wallace et al, 1979; Rybosova yet known. The work of H6rnicke et al et al, 1984), its involvement in the long-term (1972) showed that the ammonia absorp- regulation of urea flux is controversial. tion can modify haemodynamics in the rumi- The transfer of urea across the rumen nal mucosa; this could also explain the effect wall varies in the course of a feeding cycle of ammonia on urea transfer. (Rémond et al, 1993a), and so is evidently The main factors acting on urea transfer governed by a system of short-term regu- through the ruminal epithelium, and their lation. Bubbling C02 in the rumen signifi- modes of action, are summarized in figure 3. cantly increases urea flux across the rumen Other factors may also be involved in the wall (Thorlacius et al, 1971; Engelhardt et regulation of transepithelial urea flux. al, 1978; Remond et al, 1993b). Likewise, Increasing osmotic pressure in an isolated increasing the butyric acid concentration in pouch of the rumen with mannitol (Houpt, an isolated pouch of the rumen favours urea 1970) stimulates urea transfer. However, transfer (Engelhardt et al, 1978). The action Rémond et al (1993b) raised the intrarumi- of these two intraruminal factors does not nal osmotic pressure with NaCl injections, involve modifications to the ruminal urease and observed no effect on urea flux despite activity (Thorlacius et al, 1971; Remond et a lowering of the water absorption from the al, 1993b). Although C02 and butyric acid rumen. Under normal feeding conditions, stimulate subepithelial blood flow (Dobson, the osmotic pressure seems to have little 1984), the permeability of the capillary walls effect on urea transfer. Hormonal regula- to urea is too low for the blood flow to affect tion of the urea flux has also been consid- urea diffusion (Landis and Pappenheimer, ered. According to Houpt (1970), vaso- 1963). In addition, according to the results pressin may modify the permeability of the of Dobson et al (1971 urea clearance rumen wall to urea. However, Thorlacius et seems virtually independent of blood flow. al (1971) observed no modification of the VFA and C02 could affect the amount of urea clearance in response to a vasopressin epithelial surface irrigated by acting on the injection. The work of Harrop and Phillip- closing of anastomoses between arterial son (1970) and Remond et al (1993b) also suggests that gastrin might play a role in absorption of free amino acids from the the regulation of the urea flux across the rumen is, however, slight, given their very rumen wall. low levels there. The concentrations of peptides in the rumen are higher than those of free amino Amino acids acids. These concentrations vary accord- ing to the origin of the food proteins, peak- The absorption of free amino acids across ing about 1 h after feeding (Williams and the rumen wall was first observed by Cockburn, 1991). The possibility of peptide Demaux et al (1961 The permeability of absorption from the rumen by passive dif- this epithelium to amino acids has since fusion cannot, therefore, be ruled out (Webb been confirmed, with different methods, by et al, 1992). Cook et al (1965) and Leibholz (1971 a, b). The work of Webb et al (1992) even sug- gests permeability to peptides of a small Vitamins size. The concentrations of free amino acids in The work of R6rat et al (1958b) has shown the rumen are generally very low (Annison, that the rumen wall is also permeable to B- 1956; Williams and Cockburn, 1991 They group vitamins. Permeability to vitamin B1 vary during the course of a feeding cycle, seems extremely low, however (Hbller et peaking 1 h after ingestion of a meal (Leib- al, 1977). Although the vitamin concentra- holz, 1969; Williams and Cockburn, 1991). tions in the rumen are high under normal During this period, for diets rich in nitrogen, feeding conditions, it is unlikely that the free amino acid concentrations in the rumen rumen is a major absorption site for B vita- can exceed those in the plasma, resulting in mins (R6rat et al, 1958a) since these are passive diffusion through the ruminal epithe- mainly located inside microorganisms and lium (Leibholz, 1969). For most diets, the so are not available for absorption (R6rat et al, 1958b). With heavy supplementation, It is generally agreed that the main factor this absorption can become significant, as responsible for water movement through shown for niacin, which is absorbed mainly the ruminal epithelium is the osmotic gra- as nicotinamide (Erickson et al, 1991). dient between the blood in the subepithe- lial region and the rumen contents. In con- trast, the mechanism by which increasing TRANSEPITHELIAL MOVEMENT ruminal butyric acid concentration causes OF WATER AND MINERALS an increase in water absorption through the rumen wall (Dobson et al, 1976; Remond et al, 1993a) is controversial. The absorption Water of butyric acid may increase the rate of water absorption across the epithelium by causing an increase in ruminal blood flow et There is very little quantitative data con- (Dobson al, the blood cerning the net water transfer across the 1976). Increasing subepithelial a washout rumen wall. From the differences observed flow, by effect, would, however, between inflow through the oesophagus and tend to lower the concentration of solutes outflow through the reticulo-omasal orifice, absorbed in the interstitial fluid and so Warner and Stacy (1968) estimated that reduce the osmotic pressure gradient between the contents and the water absorption from the rumen could vary digestive in the sheep from 50 ml/h at rest to 300 mUL blood. The coupling of the absorption of during the hours following water intake. sodium with that of VFA (Gabel and also the stim- Using the isolated rumen method, combined Martens, 1991) may explain effect of acid on water flow. with the use of tritiated water, Willes et al ulating butyric (1970) observed that for sheep fed cut This effect may thus be linked to modifica- lucern hay twice daily, this absorption tions to the osmotic pressure gradient via ranged from 300 to 800 mUh, the highest regulation of the absorption of electrolytes values being observed 2 to 3 h after feeding. such as sodium. By lowering the intrarumi- Comparable results (360 to 720 mUh peak- nal pH with hydrochloric acid, Willes et al also observed an increase in water ing 1 h after feeding) were obtained by (1970) Remond et al (1993a) in sheep fed chopped absorption across the rumen wall. This could dactylis hay, calculating the water flux from also be explained by an increased VFA the haemoglobin levels in the arterial and absorption (linked to the fall in pH) or an venous blood and the ruminal blood flow. increased absorption of CI- supplied by HCI. On into the rumen two acids The daily net flux of water during this exper- injecting and that can be iment showed an absorption of 10 L/day. (butyric acetohydroxamic) For sheep fed continuously with compressed absorbed through the rumen wall, increased feed, Faichney and Boston (1985) estimated water absorption was observed with or with- the ruminal water absorption at 5 L/day. out increased blood flow (Remond et al, Variations in water movement Ingestion of compressed feed results in a 1993b). markedly lowered mastication time (inges- across the wall thus appear to be more tion plus rumination), which in turn results in closely linked to variations in electrolyte a lower saliva secretion. The differences in movement than to fluctuations in blood flow. saliva flow between the experiments of Dobson et al (1970) observed that C02 Faichney and Boston (1985) and Remond et stimulated water re-absorption from the ven- al (1993a) probably explain the differences tral sac of the washed isolated rumen. Its in water flux observed through the rumen mode of action was discussed by Dobson wall. (1984), who suggested the existence of a counter-current exchange system that main- The absorption of minerals in the diges- tains high concentrations of bicarbonate and tive tract can involve several pathways. C02 in the capillary blood, resulting in a high Practically all the mineral elements that are osmotic pressure. However, Remond et al present in bioavailable forms can cross the (1993b) showed that in a normally filled walls of the digestive tract along an elec- rumen, increasing the C02 did not signifi- trochemical gradient. This process is usually cantly modify the water flux across the wall.l . nonsaturatable and is not subject to any It may therefore be concluded that varia- physiological or nutritional regulation. This tions in C02 concentration in the rumen transfer can also be facilitated by transport have minimal impact on transepithelial water systems (Mg, Na, Cl, P). It can also involve flow. primary (Ca, Na) or secondary (Mg) active Finally, the rate of water absorption from transport involving energy consumption. the rumen is liable to vary according to the animal’s state of hydration. Several researchers have shown that in dehydrated Magnesium ruminants, the absorption is paradoxically much lower than in hydrated animals, Magnesium absorption from the reticuloru- despite an osmotic gradient that is particu- men accounts for about 80% of its absorp- larly favourable to absorption (Silanikove, tion from the entire digestive tract (Tomas 1994). However, these observations are and Potter, 1976). Many studies have shown controversial and have not yet been satis- that the efficiency of Mg absorption is factorily explained. strongly influenced by the physical and chemical conditions prevailing in the rumen. Thus, increasing intraruminal K concentra- Minerals tion, or reducing that of Na produces a marked decrease in Mg absorption (reviews As in single-stomach animals, the intestine, by Fontenot et al, 1989; Leonhard et al, and particularly the small intestine, was 1989). Increasing the phosphate concen- long considered to be the main or even tration favours Mg absorption (Beardsworth exclusive site of mineral absorption from et al, 1989b). The results for the effect of ruminants. Over the last 15 years, this increasing ammoniacal nitrogen are con- assumption has been challenged for mag- flicting: a decrease, sometimes marked, in nesium, for which the forestomachs are the Mg absorption (Martens and Rayssiguier, main absoptive site in the gastrointestinal 1980; Care et al, 1984; Martens et al, 1988) tract, and also for phosphorus, calcium, or no effect (Moore et al, 1972; Grings and copper, iron and zinc for which absorption Males, 1987). The experimental methods can occur before the duodenum (Kirk et al, used probably partly account for these dif- 1994; Rahnema et al, 1994). Most of the ferences; the results of Moore et al (1972) quantitative results were obtained from cal- and Grings and Males (1987) were obtained culating ruminal input/duodenal output bal- by the balance method on animals accus- ance. This type of method has the disad- tomed to a nitrogen-rich diet, while the stud- vantage of giving no indication of the exact ies showing an effect of NH3 concentration absorptive site (reticulorumen, omasum, on Mg absorption were carried out by per- abomasum) and does not take into account fusing buffer solutions in isolated rumen the minerals introduced by the saliva which pouches. The effect of NH3 concentration could mask possible absorption from the on Mg absorption therefore appears to be forestomachs. limited in time and it may occur at the time of the postprandial ruminal NH3 peak. The (Giduck and Fontenot, 1987; Giduck et al, absence of any effect due to changes in diet 1988), causes an appreciable increase in may be linked to the adaptive mechanisms the ruminal Mg absorption. This effect of the epithelium (Martens et al, 1991). observed in vivo is difficult to interpret since A fall in pH causes a drop in Mg absorp- supplementation with fermentable energy tion, which is less marked, however, in ani- substrates simultaneously produces modi- mals that have received a concentrate-rich fications to pH, and to VFA and NH3 con- diet (G5bel et al, 1987). Increasing the centrations, and modifications to the mor- intraruminal osmotic pressure from 240 to phology of the epithelium. 367 mosmol/L (Martens, 1985) or from 3155 In vitro studies of the mechanisms of Mg to 422 mosmol/L (Gabel et al, 1987) does absorption have shown that this absorption not affect the Mg absorption, although it results both from paracellular and transcel- modifies the transepithelial flux of water. lular diffusion of Mg along an electrochem- The available energy in the rumen is a ical gradient, and from electroneutral trans- factor favouring Mg absorption. In particular, port that may take place via a Mg++/2H+ the VFA concentration is positively corre- exchanger in the apical membranes lated with Mg absorption (Martens and (Martens et al, 1991; fig 4). The existence of Rayssiguier, 1980; Martens et al, 1988). an electrogenic diffusion mechanism pro- Forage supplementation with starch (Thom- vides an explanation for the inhibiting effect son et al, 1984; Giduck and Fontenot, 1987), of K on Mg absorption. In vitro methods, lactose (Rayssiguier and Poncet, 1980; which allow the K concentration and the Giduck and Fontenot, 1987) or glucose transmembrane potential (ddpt) to be varied independently, have shown that the effect of et al, 1988b; Beardsworth et al, 1989b). A K is linked to an increase in the ddpt and concentration of about 15 mmol/L of Pi is not to a direct K effect (Martens et al, 1987, optimal for Ca absorption (Care et ai, 1989). 1991 An increase in intraruminal K may This corresponds to the usual physiologi- have several effects, reducing the para- cal values in animals with no P deficiency. In cellular diffusion of Mg by raising the ddpt, contrast, the absorption of Ca is correlated and reducing the transcellular diffusion by negatively with the Mg concentration in the lowering ddpt in the apical membranes rumen; an increase in the Mg concentration (Martens et al, 1991). Reduced Mg absorp- from 1 to 5 mmol/L, spanning the normal tion resulting from a drop in intraruminal pH physiological range of 2.6 to 3.5 (Grace et al, may also be explained by a decrease in 1988), causes an appreciable decrease in ddpt linked to a decrease in Na/K-ATPase Ca absorption (Care et al, 1989). activity (Gäbel et al, 1987). The absorption of Ca in the digestive tract The electoneutral transport of Mg in the takes place simultaneously by diffusion in apical membranes depends on the intra- the intercellular spaces and by transcellu- cellular proton concentration, itself partly lar movement. In the small intestine, the lat- determined by the amount of intracellular ter makes use of a three-stage process: dif- dissociation of the absorbed VFA and the fusion through the apical membranes, hydration of C02 (Martens et al, 1991). The transport inside the cytosol by a vitamin D- absorption of Mg is therefore partly deter- dependent Ca binding protein and primary active in the basal membranes. mined by the operation of the Na/K pump transport in the basal membranes, which influences Less is known about the mechanisms of Ca the ddpt values, and the operation of the absorption from the ruminal epithelium than electroneutral exchangers that evacuate about that of Mg. In vitro studies indicated intracellular HC03 (Martens et al, 1991).). that the net flux of Ca is dependent upon active Na transport accomplished by the Na/K-ATPase system, and that the transep- Calcium ithelial fluxes consisted in both ddpt-depen- dent and ddpt-independent components The possibility of a net absorption of cal- (Holler et ai, 1988a). Adding 1-25-dihy- cium by the ruminal epithelium is now firmly droxycholecalciferol increases the net established. The quantitative importance of absorption of Ca without, however, reducing this seems to on the con- absorption depend serous-mucous transport in goats (Breves et form and of the and centration, solubility Ca, al, 1989), suggesting, as at intestinal sites, on the nutrient/Ca interactions. a regulation of active Ca transport by this The net transport of calcium through the vitamin D metabolite. rumen wall depends on the concentration of Ca++ in the rumen: net secretion up to Sodium and chloride about 1 mmol of Ca++ per L, and net absorp- tion for concentrations et higher (H61ler al, Large quantities of Na enter the rumen as 1988a; Beardsworth et al, 1989a). Under sodium bicarbonate contained in saliva (from this concentration physiological conditions, 3 to 3.5 g/L of Na in animals with no defi- is situated between 1 and 4 mmol/L (Grings ciency); about 50% of the salivary Na is and Males, 1987; Holler et al, 1988a). reabsorbed before the duodenum (Pfeffer In addition, the net absorption of Ca is et al, 1970; Gabel and Martens, 1991 ). All closely linked to the concentration of inor- the epithelia of the forestomachs display ganic phosphate (Pi) in the rumen (Holler high Na absorptive capacities, but this capacity seems to be greatest in the rumen. Potassium Chloride is also absorbed in the rumen (Martens and Blum, 1987). The amount of Cl The absorption of K in the different com- absorbed is about half that of Na (Gabel et partments of the ruminant’s stomach is al, 1987). slight; K is absorbed mainly in the small The mechanisms of the preintestinal intestine (Kirk et al, 1994; Rahnema et al, This does occur absorption of Na and Cl (fig 4) gave rise to 1994). absorption passively, the absorbed to a review (Gabel and Martens, 1991 The quantity being proportional concentration. Inversion of flow can even absorption of Na in the rumen takes place occur if the K concentration is in the against an electrochemical gradient (Gabel greater et al, 1987). In vivo studies have not plasma than in the rumen. demonstrated any such mechanism for Cl insofar as the electrochemical gradient Sulphur favours its passive absorption (Gabel et al, 1989). The transfer of Na and Cl across the A large fraction of the absorbed S is apical membrane of the epithelial cells absorbed from the rumen, mainly as sul- responds to Na+/H and CI-/HC03- phides derived from the reduction of food exchanges, respectively. The transfer of sulfates, or as inorganic S resulting in par- Na across the basal membrane involves ticular from the breakdown of sulphur-con- an Na/K-ATPase system. The mechanism taining amino acids by rumen microorgan- of the CI- exit remains to be ascertained isms, especially sulphate-reducing bacteria. (Gabel and Martens, 1991 ). In the rumen, At the usual plasma pH, the sulphides are the Na+/H exchange may be stimulated very weakly dissociated. Absorption there- by the absorption of weak acids, which fore concerns the nondissociated form would consequently affect the efficiency of (hydrogen sulphide), which is liposoluble. Na absorption. This absorption is rapid; the half-life of sul- phides in the rumen is 10 to 22 min (Bray, It is a related Phosphorus 1969). simple diffusion, directly to the sulphide concentration in the medium and and Moir, For P, most of the work done in vivo in (Bray Till, 1975; Doyle 1980). The quantity of S absorbed as sulfate can be sheep reports a net secretion of 1 to considered as 15 g/day before the duodenum (Grace et negligible. al, 1974; BenGhedalia et al, 1975; Dillon and Scott, 1979; Greene et al, 1983; Wylie Copper, iron and zinc et al, 1985). These results vary widely according to the level of feeding and the The in vivo balances indicate that the absorp- P return by the saliva. This abundant tion of Cu, Fe and Zn mainly takes place before the duodenum et al, The endogenous return by the saliva can mask (Kirk 1994). of this a net P absorption through the ruminal quantitative importance absorption the rumen and the mech- epithelium (Breves et al, 1988; Yano et al, through epithelium, anisms involved are still unknown. 1991), although this seems minor (Yano et al, 1991). The absorption of P through this epithelium does not make use of active CONCLUSION transport; it may take place via elec- troneutral transport or by simple transcel- lular or paracellular diffusion (Breves et al, This review shows that the rumen cannot 1988). be reduced to a simple microbial fermenta- tion compartment, but that it is also the site A Dobson, eds), Reston Publishing Co, Reston, VA, USA, Chapter 3 for the exchange of numerous substances Beardsworth LJ, Beardsworth PM, Care AD the rumen wall. This can (1989a) through exchange Calcium fluxes across the wall of the ovine reticulo- take the form of either absorption from the rumen in vitro. Res Vet Sci 47, 404-405 rumen diffusion into the rumen or, less often, Beardsworth LJ, Beardsworth PM, Care AD (1989b) The from the blood. Numerous studies have effect of phosphate concentration on the absorption shown that the anatomical, histological and of calcium phosphorus and magnesium from the retic- ulo-rumen of the sheep. BrJ Nutr61, 715-723 functional characteristics of the rumen Ben-Ghedalia D, Tagari H, Zamwel S, Bondi A (1975) mucosa enable these exchanges. Solubility and net exchange of calcium magnesium The most recent studies have focused and phosphorus in digesta flowing along the gut of sheep. Br J Nutr 33, 87-94 in particular on evaluating the extent of these transfers in vivo, and, using in vitro meth- Bergman EN (1990) Energy contributions of volatile fatty acids from the gastrointestinal tract in various ods, on ascertaining the mechanisms species. Physiol Rev70, 567-590 involved in these transfers the through Bbdeker D (1994) Participation of NH4’ in ammonia rumen wall. transport across sheep rumen epithelium. Proc Soc Nutr 899 Through the absorptive capacities of its Physiol3, Bbdeker D, Shen Y, Hbller H (1990) Influence of short wall, the rumen thus an role plays important chain fatty acids and HC03-on ammonia absorp- in helping to provide some of the ruminant’s tion through the sheep rumen wall. In: Proceedings energy (absorption of VFA), water and min- of the Third International Symposium on the Nutrition of Herbivores. MSAP, 36 eral needs. In addition, the absorption pro- Malaysia, Bbdeker H61ler H Involvement cess ensures a certain homeostasis of the D, Oppelland G, (1992a) of carbonic anhydrase in ammonia flux across rumen rumen excessive contents, by preventing mucosa in vitro. Exp Physiol77, 517-5199 of VFA and ammonia. This also build-up B6deker D, Shen Y, Kemskowski J, Hbller H (1992b) enables the rumen to participate in recy- Influence of short-chain fatty acids on ammonia cling processes of varying complexity, for absorption across the rumen wall of sheep. Exp Physiol77, 369-376 example, of urea or certain minerals, through rumen-saliva Bray AC (1969) Sulphur metabolism in sheep. II. The cycles. absorption of inorganic sulphate and inorganic sul- Finally, it is quite clear that absorption phide from the sheep’s rumen. Austr J Agric Res 20, 739-749 through the rumen wall can constitute a good in vivo and in vitro model for the study Bray AC, Till AR (1975) Metabolism of sulphur in the gas- tro-intestinal tract. In: Digestion and metabolism of of mechanisms of general digestive absorp- ruminants (IW McDonald, ACI Warner, eds), Univer- tion. sity of New England, Armindale, Australia, 243-260 Breves G, Hbller H, Packheiser P, Gabel G, Martens H (1988) Flux of inorganic phosphate across the sheep REFERENCES rumen wall in vivo and in vitro. Q J Exp Physiol73, 343-351 Breves G, G!bel G, Pfeffer E, Martens H (1989) Unidi- Annison EF (1956) Nitrogen metabolism in the sheep. rectional calcium fluxes across the isolated rumen Protein digestion in the rumen. Biochem J 64, 705- mucosa of goats as affected by 125-(OH)-D3. Proc 714 Nutr Soc 48, 163A Barcroft J, McAnally RA, Phillipson AT (1944) Absorption Care AD, Brown RC, Farrar AR, Pickard DW (1984) of volatile acids from the alimentary tracts of the Magnesium absorption from the digestive tract of sheep and other animals. J Exp Biol 20. 120-129 sheep. Q J Exp Physiol69, 577-587 Barnes RJ, Comline RS, Dobson A (1983) Changes in Care AD, Beardsworth LJ, Beardsworth PM, Breves G the blood flow to the digestive organs of sheep (1989) The absorption of calcium and phosphate induced by feeding. Q J Exp Physio168, 77-88 from the rumen. Acta Vet Scand 86, 152-158 Barnes RJ, Comline RS, Dobson A (1986) The control of Chalmers MI, Jaffray AE, White F (1971) Movements splanchnic blood flow. In: Control of digestion and of ammonia following intraruminal administration of metabolism in ruminants (LP Milligan, WL Grovum, urea or casein. Proc Nutr Soc 30, 7-177 Cheetham SE, Steven DH (1966) Vascular supply of Beever, DJ Thomson, eds), Agriculture Research the absorptive surface of the ruminant stomach. Council, London, UK, 4.1-4.122 P J Physiol 186, 56P Erickson PS, Murphy MR, McSweeney CS, Trusk AM Cheng KJ, Wallace RJ (1979) The mechanism of pas- (1991) Niacin absorption from the rumen. J Dairy sage of endogenous urea through the rumen wall Sci 74, 3492-3495 and the role of bacteria in the urea ureolytic epithelial Faichney GJ, Boston RC (1985) Movement of water flow. BrJ Nutr42, 553-557 within the body of sheep fed at maintenance under Cheng KJ, Costerton JW (1980) Adherent rumen bac- thermoneutral conditions. Aust J Biol Sci 38, 85-94 teria - their role in the digestion of plant material, Fell BF, Weekes TEC (1975) Food intake as a mediator urea and cells. In: epithelial Digestive physiology of adaptation in the ruminal epithelium. In: Diges- and metabolism in ruminants (Y Ruckebusch, tion and metabolism in the ruminant (IW McDonald, P MTP 227-250 Thivend, eds), Press, Lancaster, ACI Warner, eds), University of New England, Armi- Colin G (1853) Physiologie compar6e des animaux dale, NSW, 2351, Australia, 101-1188 I. 667 domestiques, tome Baillibre, Paris, p Fontenot JP, Allen VG, Bunce GE, Goff JP (1989) Fac- Cook AR (1976) Urease activity in the rumen of sheep tors influencing magnesium absorption and and the isolation of ureolytic bacteria. J Gen Micro- metabolism in ruminants. JAnim Sci67, 3445-3455 32-48 biol92, Gabel G, Martens H (1991) Transport of Na and Cl Cook RM, Brown RE, Davis CL (1965) Protein across the forestomach epithelium: mecanisms and metabolism in the rumen. I. Absorption of glycine interactions with short chain fatty acids. In: Pro- and other amino acids. J Dairy Sci 48, 475-483 ceedings of the Vllth international symposium on Demaux G, Le Bars H, Molle J, R6rat A, Simonnet H ruminant physiology, Sendai 1989, Academic Press, 129-147 (1961) Absorption des acides amines au niveau du London, UK, rumen, de l’intestin grble et du cxcum chez le mou- Gabel G, Martens H, Suendermann M, Galfi P (1987) ton. Bull Acad Vet Fr 34, 85-88 The effect of diet intraruminal pH and osmolarity on sodium chloride and from Dijkstra J, Boer H, van Bruchem J, Bruining M, Tam- magnesium absorption minga S (1993) Absorption of volatile fatty acids from the temporarily isolated and washed reticulo-rumen of Q J and Medical Sci- the rumen of lactating dairy cows as influenced by sheep. Exp Physiol Cognate 1 volatile fatty acid concentration, pH and rumen liquid ences72, 501-511 volume. BrJNutr69, 385-396 Gabel G. Bell M, Martens H (1989) The effect of low Dillon J, Scott D (1979) Digesta flow and mineral absorp- mucosal pH on sodium and chloride movement tion in lambs before and after weaning. J Agric Sci across the isolated rumen mucosa of sheep. QJ 92, 289-297 Exp Physiol74, 35-44 Dobson A (1979) The choice of models relating tritiated Galfi P, Neogrady S, Sakata T (1991) Effects of volatile water absorption to subepithelial blood flow in the fatty acids on the epithelial cell proliferation of the tract and its hormonal mediation. In: rumen of sheep. J Physiol297, 111-121 digestive Phys- iological aspects of digestion and metabolism in Dobson A Blood flow and from the (1984) absorption ruminants (T Tsuda, Y Sasaki, R Kwashima, eds), rumen Q J Exp Physio169, 599-606 Academic Press, San Diego, CA, USA, 49-59 Dobson A, Sellers AF, Shaw GT of (1970) Absorption Giduck SA, Fontenot JP (1987) Utilization of magne- water from isolated ventral sac of rumen of the cow. sium and others macrominerals in sheep supple- 100-104 J Appl Physiol 28, mented with different readily-fermentable carbohy- Dobson A, Sellers AF, Thorlacius SO (1971) Limitation drates. J Anim Sci 65, 1667-1673 of diffusion by blood flow through bovine ruminal Giduck SA, Fontenot JP, Rahnema SH (1988) Effect of epithelium. Am J Physiol 220, 1337-1343 ruminal infusion of glucose volatile fatty acids and Dobson A, Sellers AF, Gatewood VH (1976) Absorption hydrochloric acid on mineral metabolism in sheep. and exchange of water across rumen epithelium. J Anim Sci 66, 532-542 Am J 1588-1594 Physiol231, Goodlad RA (1981) Some effects of diet on the mitotic Doyle PT, Moir RJ (1980) Sulfur and methionin index and the cell cycle of the ruminal epithelium of metabolism in sheep. IV. Metabolism and absorp- sheep. Q J Exp Physiol66, 487-495 tion in the intestines. Aust J Biol Sci 303-307 33, Grace ND, Ulyatt MJ, MacRae JC (1974) Quantitative von Engelhardt W, Hales JRS (1977) Partition of capil- digestion of fresh herbage by sheep. III. The move- lary blood flow in rumen, reticulum, and omasum of ment of Mg, Ca, P, K and Na in the digestive tract. sheep. Am J Physiol232, E53-E56 J Agric Sci 82, 321-330 von Engelhardt W, Hinderer S, Wipper E (1978) Fac- Grace ND, Caple IW, Care AD (1988) Studies in sheep tors influencing the endogenous urea-N secretion on the absorption of magnesium from a low molec- and utilization in the gastrointestinal tract. In: Rumi- ular weight fraction of the reticulo-rumen contents. Br nants digestion and feed evaluation (DF Osbourn, DE J Nutr59, 93-108 Greene LW, Fontenot JP, Webb KE (1983) Site of mag- Landis EM, Pappenheimer JR (1963) Exchanges of sub- nesium and other macromineral absorption in steers stances through the capillary walls. In: Handbook of fed high levels of potassium. J Anim Sci 57, 503- physiology (WF Hamilton, ed), American Physio- 510 logical Society, Washington, DC, USA, VolII Grings EE, Males JR (1987) Effects of potassium on Leibholz J (1969) Effect of diet on the concentration of macromineral absorption in sheep fed wheat straw- free amino acids, ammonia and urea in the rumen based diets. J Anim Sci 64, 872-879 liquor and blood plasma of the sheep. J Anim Sci Harrop CJF, Phillipson AT (1970) The effect of diet and 29, 628-633 pentagastrin on the influx of urea into the rumen of Leibholz J (1971 a) The absorption of amino acids from sheep. Proc Nutr Soc 30, 3A the rumen of the sheep. 1. The loss of amino acids Henrikson RC (1970) Ultrastructure of ovine ruminal from solutions placed in the washed rumen in vivo. epithelium and localization of sodium in the tissue. Aust J Agric Res 22, 639-645 J Ultrastruct Res 30, 385-4011 Leibholz J (1971 b) The absorption of amino acids from Henrikson RC, Stacy BD (1971) The barrier to diffusion the rumen of the sheep. 2. The transfer of histidine, and ammonia across the rumen across ruminal epithelium: a study by electron glycine, epithelium microscopy using horseradish peroxidase, lan- in vitro. Aust J Agric Res 22, 647-653 thanum, and ferritin. J Ultrastruct Res 34, 72-82 Leng RA, Nolan JV (1984) Symposium: Protein nutri- Hogan JP (1961) The absorption of ammonia through the tion of the lactating dairy cows. J Dairy Sci 67, 1072- 1089 rumen of the sheep. Aust J Biol Sci 14, 448-460 H61ler H, Fecke M, Schaller K (1977) Permeability to Leonhard S, Martens H, Gabel G (1989) New aspects of in ruminants. Acta Vet Scand thiamin of the sheep rumen wall in vitro. J Anim Sci magnesium transport 44, 158-161 86, 146-151 Hbller H, Breves G, Kocabatmaz M, Gerdes H (1988a) Mailman D (1982) Blood flow and intestinal absorption. Fed Proc 2096-2100 Flux of calcium across the sheep rumen wall in vivo 41, and in vitro. 0 J Exp Physiol73, 609-6188 Martens H (1985) Magnesium absorption from the tem- Holler H, Breves G, Dubberke M (1988b) Flux of inor- porarily isolated rumen of sheep. The effect of water and osmotic Zentralbl Veterin- ganic phosphate and calcium across the isolated absorption pressure. mucosa of the sheep omasum. J Vet Med SeriesA armed !32.631-635 35, 709-7166 Martens H, Rayssiguier Y (1980) Magnesium metabolism H6rnicke H, Ehrlein HJ, Engelhardt Wv (1972) H!mo- and hypomagnesaemia. In: Proceedings of the diges- dynamische wirkungen intraruminaler ammoniak- tive physiology and metabolism in ruminants. AVII gaben bei zeigen. Zentralbl Veterinarmed [A] 19, Publishing Co, Westport, CT, USA, 117-466 822-842 Martens H, Blum I (1987) Studies on the absorption of Houpt TR (1970) Transfer of urea and ammonia to the sodium and chloride from the rumen of sheep. Comp rumen. In: Physiology of digestion and metabolism in Bioch Physiol [A} 86, 653-656 the ruminant (AT Phillipson, ed), Oriel Press, New- Martens H, Harmeyer J, Michael H (1978) Magnesium castle-upon-Tyne, UK, 119-131 transport by isolated rumen epithelium. Res Vet Sci Houpt TR, Houpt KA (1968) Transfer of urea nitrogen 24,161-168 across the rumen wall. Am J Physiol214, 1296- Martens H, Gabel G, Strozyk H (1987) The effect of 1303 potassium and the transmural potential difference Kennedy PM, Milligan LP (1980) The degradation and on magnesium transport across an isolated prepa- utilisation of endogenous urea in the gastrointestinal ration of sheep rumen epithelium. 0 J Exp Physiol tract of ruminants: a review. Can J Anim Sci 60, 205- 72, 181-188 221 Martens H, Heggeman G, Regier K (1988) Studies on the Kennedy PM, Clarke RTJ, Milligan LP (1981) Influences effect of K, Na, NH4’, VFA and COZ on the net of dietary sucrose and urea on tranfer of endoge- absorption of magnesium from the temporarily iso- nous urea to the rumen of sheep and number of lated rumen of heifers J Vet Med 35, 73-80 epithelial bacteria. BrJNutr46, 533-541 Martens H, Leonhard S, Gabel G (1991) Minerals and Kirk DJ, Fontenot JP, Ranhema S (1994) Effects of feed- digestion: exchanges in the digestive tract. In: ing lasolacid and monensin on digestive tract flow Rumen microbial metabolism and ruminant digestion and partial absorption of minerals in sheep. JAnim (JP Jouany, ed), INRA, Editions, Paris, France, 192- Sci 72, 1029-1037 216 Kramer T, Gürtler H, Gabel G (1994) Interactions McCowan RP, Cheng KJ, Costerton JW (1980) Adher- between short chain fatty acids, sodium, and chloride ent bacterial population on the bovine rumen wall: during their absorption from the reticulorumen of distribution patterns of adherent bacteria. Appl Env- sheep. Proc Soc Nutr Physiol3, 82 iron Microbiol39, 233 McDonald IW (1948) The absorption of ammonia from Pfeffer E, Thomson A, Armstrong DG (1970) Studies the rumen of the sheep. Biochem J42, 584-587 on intestinal digestion in the sheep. 3. Net move- McGavin MD, Morrill JL (1976) Scanning electron ment of certain inorganic elements in the digestive microscopy of ruminal papillae in calves fed various tract on rations containing different proportions of amounts and forms of roughage. Am J Vet Res 37, hay or rolled barley. BrJ Nutr24, 197-204 497-508 Rahnema S, Wu Z, Ohajuruka W, Weis WP, Palmquist McNeil NI, Ling KLE (1980) The mucosal surface pH of DL (1994) Site of mineral absorption in lactating the large intestine. Gastroenterology78, 1220 cows fed high-fat diets. JAnim Sci72, 229-235 M6ot F, Boivin R (1994) Rumen motility and ruminal Rayssiguier Y, Poncet C (1980) Effect of lactose sup- blood flow in sheep. Proc Soc Nutr Physiol3, 9 plement on digestion of lucerne hay by sheep. 11. Absorption of magnesium and calcium in the stom- Moore Fontenot Webb KE Effect of form WF, JP, (1972) ach. J Anim Sci 51, 186-192 and level of nitrogen on magnesium utilisation. J Anim Sci 35, 1046-1053 Rechkemmer G, Ronnau K, Kuschinsky W, Engelhardt WV (1979) pH-microclimate at the surface of the Nocek JE, Herbein JH, Polan CE Influence of (1980) intestine of guinea-pig and rat. Pfluegers Arch 382, ration form, ruminal physical degradable nitrogen R31 (abstr) and age of rumen epithelial propionate and acetate D et transport and some enzyme activities. J Nutr 110, Remond (1992) Echanges d’azote ur6ique ammo- 2355-2364 niacal a travers la paroi du rumen de mouton : cine- tique, bilan journalier et mécanismes de Nolan JV Quantitative models of regulation. (1975) nitrogen These n°467, univ Clermont-Ferrand-11, France metabolism in sheep. In: Digestion and metabolism in the ruminant (IW McDonald, ACI Warner. eds), Uni- Remond D, Chaise JP, Delval E, Poncet C (1993a) Net versity of New England, Armidale, Australia, 416-431 flux of metabolites across the rumen wall of sheep fed twice a day with orchardgrass hay. J Anim Sci 71, Nolan JV, RA of ammo- Leng (1972) Dynamic aspects 2529-2538 nia and urea metabolism in sheep. BrJNutr27, 177- 194 Remond D, Chaise JP, Delval E, Poncet C (1993b) Net transfer of urea and ammonia across the rumen wall Nolan JV, Stachiw S (1979) Fermentation and nitrogen of sheep. J Anim Sci 71, 2785-2792 dynamics in Merino sheep given a low-quality roughage diet. Br J Nutr 42, 63-80 Remond D, Poncet C, Lefaivre J (1993c) Technical note: Ruminal vein catheterization and continuous blood Nolan JV, Krebs GL, Hennessy DW (1987) Aspects of flow measurement in ruminal arteries of sheep. protein nutrition and metabolism in ruminants. In: J Anim Sci 71, 1276-1280 Isotope-aided studies on nonprotein nitrogen and agroindustrial byproducts utilization by ruminants. Remond D, Ortigues I, Jouany JP (1995) Energy sub- IAEA, Vienna, Austria, 1-177 strates for the ruminal epithelium. Proc Nutr Soc 54, 95-105 Norton BW, Janes AN, Armstrong DG (1982a) The effect of intraruminal infusions of sodium bicarbonate, R6rat A, Le Bars H, Molle J (1958a) Utilisation d’une ammonium chloride and sodium butyrate on urea m6thode de perfusion pour la mise en evidence de metabolism in sheep. BrJNutr48, 265-274 I’absorption des vitamines B chez le mouton nor- malement aliment6. Rend Acad Sci 246, Norton BW, Mackintosh JB, Armstrong DG (1982b) Urea Compt 1920-1922 synthesis and degradation in sheep given pelleted- grass diets containing flaked barley. Br J Nutr 48, R6rat A, Molle J, Le Bars H (1958b) Mise en evidence 249-264 chez le mouton de la permbabilit6 du rumen aux vitamines B et conditions de leur absorption a ce Obara Y, Dellow DW, Nolan JV (1991) Effects of energy- niveau. Compt Rend Acad Sci 246, 2051-2054 rich supplements on nitrogen kinetics in ruminants. In: Physiological aspects of digestion and metabolism Rybosova E, Javorsky P, Havassy I, Horsky K (1984) in ruminants (T Tsuda, Y Sasaki, R Kwashima, eds), Urease activity of adherent bacteria in the sheep Academic Press, San Diego, CA, USA, 515-539 rumen. Physiologia Bohemoslovaca 33, 411-4166 Oshio S, Tahata I (1984) Absorption of dissociated Sakata T, Tamate H (1974) Effect of the intermittent volatile fatty acids through the rumen wall of sheep. feeding on the mitotic index and the ultrastructure Can J Anim Sci 64 (suppl), 167-168 of basal cells of the ruminal epithelium in the sheep. Peters JP, Shen RYW, Robinson JA, Chester ST (1990) Tohoku J Agric Res 27, 133 Disappearance and passage of propionic acid from Sakata T, Tamate H (1978) Rumen epithelial cell prolif- the rumen of beef steer. J Anim Sci 68, 3337-3349 eration accelered by rapid increase in intraruminal Peters JP, Shen RYW, Robinson JA (1992) Disappear- butyrate. J Dairy Sci 61, 1109-11133 ance of acetic acid from the bovine reticulorumen Sakata T, Tamate H (1979) Rumen epithelium cell pro- at basal and elevated concentrations of acetic acid. liferation accelered by propionate and acetate. J Anim Sci 70, 1509-15 7i J Dairy Sci 62, 49-52 Sellers AF, Stevens CE, Dobson A, McLeod FD (1964) enced by diet, buffers, and pH. Can J Anim Sci 53, Arterial blood flow to the ruminant stomach. Am J 279-288 Physiol207, 371-377 Thorlacius SO, Dobson A, Sellers AF (1971) Effect of Siciliano-Jones J, Murphy MR (1989) Production of carbon dioxyde on urea diffusion through bovine volatile fatty acids in the rumen and cecum-colon of ruminal epithelium. Am J Physio1220, 162-170 steers as affected by forage:concentrate and forage Titus E, Ahearn GA (1992) Vertebrate gastrointestinal fer- physical form. J Dairy Sci 72, 485-492 mentation: transport mechanisms for volatile fatty Siddons RC, Nolan JV, Beever DE, Macrae JC (1985) acids. Am J Physiol262, R547-R553 Nitrogen digestion and metabolism in sheep con- Tomas FM, Potter BJ (1976) The site of magnesium suming diets containing contrasting forms and levels absorption from the ruminant stomach. BrJ Nutr36, of N. Br J Nutr 54, 175-187 37-45 Silanikove N (1994) The struggle to maintain hydratation Wallace RJ, Cheng KJ, Dinsdale D, 0rskov ER (1979) and osmoregulation in animals experiencing severe An independent microbial flora of the epithelium and deshydration and rapid deshydration: the story of its role in the ecomicrobiology of the rumen. Nature ruminants. Exp Physiol79, 281-300 279, 424-426 Simmonet H, Le Bars H, Mol[6 J (1957) Le cycle de Warner ACI, Stacy BD (1968) The fate of water in the l’ur6e administr6e par voie buccale chez les rumi- rumen. 2. Water balances throughout the feeding nants. Compt Rend Acad Sci 244, 943-945 cycle in sheep. Br J Nutr 22, 389-4100 Steven DH, Marshall AB (1970) Organization of the Webb KE, Matthews JC, DiRienzo DB (1992) Peptide rumen epithelium. In: Physiology of digestion and absorption: a review of current concepts and future metabolism in the ruminant (AT Phillipson, ed), Oriel perspectives. J Anim Sci 70, 3248-3257 Press, Newcastle-upon-Tyne, UK, 80-100 Weigand E, Young JW, McGilliard AD (1975) Volatile Stevens CE, Argenzio RA, Roberts MC (1986) Com- fatty acids metabolism by rumen mucosa from cattle parative physiology of the mammalian colon and fed hay or grain. J Dairy Sci 58, 1294-1300 suggestions for animal models of human disorders. Weston RH, Hogan JP (1968) The of pasture Clin Gastroenterol 15, 763-786 digestion plants by sheep. 1. Ruminal production of volatile Sydney J, Lyford J (1988) Growth and development of fatty acids by sheep offered diets of ryegrass and the ruminant digestive system. In: The ruminant ani- forage oats. Aust J Agric Res 19, 419-432 mal: and nutrition (DC Church,h , digestive physiology Whitelaw FG, Milne JS, Wright SA (1991) Urease (EC ed), A Reston Book, Prentice Hall, Canada Inc, 3.5.1.5) inhibition in the sheep rumen and its effect on Toronto, ON, Canada, 44-63 urea and nitrogen metabolism. BrJNutr66, 209-225 Tamate H, Sakata T inter- (1979) Epithelium-propria Willes RF, Mendel VE, Robblee AR (1970) Water trans- face of ruminant forestomach. Ann Rech Vet 10, fer from the reticulo-rumen of sheep. J Anim Sci 31, 482-484 85-91 Tamate Kikuchi Sakata T Ultrastructural H, T, (1974) Williams AP, Cockburn JE (1991) Effect of slowly and changes in the ruminal epithelium after fasting and rapidly degraded protein sources on the concentra- in the Tohoku J subsequent refeeding sheep. Agric tions of amino acids and peptides in the rumen of Res 25, 142-155 steers. J Sci Food Agric 56, 303-3144 Thomson Gelman Jessiman CS Effect JK, AL, (1984) Wylie MJ, Fontenot JP, Greene LW (1985) Absorption of of digestible carbohydrates on the apparent absorp- magnesium and other macrominerals in sheep tion of wether lambs. Can J Anim Sci magnesium by infused with potassium in different parts of the diges- 64, 219-220 tive tract. J Anim Sci 61, 1219-1229 Thorlacius SO (1972) Effect of steam-volatile fatty acids Yano F, Yano H, Breves G (1991) Calcium and phos- and carbon dioxyde on blood content of rumen papil- phorus metabolism in ruminants. In: Proceedings of lae of the cow. Am J Vet Res 33, 427-430 the Vllth international symposium on ruminant phys- Thorlacius SO, Lodge GA (1973) Absorption of steam- iology, Sendai !969. Academic Press, London, UK, volatile fatty acids from the rumen of the cow as influ- 277-291