Ludwig's Theory of Tubular Reabsorption: the Role of Physical Factors in Tubular Reabsorption

View metadata, citation and similar papers at core.ac.uk brought to you by CORE

provided by Elsevier - Publisher Connector

Kidney International, Vol. 9 (1976) p. 313—322 EDITORIAL REVIEW

Ludwig's theory of tubular reabsorption: The role of physical factors in tubular reabsorption

From the very origins of physiology as a science, chet [2] created a sensation among physiologists and interest was focused on flow of body fluids. Beginning physicians alike. Quoting from a review [3], appear- with movement of fluid inside blood vessels and later ing in 1828 in the first volume of the American Jour- extending to that across membranes, physiologists nal of Medical Science, of Dutrochet's experiments: have sought mechanical explanations for these phe- "M. Dutrochet has recently published a work on vital nomena. The action of physicochemical forces across motion, in which he details experiments and discov- endothelial membrane structures is reasonably well eries, of a most interesting and extraordinary char- understood, at least in a qualitative sense; it is essen- acter, calculated to throw a new light upon an tially the same as for certain collodion membranes. important portion of physiology. ... M.Dutrochet On the other hand, there is as yet no general con- was, as yet, unable to assign a cause for this physico- sensus regarding the means by which physicochemical organic phenomenon, to which he applied the name forces influence convective flux (volume flux) across of endosmose." It appears to have been Graham who epithelial membranes. In fact, it is only in the first demonstrated that for colloidal solutions, mem- last decade or so that most workers in the field branes more or less completely impermeant to the have come to recognize that physical forces, gen- macromolecule could be prepared, so that osmosis erated outside of the activity of the epithelial cells could be thought of as a unidirectional flow of sol- themselves, may be important determinants in con- vent into the solute-containing compartment [4], and vective flux across such epithelial structures as the first suggested substituting the term osmosis in place proximal tubule and the mucosal membrane of the of endosmosis. intestine. In the hope of providing a broader per- It was during this early period (1826-1846) of in- spective with which to view present-day problems tense interest in osmosis that Carl Ludwig was being with respect to transepithelial movement of fluids, it educated. In the year 1842 he delivered two inaugural may be appropriate to recall some aspects of the addresses on the kidney in Marburg [5]. The first development of thought in regard to the nature of the dealt with the microanatomy of the kidney, the sec- responsible mechanisms. ond with the chemical composition of plasma and The experimental or observational threads which urine; in the latter he made the first rational attempt first provided the basis for a rational explanation of to clearly specify the forces involved in the formation fluid flow across biological membranes were two. The of urine. origin of the first of these is lost in antiquity and Ludwig sought to describe the elaboration of urine undoubtedly was known to man from the simple in terms of two processes which had been observed observation that water can be forced through certain in inanimate systems: ultrafiltration and osmosis. solid materials even though pores or channels are not The chemical differences between plasma and urine visible to the unaided eye; and in this filtration pro- were conceived by him to be due to differential cess certain larger particles can be held back or sieved. permeability characteristics between the glomerular The second observation was the discovery of osmosis. and tubular membranes; i.e., certain solutes present The discovery of this latter phenomenon is gen- in glomerular ultrafiltrate are unable to penetrate the erally credited to Abbé Nollet in the year 1748 [1]. tubular membrane across which the fluid must pass in However, it was not until many years later, between its return to the capillaries. The force for return of 1826 and 1840, that the experiments of Henri Dutro- fluid to the peritubular capillary bed he described as arising from two circumstances: (a) the fall in hydrostatic pressure in the peritubular capillaries to low levels as a consequence of the blood having traversed Receivedfor publication May 26, 1975; and in revised form December 1, 1975. two arteriolar resistances in series, and (b) the rise in © 1976, by the International Society of Nephrology. postglomerular oncotic pressure above that in sys-

313 314 Bresler

the tubular cells. The term secretion says nothing about the nature of the forces involved in the formation of glomerular fluid nor in the deposition of solutes in the lumen. Therefore, it had little predictive power and could scarcely be tested experimentally. On the other hand, Ludwig's theory attempted to come to grips explicitly with the nature of the forces involved, was predictive in nature and consequently capable of being tested, Often, the better a theory is, the more vulnerable it is to experimental assault; assault on Ludwig's theory must have been active from the outset: first on philosophical and later on experimental grounds. It must be recalled that at the beginning of, and well into the 19th century, a raging controversy prevailed between the vitalists, who were convinced that phenomena relating to living systems could never be understood in terms of laws relating to inanimate systems, and the mechanists, who entertained quite the opposite view. While Bowman's de- scriptive theory grated on no vitalistic nerve, Lud- wig's theory must have irritated it maximally. Viewed from our present-day vantage point, the inadequacies of Ludwig's theory are quite evident. Though the Tubule glomerular ultrafiltration portion of Ludwig's theory Fig. 1. Schematic representation of glomerular and peritubular cap- is well accepted at present, the role he gave to the illary beds with adjacent tubule (lower portion of the figure), hydro- tubular membrane as merely a selective sieve was too static pressure in the various vascular segments (upper portion, continuous line) and oncotic pressure (upper portion, dashed line). limited and resulted in that greatest of scientific trage- The units in the ordinate are in mm Hg. dies—the destruction of a beautiful theory by a fact. In this case there were several facts, but we shall temic plasma by virtue of glomerular ultrafiltration1 focus on two which surfaced early and appeared irrefutable. The theoretical foundations for what ap- (see Fig. 1). The virtuosity of this theory can best be appeared to be the fatal assault were laid down in the laws of thermodynamics, which were more or less preciated by comparing it with Bowman's [8], ad- vanced at about the same time. It will be recalled completely formulated by the latter part of the 19th that Bowman suggested that the formation of urine century, and the incorporation of the laws of solu- was achieved by secretion of a watery fluid at the tions into these by Van't Hoff. Ludwig's theory in the glomerulus which washed down solutes secreted by simple form given by him could only account for the formation of a urine isosmolal with plasma. In 1859 Hoppe [9] demonstrated that when urine and serum from the same animal were separated by a pig 'It has been stated by Homer Smith [6] that in 1843 Ludwig had only the vaguest idea regarding which solutes contribute to endos- bladder membrane, the direction of flow was from mosis and that it is probable that he had in mind total solids, plasma to urine. In 1892 H. Dreser [10] pointed out including the proteins. The following excerpt from Ludwig's Lehr- that urine osmotic pressure may be far in excess of buch der Physiologie des Menschen [7] indicates that by 1861 he clearly had in mind proteins and not total solids. that in plasma and that such a result cannot arise Another hypothesis takes into consideration the characteristic through equilibration of oncotic forces. These find- type of circulations through the kidney and the phenomenon ings seemed to have the seal of thermodynamic final- that the wall of numerous capillary systems of the animal's body is endosmotically impermeable to proteins and fats. ity and apparently dealt the death blow to Ludwig's Proceeding from this basis it makes the supposition that it is postulate concerning the nature of tubular reabsorp- the blood pressure, which obtains on the inner surface of the tive forces. vessels of the glomerulus, which drives the entire blood serum (minus the proteins, fats and the salts bound to these sub- Both arguments had subtle flaws. Hoppe's experi- stances) into the lumen of the uriniferous tubules. The fluid ment was inconclusive inasmuch as the reflection which has arrived here would gradually course through the coefficients of urinary and plasma solutes across uriniferous tubules and in this way come into endosmotic relationship to the concentrated blood which courses in the a pig's bladder probably differ radically from those capillaries surrounding the tubules beyond the glomeruli. across renal tubular membranes. The importance of Role of physical factors in tubular reabsorption 315 the membrane structure and the qualitative nature of to have been more or less resolved. Even at this stage the solutes, rather than the thermodynamic potential there was strong disagreement over the quantitative of water alone in determining the direction of os- aspects of glomerular filtration. Some observers, in- motically induced volufne flux across membranes, cluding Starling [17], felt that glomerular filtration was clearly appreciated in 1895 by Lazarus-Barlow rate (GFR) did not greatly exceed the rate of urinary [11]. A paper on these matters written by Max Oker- flow. It was not until the publication of studies on Blom in 1907 represents to my mind the clearest creatinine clearance by Rehberg [18] and inulin clear- exposition of these problems to be found anywhere in ance by Smith and Clarke [19] that the concept of the literature [12]. In the case of Dreser's argument, filtration of large quantities of fluid with subsequent the defect is even more subtle. The laws of thermo- reabsorption of most of this by the tubules was uni- dynamics apply to systems at equilibrium and versally accepted. there is no assurance that at any point in the tubule the urine comes into full equilibrium with plasma, Development of Homer Smith's natriocentric theory especially in view of the changing anatomical structure of the tubular membrane and the luminal and Early advocates of the view that large volumes of capillary flows. This theoretical loophole remained fluid are reabsorbed were confronted with the diffi- unnoted and it was not until the advent of Werner cult problem of describing the nature of tubular reab- Kuhn's work that this particular issue was dealt with sorptive mechanisms. Cushny [20], impressed by the properly. Kuhn, Ryffel and Hargitay [13, 14] devised fact that the tubular reabsorbate differed only slightly a method and a working model for the production of in composition from glomerular filtrate, spurned the concentrated solutions from dilute solutions by small proposition that each molecular species would be forces. This consisted of a countercurrent flow ar- absorbed by a separate transport system inasmuch as rangement which brought adjacent solutions, sepa- nature would not likely be so extravagant as to dis- rated by a membrane and flowing slowly in a direc- assemble and then reassemble the components of tion opposite to one another, into a steady-state ultrafiltrate to form a reabsorbate of virtually ident- contact. By this means small forces acting in step- ical composition. He therefore proposed that some wise fashion could contrive to produce large con- nonspecific force, generated by cellular metabolism, centration differences in a direction axial to flow, creates a flow of an ideal Locke's solution across the a biological lever arm as it were. This raised the cell. It was precisely on the point of the ideal nature possibility that small physical forces acting in some of the reabsorbed fluid that Wesson, Anslow and such system could theoretically account for forma- Smith [21] chose to criticize Cushny's formulation. tion of concentrated urine, especially in view of the They correctly pointed out that it would appear to known required presence of Henle's loop for urine require just as elaborate a physicochemical system to concentration. Ludwig's reabsorption theory was, reabsorb a fluid of specUied composition as to reab- implicitly at least, disinterred by Kuhn only to be sorb the separate components by individual transport reburied by Kuhn and Ramel when later calculations systems.2 They then proposed the view of tubular [15] revealed that the minimal single-step force re- reabsorption which dominates current thinking. I quired in the case of the kidney exceeded available shall call it the "natriocentric theory." physical forces. By this time the discovery of active It is important to review the state of existing cellular absorptive and secretory processes had al- knowledge when Wesson et al set about formulating ready rendered Ludwig's theory in its simple form a theory for tubular reabsorption. The following were inadequate. known to them: (a) large volumes of fluid are filtered But we have gotten slightly ahead of the story as it and then reabsorbed, (b) the osmolality and composi- evolved. Even the glomerular ultrafiltration portion tion of urine can differ radically from that of plasma of Ludwig's theory was long delayed in its final ac- depending on body needs, (c) large volumes of vir- ceptance. Proponents of glomerular secretion of wa- tually sodium-free urine can be formed, (d) sodium ter, which was said to provide force for washout of concentration inside cells is very low, (e) there are secreted solutes, were not convinced when it was pointed out that in order to accomplish this a secre- 2Theobjection of Wesson et alto Cushny's view applies only when one supposes that reabsorbate constitutes an "ideal Locke's tory force in excess of 5000 mm Hg would be re- solution." The concept of passive reabsorption as proposed by quired. It apparently was not until Richards [16] Ludwig does introduce a vast simplification and could theoretically demonstrated that fluid collected from Bowman's be entertained for a large fraction of tubular reabsorption provided one were willing to make the appropriate assumptions concerning capsule by micropuncture approximated in composi- the freedom of sodium and other solutes to cross some hydauli- tion an ultrafiltrate of plasma that the issue appears cally conductive area of the membrane. 316 Bresler specific transport systems for active reabsorption such distal sodium reabsorption would not exceed 20% of as for glucose and active secretory transport sys- filtered sodium) by directly sampling proximal fluid tems as for PSP and (f)cellsin general appear to during mannitol diuresis. They found that sodium be permeable to water. concentration in luminal fluid, under such condi- They therefore conceived that the columnar epithe- tions, fell significantly below plasma values. hal cells in the kidney formed an impermeant barrier The presumed impermeability of the tubular mem- to sodium and perhaps other electrolytes while allow- brane to sodium immediately foreclosed a number of ing free diffusion of water and somewhat hindered considerations: for example, Ludwig's earlier pro- diffusion of some smaller neutral molecules as, for posal that peritubular oncotic forces could lead to example, urea. They suggested that the main motive significant reabsorption [7], and any conception of force for fluid transport across the tubule arises from nonspecific forces (i.e., forces not created by a so- the action of a specific transport system for sodium dium pump) generated within a cell which would with water following passively to achieve osmotic account for movement of water, sodium chloride and equilibration and chloride following passively to other solutes across it, en masse, as suggested by achieve electropotential equilibration. Cushny. This concept of nonpermeability to sodium Also known to them, from the micropuncture (if not in the absolute sense, then at least in a relative studies of Walker et al [22],wasthat despite the sense) still remains as the bedrock upon which the formation of a urine differing markedly from plasma sodium-pump model for transcellular transport is in osmolality and sodium content, tubular fluid, from based. Having precluded appreciable permeability as far down in the proximal tubule as could be sam- for sodium, there remained no other possibility save pled and despite a marked diminution in its volume, the sodium pump to account for the reabsorption of remained essentially isonatric and isosmolal with fluid in the proximal tubule inasmuch as no other plasma. For this reason Smith coined the terms solutes are available in sufficient quantity to attract "obligatory" and "facultative" reabsorption of wa- appreciable amounts of water across a sodium-im- ter [23]. In the proximal tubule the process of water permeant membrane. reabsorption appeared blind to the osmoregulatory Smith's theory of proximal tubular reabsorption aspects of homeostasis inasmuch as tubular fluid re- was soon adopted for other epithelial membranes, mained isosmolal under all conditions (the process such as those of the intestine, across which reabsorp- appearing only to be concerned with reducing the tion was known to take place with luminal fluid volume of tubular fluid insofar as strong electrolytes remaining essentially isosmolal. Curran and Solomon and water are concerned), and so the term "obliga- [26] showed that when the mannitol content of a tory" for water reabsorption seemed appropriate. series of luminal saline solutions was increased step- On the other hand, in the distal tubule osmolal- wise (maintaining total osmolality equal to plasma), ity and sodium composition may undergo marked a close correlation of solute (mainly sodium) with changes appropriate to the homeostatic needs of water transport occurred. These findings were taken the body, whence the term "facultative". Smith to provide further corroborative evidence for the thought the isonatricity and isosmolahity of proximal primacy of a sodium pump, inasmuch as the reduc- fluid resulted from the relatively high water permea- tion of net sodium flux and, presumably secondarily, bility of the membrane in this portion of the tubule. water flux from solutions of successively higher These ideas, with respect to the nature of proximal mannitol concentration were considered to arise reabsorption, were put to the test in Smith's labora- as a result of the increasingly adverse sodium con- tory by Wesson and Anslow [24]. Inducing a strong centration gradient.3 osmotic diuresis by mannitol infusion, volumes of When one adds to these studies the results of other urine up to about 70% of GFR were obtained. From studies—as for example the demonstration of an ap- the sodium concentration of urine, and the as- parent salt pump operative in the ascending thick sumption that distal sodium reabsorption would not limb of Henle [28], and the behavior of the salt- exceed 20% of that filtered, they inferred that the secreting gland of certain marine birds [291, as well as sodium concentration of proximal fluid fell consid- the general feeling that the membrane is largely im- erably below that of plasma. This was the crucial permeant to sodium, and finally with a large liter- evidence they sought to support the view that sodium ature of observations, all compatible with the natrio- reabsorption was active and primary, with water following passively. Some years later Windhager and 3Analternate explanation of these results would be that a solu- Giebisch [25], using micropuncture, plugged one of tion pump, moving salt water in bulk, is increasingly inhibited by the loopholes in this experiment (the assumption that stepwise increments in mannitol concentration [271. Role of physical factors in tubular reabsorption 317

centric theory—it is not surprising that evidence for sistent change in GFR. (c) Vogel and Heym [38] had determinants of fluid reabsorption other than the shown that comparison of colloid with noncolloid- operation of a sodium pump across a sodium- containing solutions perfusing the renal portal vein of impermeant or relatively impermeant membrane has the frog revealed that the presence of colloid en- received scant attention until very recently. hanced tubular reabsorption. (d) Patients in the edema-accumulatory phase of congestive heart fail- Evidence supporting a role for physical factors ure were known to have high filtration fractions (FF) [39] which fell during compensation. (e) Several stud- The original basic premise of the natriocentric the- ies had been reported in which ECF or plasma vol- ory, the impermeance of the membrane to sodium, ume expansion or both were associated with lowering would of course preclude any appreciable direct ac- of filtration fraction and enhanced urinary excretion tion of oncotic forces as originally conceived by Lud- [40, 41]. However, the evidence on this point was not wig or even by any force not generated by a sodium conclusive as, for example, in one study plasma ex- pump.4 It is understandable why proposals by Bresler pansion with dextran was reported not to signif- [3 1-33] that proximal tubular reabsorption might be icantly alter FF [42]. generated in large part by peritubular Starling forces, Thus, scattered as it was, there appeared to be perhaps aided and abetted by active transport of sufficient evidence to reconsider Ludwig's original substances as glucose, etc., met with little favor. Sev- proposal with respect to peritubular control of prox- eral years later a similar proposal, made by Vander et imal reabsorption.5 The explanation of ECF was al [34], was rejected at least in part because the known to be accompanied by protein dilution and method employed (stop-flow) was not deemed appro- low filtration fraction, the contraction of ECF by priate to describe proximal tubular events. high filtration fraction and, as a consequence, high The proposal [31] that Ludwig's model might ap- peritubular protein concentration. It was therefore ply to the proximal tubule arose from an attempt to suggested [31] that peritubular oncotic pressures, in define a model of the renal function appropriate for particular as they might be conditioned by FF, volume regulation. It was pointed out that volume played an important role in absorption across the regulation differs fundamentally from regulation of proximal tubular membrane inasmuch as absorption the electrolyte composition of body fluids. The latter was then known to proceed with the luminal fluid is regulation of an intensive property of extracellular remaining isosmolal and isonatric. Supportive of fluid (ECF) while the first is a regulation of an exten- this concept were studies reported independently by sive property. It was therefore proposed that the na- Toussaint and Vereerstraeten [44] and Bresler [32] ture of renal regulatory mechanisms might also differ which indicated that a hypernatric filtrate obligated a fundamentally. Ludwig's theory for tubular reab- hypernatric reabsorbate and that more salt was absorption seemed to be ideally suited to volume regu- sorbed during hypernatremia and total body surfeit lation and the abrupt changes in urinary flow asso- of salt. These were interpreted as supporting a prox- ciated with postural changes. Moreover, insofar as imal tubular control on bulk fluid reabsorption was known at the time, the principal objection to rather than sodium reabsorption. In 1964 Giebisch, Ludwig's theory of tubular reabsorption did not nec- Klose and Windhager [45] and Lassiter, Mylle and essarily appear to apply to the proximal tubule, Gottschalk [46] using micropuncture studies reported where reabsorption in the ordinary course of events a seemingly similar paradoxical behavior for the remained isosmotic and isonatric. proximal tubule of rats. Differing observations have There was considerable early evidence in support been provided by two studies claiming to have de- of the view that oncotic pressure significantly in- monstrated a tubular maximum (Tmax) for sodium in fluenced absorption across epithelial membranes: (a) the proximal tubule [47, 48]. A series of experiments by Herbert Wells in the This attempt at revival of Ludwig's theory of tubu- 1930's had provided evidence favoring oncotic pres- lar reabsorption for the proximal tubule was put to a sure as an important determinant in the rate of reab- more direct experimental test using micropuncture sorption across the intestinal mucosal membrane [35, techniques. Plasma or colloid-containing solutions 36]. (b) Podhradszky [37] had shown that infusion of were placed in the lumen of the proximal tubule saline, in dogs, led to a greater diuretic response than between oil droplets. By measuring the decrease in identical solutions with acacia added despite no con- size and estimating the volume change of the aqueous

Theassumptionof absolute impermeability to sodium has undergone a steady erosion [301, particularly with regard to inter- It is recognized that tubular localities other than proximal cellular "shunt" pathways. tubule may be importantly involved in volume regulation [43J. 318 Bresler

droplet, one could compare rates of reabsorption in Brenner and co-workers [67-73], who have subjected the presence and absence of colloid. As had been the problem of Ludwig-Starling factors to the most shown much earlier for the case of the intestine [49, thorough theoretical and experimental scrutiny at- 50], absorption continued despite the installation of tempted thus far. Deen, Robertson and Brenner [74, plasma, indicating that forces other than and much 75] demonstrated that filtration of fluid in the stronger than oncotic forces were at play in proximal glomerulus and uptake of fluid by the peritubular tubular reabsorption [51-53]. Even though intra- capillaries must be correlated with effective filtrative luminal colloid, in two of the three studies, partially or absorptive forces which, in turn, must be an in- inhibited transport rate, if all forces involved in tegrated mean force (i.e., averaged over the length of transport out of the lumen were passive (conditioned the capillary). Theoretical analysis reveals that the by physical factors), tubular reabsorption should equations are in general highly nonlinear and in cer- have been completely inhibited by plasma. It has tain ranges the mean force may be highly flow-depen- been subsequently shown that reabsorption persists dent. This important point had eluded the attention up to luminal protein concentrations of 27 g/100 ml of most investigators who like myself had only a [54]. It is clear from such studies that forces other qualitative feel for the problem and thought mainly than and of much greater potential intensity than in linear and nonflow-dependent terms.6 Moreover, peritubular physical forces can exist in the transport Brenner argues convincingly that a proper experi- pathways of the proximal tubule. In fact, however, mental evaluation of whether one factor (i.e., oncotic the first three mentioned studies were given a much pressure) is well correlated with absorptive rate re- stronger interpretation, viz., that Starling forces quires either exact knowledge of other pertinent exerted no or a vanishingly small influence on the variables of Starling's equation or, failing this, an overall transport process. However, a number of experimental design in which these can be reason- studies subsequently appeared supporting for the ably assumed to remain constant between control proximal tubule what Herbert Wells [35, 36] had long and perturbed conditions. Since the forces involved ago claimed to show for the jejunum: the rate of are small and delicately interrelated, great care must reabsorption is directly and importantly related to be taken in experimental design and execution in the oncotic pressure of plasma in adjacent capillaries. order to obtain meaningful results. Failure to fully Credit for keeping alive the concept of the impor- appreciate one or more of these difficulties may ac- tance of the level of peritubular oncotic pressure count for some of the discordant results among work- as a determinant of renal tubular reabsorption must ers in the field. be given first of all to Vereerstraeten and coworkers Recently l-lolzgreve and Schrier [77] using micro- [55-58]. Using clearance techniques and estimating puncture techniques have compared the response of peritubular oncotic pressure from plasma protein proximal tubule segments, artificially perfused with a concentration and filtration fraction, they showed 9 g/l00 ml of protein-containing, Ringer-like solu- an excellent correlation between the rise in oncotic tion, and tubules whose capillaries were allowed to be pressure and the fall in sodium excretion [58], perfused by blood in an undisturbed fashion to two It was Earley, Martino and Friedler who first pro- experimental perturbations: (1) aortic constriction vided strong evidence that this effect is exerted (in and (2) saline expansion. Since they were unable to part at least) at the level of the proximal tubule [59] find significant differences in the response of the two and did much to revive interest in physical deter- sets of tubules, they concluded that peritubular on- minants of tubular reabsorption [60-63]. Somewhat cotic pressure is not an important determinant of earlier, Shipley and Study [64] and Selkurt [65] had absolute tubular reabsorptive rate. Somewhat dis- suggested that changes in interstitial pressure might turbing in these studies is their failure to find a de- exert reciprocal effects on tubular reabsorption. pression of reabsorptive rate in their blood-perfused However, undoubtedly due to the fact that these tubules during saline expansion, a finding universally results required strained interpretation in terms of found by other workers. This may be related to the the natriocentric theory, it was not until Lewy and extremely wide variation in nephron filtration rates Windhager similarly interpreted their micropuncture between kidneys and in the same kidney which was data [66] that the full impact of these findings be- present in their studies. Perhaps their inability to find gan to be appreciated widely. Many additional an effect ascribable to peritubular oncotic pressure is micropuncture studies followed this paper, most likewise related to highly unfavorable "signal to noise but not all of which confirmed the importance of ratio" problems. Starling forces in determining the rate of reabsorption across the proximal tubular membrane. 6Anexception is Earley and Schrier, who also recognized the Particularly noteworthy among these are studies by importance of flow dependency [761. Role of physical factors in tubular reabsorption 319

In two studies, using "shrinking drop" techniques [84], who demonstrated that leakage of a molecule as of micropuncture, no change was found in reab- large as inulin from blood to lumen across the gut sorptive rates from the proximal tubule when oncotic could be induced by expansion of ECF volume by i.v. forces were altered by artificial perfusion of the ad- saline infusion. (2) The second explanation contends jacent capillary bed [47, 78]. However, a more recent that oncotic force acts, to a significant degree, directly study from the first laboratory, using similar tech- in the manner first visualized by Ludwig. Studies niques, shows a definite effect of changes in oncotic cited earlier which revealed no or small effects of in- force [79]. traluminally placed colloid solution on transport lmai and Kokko [80] found that the volume of argue against this last possibility. However, more fluid transported out of isolated perfused rabbit prox- recently Perrson, Agerup and Schnermann [54] have imal tubules was markedly decreased when serum found much more marked inhibition of transport in ultrafiltrate was substituted for serum in the bath such experiments and suggest that from 20 to 30% of fluid. Conversely, when serum concentration to 12.5 tubular reabsorption may be in direct answer to per- g/lOO ml of protein was substituted for normal se- itubular forces. Yet more recently studies by Green, rum, a marked increment in reabsorptive rate was Windhager and Giebisch [85] have yielded results observed. Similar findings were reported by Grant- indicating that such passive forces cannot account for ham, Qualizza and Welling [81], save for the fact that more than 8% of proximal reabsorption. elevation of bath from 6 to 10 g/l00 ml of albumin Critically involved in these estimates is the value was without effect. On the other hand, Horster et al assigned to the hydraulic conductivity (L0) of the [82], using similar preparations, were unable to find membrane. The marked discrepancy in values ob- an effect of oncotic pressure. However, more recently tained for hydraulic conductivity of the same bio- results obtained in this last laboratory, in studies in logical membrane, when driving forces of a different which artificial solutions free of and containing bo- nature are used, counsels caution in attributing great vine albumin were added to the bath solutions, have significance to them. Schnermann's discussion [83] of revealed an increment in tubular reabsorption when these matters is well worth reading. protein was present (M. Burg, personal communi- Bearing importantly upon the question of direct vs. cation). indirect effect of oncotic pressure is a report by Mur- For a listing of many other studies both pro and rish and Schmidt-Nielsen which appeared in 1970 in con bearing on the question of whether Starling Science [86]. Since it involves transport across the forces influence the reabsorptive rate across the epithelial membrane of the cloaca, it has perhaps renal tubules and the proximal tubule in particular, escaped the notice of many renal physiologists. Using the interested reader is referred to the review of the wick technique, devised by Scholander, Hargens Schnermann [83]. In view of the technical difficulties, and Miller [87] for measurement of tissue pressure, the lack of unanimity is not at all surprising. In they were able to measure the intensity of the reab- final analysis, though the exact quantitative role of sorptive force across the cloaca of the desert iguana. physical forces has yet to be established in various This turned out to be surprisingly low, about 210mm physiological circumstances, a substantial body of H20, and corresponded closely with measured plasma evidence now exists which supports the view that oncotic pressure. When the animals were dehy- Starling forces in general and peritubular capillary drated, both oncotic pressure and intensity of ab- oncotic forces in particular may exert an important sorptive force rose to about 250 mm H20. These influence on the rate of reabsorption from proximal findings indicate a direct transepithelial action of on- tubules. cotic force. Moreover, they signify that, for some pathway across certain epithelial membranes, the Direct versus indirect action of physical forces reflection coefficient for sodium must near zero. This need not be in conflict with reported reflection There remains the problem of explaining how coefficients ranging from about 0.4 to 0.7 for the such small forces as hydrostatic and oncotic pres- whole membrane in mammals as found by others sure can be operative within the framework of the [83]. Attempts to measure reflection coefficients in natriocentric theory. Generally the explanations have complex mosaic membranes, much less living mem- proceeded along two lines: (1) The first proposes that branes, are fraught with dangers very much the same reduction of oncotic force leads to accumulation of In a mosaic membrane such as that of epithelial cells, we are interstitial fluid, particularly in intercellular spaces dealing with at least two pathways: (a) transcellular and (b) inter- where it induces a back-leak of sodium or salt water. cellular. A reflection coefficient determined across the whole membrane by inducing a gradient in total osmolality does not neces- Experimental evidence favoring such a view has been sarily provide information concerning the reflection coefficient presented for rat intestine by Humphreys and Earley across the zona occludens itself. 320 Bresler as those described by Schnermann for measurement Reprint requests to Dr. Emanuel H. Bresler, Veterans Adminis- tration Hospital, 1601 Perdido Street, New Orleans, Louisiana of hydraulic conductivity. 70146, U.S.A. Regardless of the exact details of the transepithelial References step in transport, Earley and Schrier [76], in a de- 1. NOLLET JA: Recherches sur les causes du bouillonment des tailed analysis of the evidence relating to glomerulo- liquides. MemAcad (Paris), 101,1748 tubular balance, indicated that perhaps this phe- 2. DUTROCHET RJH: Endosmosis, in Cyclopediaof Anatomy and Physiology, editedby Toon RB, London, Longman, Brown, nomenon is related to a balance of physical forces Green, Longmans and Roberts, 1836—1839, vol. 2, PP. 98—111 generated in the glomerular capillary filtrative bed 3.QuarterlyPeriscope: On vital motion. AmJ Med Sci 1:423, and dissipated in the peritubular capillary reab- 1828 sorptive bed, inasmuch as under a wide variety of 4. GRAHAMT:On osmotic force (Bakerian lecture). PhilosTrans, conditions the absolute rate of tubular reabsorption 177—228,1854 5. LUDWIGC:Beitthgezur Lehre vom Mechanismus der Harnse- is closely correlated with the oncotic force generated cretion. Marburg,NG E!wert, 1843 by the filtrative process. Thus, not only is capillary 6. SSIITH H: Highlights in the history of renal physiology. George- uptake the final common pathway for reabsorption, town Med Bull 13:4—48,1959 butthislast step appears to be rate-limiting under the 7.LUDWIG C:Lehrbuchder Physiologic des Menschen. Heidel- conditions analyzed by them and in the studies of berg, CF Wintersche Verlagshandlung, 1861, p. 427 8. BOWMAN W: On the structure and use of the Malpighian Brenner and associates. bodies of the kidney. PhilosTrans 132:57—80,1842 (Reprinted in MedClassics 5:258—291, 1940) 9. HOPPEF:Ueber die Bildung des Hams. ArchPathol Anat Conclusion 16:412—413,1859 The historical development of the concept that 10. DRESERH:Ueber Diurese und ihre Beeinflussung durch phar- makologische Mittel. ArchExp Pathol Pharmacol 27:303—319, physical forces in the peritubular surroundings in- 1892 fluence absorption across certain epithelial mem- II. LAZARUS-BARLOWWS:Observations upon the initialrates of branes has been traced. The question of whether osmosis of certain substances in water and in fluids containing these forces act in a permissive manner by influencing albumin. JPhysiol 19:140—166,1895 the permeability of the zona occiudens (or some other 12. OKER-BLOMM:Tierische Säfte und Gewebe in Physikalisch- chemischer Beziehung: IX Mitteilung. Die physiologische barrier) to various solutes or whether these forces Bedeutung der tierischen Membranen für die Resorption- act directly or both remains moot. It should be serscheinung. SkandinArch 19:162—170,1907 reemphasized that the quantitative aspects of this 13. KUHN W, RYFFEJ,K:Herstellung konzentrierter Losungen aus problem remain to be resolved as well as the nature of verdOnnten durch blosse Membranwirkung: Em Modellver- their participation in the phenomenon of glomerulo- such zur Funktion der Niere. ZPhysiol Chem 276:145—178, 1942 tubular balance and volume regulation. However, 14. HARGITAY B, KUHN W: Das Multiplikationsprinzip als Grund- Ludwig's original vision of the nature of urine forma- lage der Ham Konzentrierung in der Niere. ZElektrochem tion appears to be, at least in part, applicable to the 55:539—558,1951 reabsorption of fluid from the proximal tubule just as IS. KUHN W, RAMEL A: Aktiver Salztransport als mOglicher (und it has long been recognized to be valid for the forma- wahrscheinlicher) Einzeleffekt bei der Harnkonzentrierung in derNiere. I-letsChim Acta 42:628—660,1959 tion of glomerular ultrafiltrate. Should future studies 16. RICHARDS AN: Process ofurine formation: Croonian lecture. establish a direct transmembrane action of oncotic ProcR Soc Lond/Biol] 126:398—432,1939 forces of significant magnitude then, in my opinion, 17. STARLING EH: The glomerular functions of the kidney. JPhys- given the information available at the time, Carl Lud- jot 24:317—330,1899 wig's conception of the urine formation must rank as 18. REHRERG PB: Studies on kidney function. BiochemJ 20:447—482, 1926 one of the most brilliant intuitive insights in the his- 19. SMITH HW, CLARKE RW: The excretion of inulin and creati- tory of physiology. nine by the anthropoid apes and other infrahuman primates. AmfPhysiol 122:132—139, 1938 EMANUEL H. BRESLER 20. CUSIINY AR: TheSecretion of Urine. London,Longmans, New Orleans, Louisiana Green and Co., 1917, p. 28 21. WESSON LG JR, ANSLOW WP, Sairi-i HW: The excretion of strong electrolytes. BullNYAcadMed 24:586—606,1948 Acknowledgments 22. WALKER AM, Boil PA, OLIvER J, MAC D0wEL1. M: Collec- tion and analysis of fluid from single nephrons of the mamma- Ms. Kristin Nielsen and Meredith Clark helped in lian kidney. Am J Physiol134:580—595,1941 the preparation of this manuscript, Dr. Edward Fer- 23. SMIEH HW: ThePhysiology of the Kidney. Oxford,Oxford University Press, 1937, p. 233 guson engaged in discussions related to kidney func- 24. WESSON LG, ANSLOW WP JR: Excretion of sodium and water tion over the years, and Dr. Barry Brenner supplied during osmotic diuresis in the dog. Am J Physiol153:465—474, an English translation of Ludwig's inaugural address. 1948 Role of physical factors in tubular reabsorption 321

25. WINDUAGER EE,GIEBISCUG:Micropuncture study of renal LICI-I E, ULLRICH KJ, WIDERHOLT M: Natriumtransport in den tubular transfer of sodium chloride in the rat. Am J Physiol proximalen Tubuli und den Sammelröhren bei Variation der 200:581—590, 1961 Natriumkonzentration in umgebenden Interstitium. Pflugers 26. CURRANPF,SOLOMONAK:Ion and water fluxes in the ileum Arch 310:354—368, 1969 of rats. J Gen Physio/ 41:143—168, 1957 48. PUSCIIETT JB, GOLDSTEIN 5, GODSHALL S, STAUM BR, GOLD- 27. BRESLEREl-I:Aspects of tubular reabsorption revisited. Chest BERG M: Effects of filtration rate and plasma sodium on prox- 58:261—270, 1970 imal sodium transport. Am J Physio/ 221:788—794, 1971 28. KASI-IGARIAN MH, STOCKLE H, GOTTSCI-IALK CW, ULLRICH 49. VISSCHER MB, ROEPKE RR, LIFSON N: Osmotic and elec- KJ: Transtubular electrochemical potentials of sodium and trolyte concentration relationships during the absorption of chloride in proximal and distal renal tubules of rats during autogenous serum from ileal segments. Am J Physio/ 144:457— antidiuresis and water diuresis (diabetes insipidus). Pfl0gers 463, 1945 Arch Ges Physio/ 277:89—106, 1963 50. RABINOVITCH J: Factors influencing the absorption of water 29. SCHMIDT-NEILSEN K, JORGENSEN CB, O5AKI H: Extrarenal and chlorides from the intestine. Am J Physiol 82:279—289, salt excretion in birds. Am J Physiol 193:101—107, 1958 1927 30. ULLRICH KJ: Permeability characteristics of the mammalian 51. WHITTEMBURY 6, OKEN DE, WINDFIAGER EE, SoLOMON AK: nephron, chapter 12, in Handbook of Physiology, section 8, Single proximal tubules of necturus kidney: IV. Dependence Renal Physiology, edited by ORLOFE J, BERLINER RW, Wash- of H2O movement on osmotic gradients. Am J Physio/ 197:1121 - ington D.C., American Physiological Society, 1973, p. 394 1127,1960 3,. BRESLER EH: The problem of the volume component of body 52. KASIIGARIAN M, WARREN Y, MITCHELL RL, EPSTEIN FH: fluid homeostasis. Am J Med Sci 232:93—104, 1956 Effect of protein in tubular fluid upon proximal tubular ab- 32. BRESLER EH: Reabsorptive response of renal tubules to ele- sorption. Proc Soc Exp Bio/ Med 117:848—850, 1964 vated sodium and chloride Concentrations in plasma. Am J 53. GIEBISCH G, KLOSE RM, MALNIC G, SULLIVAN WJ, WINGIIA- Physiol 199:517—521, 1960 GER FE: Sodium movement across single perfused proximal 33. BRESLER EU: Reflections on the nature of renal tubular reab- tubules of rat kidneys. J Gen Physiol 47:1175—1194, 1964 sorption. Am Heart J 62:1—6, 1961 54. PERRSON AEG, AGERUP B. SCHNERMANN J: The effect of lu- 34. VANDERAJ,MALVIN RL, WIDLE WS, SULLIVAN LP: Re-exam- minal application of colloids on rat proximal tubular net fluid ination of salt and water retention in congestive heart failure; flux. Kidney mt 2:203—2 13, 1972 significance of renal filtration fraction. Am J Med 25:497—502, 55. VEREERSTRAETEN P, TOUSSAINT C: Excretion urinaire du 1958 chlorure de sodium chez le chien: Arguments en faveur de 35. WELLS US: The passage of materials through the intestinal transport tubuaire passif de l'ion Na. Rev Fr Etudes C/in wall. Part I. Am J Physio/ 99:209—220, 1931 Bio/ 7:949—955, 1962 36. WELLS US: The passage of materials through the intestinal 56. VEREERSTRAETEN P, TOUSSAINT C: Modalites de l'excretion wall. Part II. Am J Physiol 101:434—445, 1932 urinaire d'une surcharge en chlorure de sodium chez Ic chien. J 37. PODHRADSZKYL:Kristalloidosan és kolloidosan isotoniás old- Urol Nephro/68:157—163,1962 atok hatbsa a vesemiiködksre. Orvoskepzes, Herzoz Memorial 57. VEREERSTRAETEN P, DE MYTTENAERE M, LAMBERT PP: Reduc- Vol., 1939, pp. 63—66 tion de Ia natriurese par Ia perfusion de proteines dans l'artere 38. VOGEL G, HEYM F: Untersuchungen zur Bedeutung kolloi- renale de chien. Nephron 3:103—122, 1966 dosmotiseher Druckdifferenzen für den Mechanismus der 58. VEREERSTRAETEN P, DE MYTTENAERE M: Effect of raising the isosmotischen FlOssigkeitsresorption in der Niere. Pflflgers transtubular oncotic gradient on sodium excretion in the dog. Arch Ges Physiol 262:226—232, 1956 PflOgers Arch Ges Physio/ 302:1—12, 1968 39. HELLER BI, JACOBSON WE: Renal hemodynamics in heart 59. EARLEY LE, MARTINO JA, FRIEDLER RM: Factors affecting disease. Am Heart J 39:188—204, 1950 sodium reabsorption by the proximal tubule as determined 40. WILSON JR JR, HARRISON CR: Cardiovascular, renal, and during blockade of distal sodium reabsorption. J Clin Invest general effects of large, rapid plasma infusions in convalescent 45:1668—1684, 1966 men. J C/in Invest 29:25 1—257, 1950 60. EARLEY LE, FRIEDLER RM: Studies on the mechanism of 41. BRADLEY SE, BRADLEY GP, TYSON CJ, CORRY JJ, BLAKE WD: natriuresis accompanying increased renal blood flow and its Renal function in renal diseases. Am J Med 9:766—798, 1950 role in the renal response to extracellular volume expansion. J 42. FLEMING JW, CARGILL WH, BLOOMWL:Effects of in- C/in Invest 44:1857—1865, 1965 travenous administration of dextran on renal function. Proc 61. EARLEY LE, FRIEDIER RM: The effects of combined renal Soc Exp Bio/ Med 79:604—606, 1952 vasodilation and pressor agents on renal hemodynamics and 43. STEIN JH, OSGOOD RW, BOONJARERN S, FERRIS TF: A Com- the tubular reabsorption of sodium. iC/in Invest 45:542—55 1, parison of the segmental analysis of sodium reabsorption dur- 1966 ing Ringer's and hyperoncotic albumin infusion of the rat. J 62. MARTINO JA, EARLEY LE: Demonstration of a role of physical C/in Invest 52:23 13—2323, 1973 factors as determinants of the natriuretic response to volume 44. TOUSSAINT CH, VEREERSTRAETEN P: Renal tubular transport expansion. J C/in Invest 46:1963—1978, 1967 of sodium during sodium chloride and sodium bicarbonate 63. MARTINO JA, EARLEY LE: Relationship between intrarenal loadings in the normal dog. Experientia 16:309—311, 1960 hydrostatic pressure and hemodynamically induced changes in 45. Gisnsi-i 6, KLOSE RM, WINDHAGER EE: Micropuncture sodium excretion. Circ Res 23:37 1—386, 1968 study of hypertonic sodium chloride loading in the rat. Am J 64. SIIIPLEY RE, STUDY RS: Changes in renal blood flow, extrac- Physio/ 206:687—693, 1964 tion of inulin, glomerular filtration rate, tissue pressure and 46. LASSITER WE, MYLLE M, GOTTSCHALK CW: Net transtubular urine flow with acute alterations of renal artery blood pres- movement of water and urea in saline diuresis. Am J Physio/ sure. Am) Physio/ 167:676—688, 1951 206:669—673, 1964 65. SELKURT EE: Effect of blood pressure and mean arterial pres- 47. BALDAMUS CA, HIERI-1OLZER K, RUMRICH G, STOLTE H, Uu- sure modification on renal hemodynamics and electrolyte and 322 Bresler

water excretion. Circulation 4:541—551, 1951 77. HOLZGREVE I-I, SCHRIER RW: Variation of proximal tubular 66. LEWYJE, WINDHAGER EE:Peritubular control of proximal reabsorptive capacity by volume expansion and aortic con- tubular fluid reabsorption in the rat kidney. Am J Physiol striction during constancy of peritubular capillary protein 214:943—954, 1968 concentration in rat kidney. PflOgers Arch 356:73—86, 1975 67. BRENNERBM,FALcIIUKKH,KEIrnowITzRI,BERLINERRW: 78. LOWITZ HD, STUMPEKO,OCHWALDB:Micropuncture study The relationship between peritubular capillary protein concen- of the action of angiotensin II on tubular sodium and water tration and fluid reabsorption by the renal proximal tubule. J reabsorption in the rat. Nephron 6:173—187, 1969 C/in Invest 48:15 19—1531, 1969 79. SATOK:Reevaluation of some micropuncture techniques: 68. BRENNER TROYJL:Postglomerular vascular protein con- BM, Some of the factors which affect the rate of fluid absorption by centration: Evidence for a causal role in governing fluid reab- the proximal tubule, in Biochemical Aspects of Renal Function, sorption and glomerulotubular balance by the renal proximal edited by ANGIELSKIS,DUBACHNC,Bern, Hans Huber, tubule. J C/in Invest 50:336—349, 1971 1975, pp. 175—187 69. FALCHUKKH,BRENNERBM,TAD0K0R0M,BERLINERRW: 80. IMAIM,KOKKOJP:Transtubular oncotic pressure gradients Oncotic and hydrostatic pressures in peritubular capillaries and net fluid transport in isolated proximal tubules. Kidney mt and fluid reabsorption by proximal tubules. Am J Physiol 6:138—145, 1974 220:1427—1433, 1971 81. GRANTI-IAMJJ, PB,WELLINGLW:Influence of se- 70. BRENNER TROY JL, DAUGHARTYTM:On the mechanism QUALIZZA BM, rum proteins on net fluid reabsorption of isolated proximal of inhibition in fluid reabsorption by the renal proximal tubule tubules. Kidney mt 2:66—75, 1972 of the volume-expanded rat. J Clin Invest 50:1596—1602, 1971 82. HORSTERM.BURGM,POTTSD,ORLOFF J: Fluid absorption 71. DAUGHARTYTM,UEKIIF,NICI-IOLASDP,BRENNERBM: Comparative renal effects of isoncotic and colloid-free volume by proximal tubules in the absence of a colloid osmotic expansion in the rat. Am J Physiol 222:225—235, 1972 gradient. Kidney mt 4:6—11, 1973 72. BRENNERBM,GAILAJH:Influence of postglomerular he- 83. SCI-INERMANN J: Physical forces and transtubular movement of matocrit and protein concentration on rat nephron fluid trans- solutes and water, chapter 6, in MTP International Review of fer. Am J Physiol 220:148—161, 1971 Science, Kidney and Urinary Tract Physiology, edited by 73. MYERSBD,DEENWM,BRENNERBM:Effects of norepineph- TI-IURAUK,GUYTONAC,Baltimore, University Park Press, rifle and angiotensin II on the determinants of glomerular 1974, vol. 6, pp. 157—198 ultrafiltration and proximal tubule fluid reabsorption in the 84. HUMPI-IREYSMH,EARLEYLE:The mechanism of decreased rat. Circ Res 37:101—110, 1975 intestinal sodium and water absorption after acute volume 74. DEENWM,ROBERTSONCR,BRENNERBM:Transcapillary expansion in the rat. J Clin Invest 50:2355—2367, 1971 fluid exchange in the renal cortex. Circ Res 33:1—8, 1973 85. GREENR,WINDHAGEREE,GIEBIscH G: Protein oncotic pres- 75. DEENWM,ROBERTSONCR,BRENNERBM:A model of per- sure effects on proximal tubular fluid movement in the rat. Am itubular capillary control of isotonic fluid reabsorption by the J Physiol 226:265—267, 1974 renal proximal tubule. Biophys J 13:340—358, 1973 86. MURRISHDE,SCHMIDT-NIELSENK:Water transport in the 76. EARIlYLE,SCHRIERRW:Intrarenal control of sodium excretion by hemodynamic and physical factors, chapter 22 in cloaca of lizards. Science 170:324—325, 1970 Handbook of Physiology, section 8, Renal Physiology, edited 87. SCHOLANDERPF,HARGENSAR,MILLER SL: Negative pres- by ORLOFF J, BERLINERRW,Washington, D.C., American sure in the interstitial fluid of animals. Science 161:321—328, Physiological Society, 1973, pp. 721—762 1968