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Kidney International, VOL II (l977)p. 1-8

EDITORIAL REVIEW

Site of action of diuretic drugs

Diuretic drugs continue to attract the interest of cardiac output. It is likely that the mechanism renal physiologists not only for their intrinsic tubular whereby ECF volume losses lead to decreases in effects but equally importantly for the insight that GFR is related to a reduction in glomerular capillary such studies provide into normal and abnormal plasma flow [8], although there have been no direct mechanisms of renal function. Much new informa- studies to establish this point. Other diuretics, such as tion has accumulated in recent years from experi- thiazides, mercurials, and carbonic anhydrase inhib- ments at a single nephron level particularly as a result itors, invariably cause a decrease in GFR whether of work on the isolated kidney tubule initiated by diuretic losses are replaced or not, clearly indicating Burg and his collaborators [1, 2]. This makes it ap- other mechanisms rather than changes in ECF vol- propriate at the present time to review current knowl- ume in the control of filtration rate. Recent evidence edge about the site of action of commonly used diu- presented by Wright and Schnermann [9, 10] sup- retics with particular attention to the results of ports the notion that these effects may in part be micropuncture studies. We plan to consider the avail- mediated by an intrarenal feedback mechanism able evidence in each segment of the nephron for the which couples distal salt delivery to filtration rate in following groups of drugs: the potent loop diuret- individual nephrons. ics—furosemide, ethacrynic acid and mercurials; the Proximal convoluted tubule. The various hemody- benzothiadiazides, or simply the thiazide group of namic and possibly intrarenal effects of diuretics have diuretics; the carbonic anhydrase inhibitors; and the made detection of primary transport effects within potassium—sparing diuretics. the proximal convoluted tubule very difficult to es- Before considering the specific action of diuretic tablish with certainty. The only drugs at the present drugs on the transport of water and electrolytes in the time for which the evidence strongly supports a major renal tubule, brief attention should be given to their inhibitory action on proximal salt and water reab- effects on glomerular filtration since such effects may sorption are the carbonic anhydrase inhibitors and influence to a major extent the magnitude of the the thiazide diuretics. The carbonic anhydrase inhib- diuresis. The use of all diuretic drugs is often asso- itor acetazolamide has been extensively studied using ciated with a decrease in filtration rate for reasons a variety of techniques. Virtually all groups using this that are not entirely clear. An important factor ap- drug have demonstrated inhibitory effects within the pears to relate to secondary changes in extracellular proximal tubule. The magnitude alone of the urinary fluid (ECF) volume. Burke, Robinson, and Clapp [3] bicarbonate losses that follow acetazolamide admin- have shown that the decrease in glomerular filtration istration attests to a reduction in proximal bicarbo- rate (GFR) that occurs following an administra- nate reabsorption and this has been subsequently tion of furosemide can be prevented if the diu- confirmed in free—flow micropuncture studies. The retic—induced volume losses are adequately replaced. aspect of much more recent interest relates to the The same laboratory [4], however, has found that effects of carbonic anhydrase inhibition on sodium another potent loop diuretic, ethacrynic acid, still and chloride reabsorption and whether or how such occasionally causes a fall in GFR despite concurrent effects can be attributed to the reduction of sodium replacement of fluid losses. In view of the action of and bicarbonate reabsorption. these drugs on peripheral venous compliance and Free—flow micropuncture studies have repeatedly venous return [5—7], which appears to be independ- shown that following carbonic anhydrase inhibition ent of their renal effects, it is not surprising that there is a rise in tubular fluid to plasma (TF/P) changes in renal hemodynamics and filtration rate bicarbonate ratios [11—17]. This has also been re- may occur despite prevention of ECF volume deple- flected by a corresponding fall in TF/P chloride ra- tion, mediated perhaps through subtle changes in tios towards unity [14, 17—19]. Whereas earlier work employing the quinhydrone electrode has reported TF/P bicarbonate values in excess of unity after Receivedfor publication April 22, 1976: and in revised form August 27, 1976. acetazolamide administration, TF/P chloride values © 1977, by the International Society of Nephrology. do not fall below one, suggesting that the calculated

I 2 Seely and Dirks bicarbonate results using this electrode were spur- tion that occurs. This scheme, though attractive, iously high. The reduction in bicarbonate reabsorp- lacks firm experimental support. Furthermore, direct tion after acetazolamide administration has been at- testing of this hypothesis in the in vitro perfused tributed to a reduction in active hydrogen ion rabbit proximal tubule failed to show any consistent secretion across the luminal cell membrane, which is relationship between fluid absorption and the trans- in line with current concepts of bicarbonate reabsorp- epithelial potential difference [35].Analternative hy- tion [20]. Malnic et al [21]'have studied the effects of pothesis recently proposed by Frömter [36] and by acetazolamide on the rate of intratubular acid- Schafer, Patlak, and Andreoli [37] appears more ification of split—drops of known buffer composition. likely. This attributes the coupling of solute and wa- Their results suggest that acetazolamide inhibits the ter transport to the fact that the transported bicarbo- active step of hydrogen ion secretion as well as reduc- nate species has a higher reflection coefficient than ing passive permeability of the epithelium to bicarbo- chloride. The positive potential difference is a con- nate. sequence of the transepithelial chloride gradient un- Several free—flow studies have demonstrated that in der in vivo conditions, but is not of itself a driving addition to the effects on bicarbonate transport the force for solute reabsorption. fractional reabsorption of sodium, chloride and Certain ones of the thiazide diuretics which have water was also reduced after carbonic anhydrase been studied by micropuncture exhibit many of the inhibition [17, 22, 23]. Kunau [17] has reported same effects as acetazolamide on the proximal tubule, that another potent carbonic anhydrase inhibitor, although their net effects on the whole kidney show benzolamide, significantly reduced proximal frac- some important differences, The early micropuncture tional reabsorption of chloride by 29%, sodium by study of Dirks, Cirksena, and Berliner [24] failed to 34%, and bicarbonate by 55%.Earlierreported stud- show inhibition of fractional reabsorption in the ies, in which volume losses were not replaced, had proximal tubule after hydrochlorothiazide adminis- given equivocal results [14, 24, 25]. Several groups tration, probably as a result of a failure to prevent have also shown a reduction in volume reabsorption diuretic—induced ECF volume losses and consequent measured by the split oil droplet technique [26, 27]. large falls in GFR. A subsequent study from the same Grantham [28] has reported a reduction in absorp- laboratory [38] has shown a small depression in prox- tion from both convoluted and straight portions of imal fractional reabsorption (10 to 15%) after chloro- the proximal tubule using a modified in vitro tech- thiazide administration in the dog under both hydro- nique to study the isolated rabbit tubule. The studies penic and mild saline—loaded conditions. When of Radtke et al [26], which have shown a pro- volume losses were replaced, Fernandez and Puschett longation of the split—droplet half—time in the ab- [39] have also shown that both chlorothiazide and a sence of bicarbonate using the organic buffer glyco- related drug, metolazone, inhibit fractional sodium diazine, were interpreted to mean that acetazolamide and water reabsorption in the proximal tubule of may also inhibit a noncarbonic anhydrase mediated hydropenic parathyroidectomized dogs, provided hydrogen ion secretory transport system. that the GFR did not drop by more than 26%. Sev- The mechanism whereby sodium and chloride eral groups have shown that chlorothiazide leads to a transport is inhibited secondary to the inhibition of depression of split—drop reabsorption [27, 28, 40]. In bicarbonate reabsorption is still unsettled at the pres- recent studies of Kunau, Weller, and Webb [41], frac- ent time. Chloride transport appears to be passively tional chloride reabsorption was reduced by chloro- driven down a favourable electrochemical gradient thiazide to the same extent as by the carbonic anhy- created by the preferential reabsorption of sodium drase inhibitor benzolamide. End proximal TF/P bicarbonate. The rise in luminal chloride concentra- chloride values were similarly reduced by both drugs. tion generates a small diffusion potential, lumen posi- These similarities suggest that the proximal effect of tive in the later portions of the proximal convoluted chlorothiazide may be attributable to inhibition of tubule [18, 29, 30]. It is possible that this could also carbonic anhydrase. However, metolazone, which is facilitate the passive reabsorption of sodium in this not a carbonic anhydrase inhibitor, resulted in a sim- segment in view of the high permeability to sodium ilar depression of proximal fractional reabsorption. and low electrical resistance of this portion of the Unfortunately, no direct studies of bicarbonate trans- nephron [31—33]. Interference with preferential bi- port in the proximal tubule are yet available with carbonate reabsorption by acetazolamide, which has either drug. Nevertheless, it is clear that the major been shown to result in lower IF/P chloride ratios in differences in the whole kidney effects of thiazides the late proximal tubule, leads therefore to a smaller and acetazolamide must be due to their action be- positive potential [18, 29, 34] and thus might account yond the proximal tubule. for the inhibition of sodium and chloride reabsorp- The evidence for a proximal tubular effect of drugs Diuretic site of action 3 known to exert major inhibitory effects on the loop of Table 1. Majorsite of diuretic action Henle, i.e. ethacrynic acid, furosemide and the mer- Proximal tubule —Carbonic anhydrase inhibitors curials, is much more conflicting. In part, the reason —Thiazides for this appears to be related to their hemodynamic Ascending limb —Furosemide effects. Dirks, Cirksena and Berliner [24] have shown —Ethacrynic acid -— that the use of all three drugs led to large decreases in Mercurials GFR and increases in proximal fractional reabsorp- Distal tubule and -— Thiazides collecting duct -— Spironolactone tion. This was attributed to the volume depletion that —Amiloride ensued, since diuretic losses were not replaced. More- over, this seemed likely in view of their previous demonstration that acute volume expansion led to a occurred which was reflected in falsely high values for reduction in proximal fractional reabsorption [42]. both single nephron GFR and fractional reabsorp- Furthermore, when losses were replaced during etha- tion. It is possible that this may account for the crynic acid administration, fractional reabsorption failure of many reported free—flow studies to show was then found to be unchanged. Several studies have inhibition of proximal reabsorption with furosemide subsequently confirmed that acute volume depletion despite attempts at volume replacement. It must also enhances fractional and absolute proximal reabsorp- be recognized, however, that the attempt to fully tion [43, 44]. replace fluid losses after furosemide administration A number of other variables may also interfere carries with it a danger of overcorrection and some with or obscure any direct effects of diuretic drugs on degree of volume expansion because of the magni- the proximal tubule. Recent studies of Burke and tude of the diuresis. collaborators [3, 4] have emphasized the role of In contrast to the somewhat conflicting results of changes in whole kidney filtration rate on proximal furosemide under free-flow conditions, almost all ex- reabsorption following administration of furosemide periments designed to test the effects of this drug on and ethacrynic acid. In the case of furosemide, altera- the intrinsic reabsorptive rate of the proximal tubule tions in GFR appeared to be related to ECF volume have indicated an inhibitory action. Studies in at least depletion since replacement of losses prevented a de- seven laboratories [25, 26, 40, 43, 45-47] have demon- crease in GFR. In this instance proximal fraction strated inhibition by the shrinking droplet technique reabsorption was observed to decrease [3]. When either after systemic or local administration of the losses were not replaced, GFR fell and fractional drug in the peritubular circulation or split droplet. reabsorption was found to increase. Similar increases The in vivo microperfusion studies of Morgan et al in fractional reabsorption could be demonstrated [48] have demonstrated both a reduction in net so- when GFR was reduced by means of partial renal dium transport as well as an elevation of the artery constriction despite volume replacement. In steady—state sodium concentration, The steady—state comparable experiments performed with ethacrynic chloride ratios have also been found to be signifi- acid, the same laboratory reported that changes in cantly lower after furosemide [49], suggesting that fractional reabsorption could still be correlated with steady state bicarbonate concentrations should also changes in GFR despite apparently adequate volume be higher. Radtke et at [26] and Malnic and Giebisch replacement [4]. Animals showing a severe reduction [20] have also shown a reduction in the rate of prox- in GFR (45 to 64%) showed a significant rise in imal hydrogen ion secretion after furosemide admin- fractional reabsorption, whereas fractional reabsorp- istration, although to a somewhat lesser extent than tion fell only in those animals in which there was a after acetazolamide administration, These results are minimal or no fall in GFR. All animals, however, in keeping with an inhibition of carbonic anhydrase showed significant increases in absolute and frac- which has been observed with this drug. In contrast tional sodium excretion whether proximal fractional to the foregoing studies performed under in vivo con- reabsorption had increased or decreased. Brenner et ditions, recent experiments of Burg et al [50] have al (45) have drawn attention to a possible artifact failed to show any inhibition of either net sodium introduced by retrograde collection of tubular fluid transport or generation of a negative potential differ- under diuretic conditions when intratubular pres- ence in the isolated rabbit proximal tubule in luminal sures are elevated. He showed in the rat that, pro- concentrations of up to 104M. Peritubular concen- vided large oil blocks were placed distal to the punc- trations of 103M also failed to alter the electrical ture site and thus prevented retrograde flow, absolute resistance or potential difference of the epithelium. reabsorption in the proximal tubule decreased after Grantham [28], however, has reported inhibitory ef- furosemide administration. When these precautions fects of furosemide when using a modified method of were not taken, a significant degree of contamination studying fluid absorption from the isolated rabbit 4 Seely and Dirks proximal tubule. With this method, one end of the sequent experiment from the same laboratory has tubule is closed on itself and the rate of absorption shown no further change in fractional reabsorption studied by the rate at which fluid leaves the can- when a mercurial diuretic was superimposed on a nulating pipette. It is possible that the composition of modest saline diuresis [52]. These few results suggest the luminal fluid may resemble more closely that that mercurials do not exert any significant effects on found under in vivo conditions when using the latter proximal tubular function. The clearcut effects of method, rather than when short segments are per- furosemide, ethacrynic acid, and mercurials in the fused in vitro, since there may be more time for loop of Henle (see following), in contrast to the prox- changes in perfusate composition to occur. If such imal tubular studies, give added support to the notion changes are important in determining the net rate of that different transport systems are involved in these salt and water transport, this might then account for two segments. the differences observed. It is also possible that in Loop of Henle. Studies of the effects of diuretics such experiments secretion of the drug into the l.umen within the loop of Henle have generated far less con- may osmotically restrict fluid movement apart from troversy than their action in the proximal tubule, any intrinsic or pharmacological effects. These con- reflecting the large degree of agreement between the flicting results make it difficult to draw any firm con- results of different laboratories. In this segment, in clusion about the extent of proximal inhibition after particular, studies of isolated perfused tubules have furosemide administration. Some of the data cited do provided new insight into mechanisms of transport suggest that salt and water absorption may be inhib- and of diuretic action. Clearance studies based on ited secondary to inhibition of carbonic anhydrase measures of urinary concentration and dilution have and that such effects may not be evident with the in previously pointed to the loop of Henle as an impor- vitro technique used to study the rabbit proximal tant site of action for many of the more potent diuret- tubule. It is clear, however, that the proximal effects ics. This has been subsequently supported by in vivo may be easily overcome since, unless extracellular micropuncture evidence of the composition of tubu- fluid volume is carefully sustained during the diuresis, lar fluid as it emerged from the ascending limb into fractional reabsorption actually increases. It is likely the distal convoluted tubule. Clapp and Robinson that this occurs in the clinical conditions for which [53], for instance, have been able to show an increase this drug is used. Nevertheless, as Clapp and his in the tubular fluid osmolality of early distal fluid colleagues have emphasized [3, 4], it is likely that an samples after administration of a number of diuretic inhibitory action in the proximal tubule may reduce drugs. Other groups subsequently have demonstrated the degree to which fractional reabsorption is in- significant increases in distal sodium and chloride creased by any given degree of volume contraction concentrations in a variety of species following thiaz- and, in this sense, may enhance the extent of diuresis ide, furosemide, and mercurial administration. The by ensuring a higher degree of delivery out of the non—reabsorbed fraction of filtered sodium has been proximal tubule than would otherwise occur. shown to be increased with the latter two despite the Ethacrynic acid, which closely resembles furose- lack of any change in proximal fractional reabsorp- mide in its overall effects on the human kidney, has tion, providing strong evidence for an inhibition of been far less studied by micropuncture techniques, in salt reabsorption within the 1oop itself. large part because it is relatively inactive in the rat Direct analysis of the effects of diuretics within compared to furosemide. Nevertheless, the studies loop structures has had to await the development of that are available indicate a close similarity of prox- techniques for perfusion of isolated segments of the imal effects. Both Deetjen [51]andClapp, Notte- loop in an in vitro system. Work in two laboratories bohm, and Robinson [4] have demonstrated that with this method has revealed evidence for an active ethacrynic acid leads to a reduction in proximal chloride transport system in the thick ascending limb fractional reabsorption provided volume losses are of the loop of Henle [54, 55]. This segment has been replaced and that filtration rate remains constant. shown to generate a small potential difference, lumen Split-drop studies in the rat [27] failed to show any positive, when an identical solution is placed on the significant inhibitory effects. two sides of the epithelium. The potential is abolished Mercurial diuretics are almost of historical inter- in the absence of chloride and is increased when est only, at the present time having largely been re- sodium is removed from the bathing solution. This placed by the more potent oral agents now available. segment is relatively impermeable to water and in the Experiments in the dog have shown that when vol- presence of slow— or stop—flow conditions can gener- ume losses are not replaced, GFR falls and fractional ate hypotonic fluid within the lumen. In this situation reabsorption in the proximal tubule rises [24]. A sub- the potential becomes more positive due to the impo- Diuretic site of action 5 sition of a dilution potential for sodiuni chloride itory action of this group of drugs on ascending limb across the epithelium in view of the fact that the chloride transport. permeability to sodium exceeds that of chloride. In The action of thiazides in the loop of Henle has contrast to the thick ascending limb, both the de- been less clear cut. The results of experiments which scending limb and thin ascending limbs of Henle's demonstrated no effect on free water reabsorption, loop have not convincingly been shown to be capable although free water clearance was inhibited, led to of active transport. The mechanism of osmotic equili- the concept of a "cortical diluting segment" which bration of fluid within the descending limb of Henle's was inhibited by these drugs [59]. The exact location loop and the role of the thin ascending limb in the of this site within the nephron, however, was not generation of inner medullary hypertonicity remain defined and could conceivably involve the outer med- controversial, but in any case are secondary to the ullary ascending limb of Henle's loop, distal con- action of the thick ascending limb which provides the voluted tubule or collecting duct system since in- ultimate source of energy for urinary concentration ability to generate or maintain hypotonic tubular and in part for urinary dilution. It is here that all the fluid within any one of these segments could account potent diuretics appear to have their major site of for the impairment in free water clearance. Free—flow action. Burg and his collaborators have shown that micropuncture experiments in the rat [41] have dem- furosemide [50], ethacrynic acid [56] and the mercu- onstrated that early distal chloride delivery after rial diuretic, mersalyl [57], are all capable of inhib- thiazide administration is increased to a very small iting both the electrical potential and net solute extent, compared to the marked increase in end prox- transport when applied in low doses to the luminal imal delivery which is very similar to the results seen bathing medium. It is of interest that these drugs ap- after carbonic anhydrase inhibition. If chloride is the pear to be far less potent or inactive when presented only actively transported anion species in the ascend- to the peritubular side of the epithelium. The ef- ing loop, this would strongly suggest that this process fect of mersalyl could be reversibly inhibited by is not inhibited by thiazides. Distal TF/P sodium p—chloromercuribenzoate, whereas the effect of mer- concentration ratios are found to be increased to a curic chloride could not, suggesting that the action of small extent after thiazides [38], which could result the mercurial group of diuretics is not simply depen- from the increased delivery of sodium and bicarbo- dent on the mercuric ion alone. Burg was also able to nate from the proximal segment to the distal tubule. show that the ethacrynic—cysteine adduct, in which Distal convoluted tubule and collecting duct. Studies form the drug is partly excreted, is active at a far of the distal portion of the nephron, distal convoluted lower concentration than the native compound. In tubule, collecting tubule, and ducts have been rela- view of the preferential action of these drugs from the tively hampered more by greater methodological luminal side of the membrane, it is relevant that problems than the more proximal portions. The dis- furosemide is secreted into the lumen of the proximal tal convoluted tubule is shorter and less easily acces- tubule and may thus exert an inhibitory action on the sible to the surface for in vivo free—flow studies and ascending limb when only small doses of the drug are few in vitro studies have been performed on this administered, Interference with the secretion of fu- segment. The cortical collecting tubules have been rosemide by the organic acid pathway may account in studied by perfusion techniques but are not generally part for the reason that larger doses of the drug need as accessible for micropuncture as are the medullary to be administered for a diuretic effect in uremic collecting ducts which have also been studied by patients. With the exception of amiloride, which is microcatheterization techniques. One of the major inactive in the ascending loop [58], there is no direct problems of studies using free—flow micropuncture information on the action of other diuretic drugs on techniques has been the fact that changes in proximal the ascending limb studied in the in vitro system. or loop function may have resulted in different rates Clearance studies suggest that acetazolamide has of volume and solute delivery to the distal tubule. little or no inhibitory effects within the loop [59].A Unless the response of this segment to such changes recent study [41] has demonstrated that net chloride in load produced by other means is known, it is reabsorption within the loop was increased after ad- impossible to detect changes in reabsorption due to ministration of the carbonic anhydrase inhibitor ben- diuretic effects from those due to alterations in load, zolamide, since fractional chloride delivery rose from apart from any other hemodynamic or other effects 54% to 68% at the end of the accessible proximal of the diuretic on the kidney. Many authors have tubule in contrast to a very small change from 4.5 to surmised that an elevation of the intratubular sodium 6.3% in fractional delivery to the early distal tubule. concentration in the distal tubule implies an inhibi- This gives further evidence for the lack of an inhib- tion of sodium reabsorption both in the loop and 6 Seely and Dirks distal tubule; however, perfusion studies make clear high urine flow. They concluded that the effects of that high rates of delivery to the loop alone may furosemide on potassium transport could be attri- result in high intratubular concentrations of sodium buted entirely to changes in flow rate along the distal and chloride. In a free—flow study of the rat distal tubule. The effects of carbonic anhydrase inhibitors tubule, Duarte, Chomety, and Giebisch [60] found on the distal transport of potassium has been studied that furosemide almost completely abolished the so- extensively by Giebisch and his collaborators. Studies dium gradient along the distal tubule; however, the of both amphibian and mammalian kidneys have fraction of the filtered sodium reabsorbed along the demonstrated that these agents lead to augmented distal tubule appeared to be 20 to 30%, which is two potassium secretion [22, 60]. Wiederholt, Sullivan, to three times greater than the normal amounts reab- and Giebisch [611 suggested that these effects resulted sorbed in this portion. It seems highly likely that such from enhanced uptake of potassium at the per- apparent increases in absolute sodium reabsorption itubular membrane and an increase in the intra- in this portion result from the higher delivery and the cellular potassium transport pool. low gradient against which sodium must be absorbed. The action of the mildly natriuretic potassium Morgan et al [48] have studied the effects of furose- sparing agents have received little study by micro- mide on distal tubular function by microperfusion of puncture. The effect of aldosterone within the distal the loop of Henle and sequential collections from two convoluted tubule and collecting duct has been well sites in the same distal nephron. They attempted to established [62, 63]. In view of the evidence that control for differences in the loop by providing higher spironolactone is a competitive antagonist of perfusion rates in the control state to achieve com- aldosterone and other mineralocorticoids, it is rea- parable flow rates in the early distal tubule. They sonable to expect any future micropuncture studies were unable to show any significant differences in under in vivo conditions or in isolated tubules to reabsorptive rates in control or furosemide—treated support such a distal action. animals; however, the scatter in the results was large Amiloride and triamterene, compounds which are and the concentrations of sodium and potassium physiologic though not specific antogonists to al- were not equal in the two situations. dosterone, also result in modest natriuresis with re- Work with carbonic anhydrase inhibitors has con- duced potassium excretion. During free—flow and firmed that distal hydrogen ion secretion is reduced microperfusion studies of the loop of Henle, amiloride [20] and that free-flow distal sodium concentrations was not found to alter early distal tubular concentra- are elevated along with concurrent increases in frac- tions of sodium or potassium [60] nor has any effect tional sodium reabsorption [60, 61]. Wiederholt, Sul- been observed with this drug in studies of the isolated livan and Giebisch [611 concluded that the increased ascending limb. However, as expected from clearance load provided additional sodium to undersaturated studies, amiloride has marked effects on the distal distal reabsorptive sites. Recent work of Kunau, convoluted tubule and collecting duct. Duarte, Weller, and Webb [41] presents strong evidence that Chomety, and Giebisch [60] observed a tendency for thiazide diuretics inhibit the process of sodium and distal sodium concentration ratios to rise along the chloride reabsorption in the distal tubule. The car- distal convoluted tubule. This was accompanied by a bonic anhydrase inhibitor, benzolamide, in contrast, small increase (2%) in fractional sodium excretion. had no inhibitory effects on distal chloride reabsorp- Conversely, the normal increase in potassium con- tion. This effect appears to account for the major centration along the distal tubule was blunted, in- natriuretic and chloruretic effect of thiazide diuretics dicating inhibition of distal potassium secretion and and probably accounts for the depression of trans- hence leading to a reduction in urinary potassium port in the "cortical diluting segment" identified by excretion. The mean distal transtubular potential dif- clearance studies. ference fell from —46 to —26 mV. They considered In contrast to the relative paucity of data regarding that amiloride directly reduced luminal sodium per- changes ii sodium reabsorption as a result of diuret- meability, a finding consistent with studies of this ics in the distal tubule, there is abundant evidence to drug on anuran membranes. The recent work of Sto- suggest that major changes in potassium transport ner, Burg, and Orloff on the perfused rabbit cortical occur in this segment. Duarte, Chometry and collecting tubule [64] demonstrated that amiloride Giebisch [60] found in their study that whereas early rapidly reversed the negative transtubular electrical distal tubular potassium concentrations appeared to potential leading to a small positive potential differ- rise following furosemide, late distal TF/P potassium ence. Sodium reabsorption and potassium secretion concentration ratios were equal or lower than con- were also sharply decreased. Further addition of trols, yet fractional secretion and urinary fractional acetazolamide abolished the positive potential differ- excretion were considerably increased owing to the ence, suggesting the presence of a current of acid- Diuretic site of action 7 ification. The small natriuretic effect compared to the 4. CLAPPJR,NOTTEBOI-IM GA, ROBINSONRR:Proximal site of antikaliuretic effect was attributed to the meagre so- action of ethacrynic acid: Importance of infiltration rate. Am J dium load normally presented to the collecting tubule Physiol 220:1355—1360. 1971 5. OciiLviE RI, RUEDY J: Hemodynamic effects of ethacrynic acid but could also reflect the possibility that sodium reab- in anephric dogs. J Pharmacol Exp Ther 176:389—396, 1971 sorption is unimpaired by the drug beyond the cor- 6. OGILvIE RI, SCHLIEPER E: Comparative effects of ethacrynic tical collecting tubule. Meng [65] has recently con- acid, furosemide, and diazoxide in the perfused dog hindlimb. firmed, using the split oil droplet technique, that Can J Physiol Pharmacol 49:1038—1043, 1971 intraluminal application of amiloride (up to lO-6M) 7. DiKSHiT K, VYDEN JD, FORRESTER iS, SWAN 1-IJC: Renal and extrarenal hemodynamic effects of furosernide in congestive strongly inhibits isotonic fluid absorption in the distal heart failure after acute myocardial infarction. N Engi J Med convoluted tubule of the rat kidney. Proximal effects 288:1087—1090, 1973 were also seen but only at very much higher concen- 8. BRENNER BM, TROY IL. DAUGI-IARTY TM, DEEN WN, ROB- trations. ERTSON CR: Dynamics of glomerular ultrafiltration in the rat: In conclusion, the availability of many different II. Plasma flow dependence of GFR. Am J Physiol 223:1184— 1190, 1972 diuretic drugs that can selectively inhibit one or more 9. WRIGHT FS: Intrarenal regulation of glomerular filtration rate. of a variety of transport processes within the nephron NEngIJ Med 291:135—141,1974 provides a wide range of options that can be ra- 10. WRiGHT FS, SCIINERMANN J: Interference with feedback con- tionally used in the treatment of edema states and trol of glomerular filtration rate by furosemide, triflocin and nonedematous disorders. Caution must be exercised cyanide. J C/in Invest 53:1695—1708, 1974 II. CLAP!' 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WEINSTEIN SW: Micropuncture studies of the effects of aceta- fects of certain diuretics upon proximal tubular trans- zolamide on nephron function in the rat. Am J Physiol port are still unsettled and the results of invivo 214:222—227, 1968 15. BERNSTEIN BA, CLAPP JR: Micropuncture study of bicarbo- experimentshave not been reconciled with invitro nate reabsorption by the dog nephron. Am J Physio/ studiesin the isolated perfused tubule. The mecha- 214:251—257, 1968 nism whereby carbonic anhydrase inhibitors affect 16. V1EIRA FL, MALNIC G: Hydrogen ion secretion by rat renal sodium and chloride transport is still unclear as is the cortical tubules as studied by an antimony electrode. Am J extent to which such effects can account for the prox- Phy.siol214:710—718,1968 17. KUNAURLJR: The influence of the carbonic anhydrase inhib- imal action of thiazide diuretics. 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