Klinische Wochenschrift
Klin. Wochenschr. 57, 1001-1008 (1979)
© Springer-Verlag 1979
Renal Tubular Control of Potassium Transport* G. Giebisch Department of Physiology, Yale University School of Medicine, New Haven, Connecticut, 06510, USA
Key words- Renal K transport - Tubular heterogeneity - Micropuncture Intracellular K activity Acid base balance - Adrenal steroids - Diuretics Homeostasis. This review will address problems related to (1) potassium transport sites within the nephron and, (2) to some of the control mechanisms modifying net transport of this ion species. Attention will be directed towards definition of the electrochemical driving forces across the luminal and peritubular cell membranes of those tubule cells which control potassium translocation. Furthermore, an effort will be made to pinpoint those transport parameters which are involved in the modulation of potassium excretion.
proximal tubule reabsorbs 60-70% of the filtered potassium. Proximal tubular potassium concentrations are close to that of plasma potassium concentrations. Normally, the rate of reabsorption of potassium along the proximal convoluted tubule is tightly coupled to that of sodium and water, although after acetazolamide [14] and after unilateral nephrectomy [15] proximal tubular sodium reabsorption is depressed whereas that of potassium continues at a fairly undiminished rate. This demonstrates that, potentially at least, proximal tubular potassium transfer may be regulated independently of sodium and of fluid transport. Studies on proximal straight tubules by perfusion of isolated segments in vitro have shown that net reabsorption takes place, although at a significantly lower rate than across the proximal convoluted tubule [16]
I. Tubular Sites of Potassium Transport
A large body of experimental evidence supports the view that most of the filtered potassium is reabsorbed in the proximal tubule and the loop of Henle. It is the distal tubule, the collecting tubule and the papillary collecting duct which determine and control the rate of urinary potassium excretion. Renal control of potassium excretion consists of either reabsorption of that amount of potassium that escapes proximal tubular reabsorption, or of net secretion, in response to specific stimuli, of variable amounts of potassium across the distal nephron. The topic of renal transport and control of potassium has been extensively reviewed [1-13].
(a) Proximal Tubule Micropuncture studies have firmly established that the accessible portion of the superficial mammalian * Studies carried out in the author's laboratory were supported by N.I.H. Grant PHS-AM-17433
(b) Loop of Henle Some recent evidence has evolved that suggests that the loops of Henle of superficial and juxtamedullary nephrons may exhibit somewhat different potassium transport patterns. Figure 1 provides a summary of what is presently known about the progression of potassium transport along cortical and juxtamedullary nephrons [12]. Micropuncture experiments carried out on superficial mammalian tubules have clearly shown that only some 5-15 % of the amount of potassium filtered reach the early distal tubule. Importantly, this fraction of potassium is not substantially altered even when the final excretion rate varies dramatically, even over as large as a two hundred fold range [17, 18]. The amount of potassium reaching the early distal tubule is usually less than that present at the end of the late proximal tubule, thus demonstrating that net reabsorption of potassium continues beyond the proxi-
1002
G. Giebisch: Renal Tubular Control of Potassium Transport
0 - ~50%
©
CORTEX
(?%)
j
f
J
MEDULLA
©
5-t50%
mal convoluted tubule along some section of the loop of Henle. The situation pertaining to the loop of Henle of juxtamedullary nephrons is less clear. Micropuncture of tubular fluid at the tip of the renal papilla in rat kidneys has shown a sharp increase in potassium concentration as fluid passes from the end of the proximal convoluted tubule to the hairpin turn of the loop of Henle [19-21]. The amount of potassium at the hairpin turn of juxtamedullary nephrons may exceed even the quantity of potassium filtered, and has been shown to be strongly dependent on the state of potassium secretion along the distal tubule and collecting tubules. Thus, acute and chronic potassium loading increase the amount of potassium at the end of the descending limb whereas administration of amiloride, a diuretic that blocks potassium secretion across the distal tubule [22] and the cortical collecting tubule [23], sharply curtails the amount of potassium at the tip of Henle's loops of juxtamedullary nephrons. The very significant entry of potassium into the descending limb ofjuxtamedultary nephrons raises the possibility that this process of potassium secretion may contribute to urinary potassium excretion. Because it is presently impossible to determine whether (1) potassium similarly enters the descending limb of superficial nephrons or (2) potassium that enters
Fig. 1. Schematic representation of tubular structures showing outer cortical and inner juxtamedullary nephrons: proximal tubule, thin and thick limbs of Henle's loop, distal tubule and collecting tubules. Numbers indicate, where known, fractional delivery rates of potassium (in % of filtered load). Arrows show direction of net potassium transport: reabsorption or secretion. Solid arrows indicate that experimental measurements have been made; open arrows indicate possibilities not yet supported by strong evidence [12]
the descending limb of deep nephrons is lost from those deep nephrons before the fluid reaches the distal tubule, it is difficult to assign a physiological rote to the process of potassium secretion into the loop of Henle of juxtameduIlary nephrons. Two possibilities must be considered. On the one hand, it is possible that the ascending limbs of juxtamedullary nephrons increase their reabsorptive rate as potassium secretion is augmented, as can be shown to be the case when superficial loops of Henle are presented with an elevated potassium load. If this were the case, the addition of potassium to the loop of juxtamedullary nephrons would not contribute much if at all to the final rate of urinary potassium excretion. Alternatively, as a significantly greater amount of potassium ions enters the descending limb of juxtamedullary nephrons, due to their deeper penetration to the papilla, than enters the descending limb of superficial nephrons, it is possible that the fraction of potassium that is delivered to the early distal tubule of juxtamedullary nephrons is also proportionally larger. Conceivably, such potassium delivery could contribute to urinary excretion of potassium [12].
(c) Distal Tubule Morphologically and functionally, the distal tubule is a heterogenous structure and different cell types
G. Giebisch: Renal Tubular Control of Potassium Transport have been described [11, 24, 25]. Morphological separation is based on differences in appearance of segments dissected in vitro [27] and on differences in ultrastructure by electron microscopy [24-26]. Significant differences in biochemical properties have also been reported [28, 29]. Typical for the distal tubule, which most inclusively applies to the nephron starting after the end of the ascending limb and ends with the junction of two distal tubules, - is a gradual cytological change in which cells typical of the cortical collecting duct gradually replace distal tubule cells. Under almost all experimental conditions it can be shown that the distal tubule contributes a major fraction of potassium to the urine and that this potassium transport operation is dramatically affected by a variety of stimuli. Thus, the increase in potassium concentration along the distal tubule varies greatly and has been shown to depend critically upon the metabolic situation. Changes in acid base balance [30], the effects of some diuretics [6], adrenal steroids [31, 32], changes in dietary potassium intake [17, 33], stimulation of potassium excretion by the delivery of increased amounts of fluid and sodium [34, 35] and adaptive changes following loss of renal mass [15, 36] have been shown to modulate the amount of potassium secretion in the distal tubule. Most of the functional adjustments of distal tubular potassium transport consist in varying the rate of tubule potassium secretion. However, complete suppression of potassium secretion in the distal tubule as well as net potassium reabsorption have also been observed, and underscore the great functional flexibility of the distal tubule with respect to potassium transport.
1003 particularly when perfused at a low rate with solutions having high sodium concentration [44]. Amiloride [23] and acidification [45] of the luminal contents inhibit potassium secretion whereas collecting tubules from rabbits maintained on DOCA show a progressive increase in their ability to secrete potassium [46]. Whereas potassium secretion is most frequently observed along cortical collecting tubules, there is a marked tendency of papillary collecting tubules to show potassium reabsorption. Studies using microcatheterization of single papillary collecting tubules have often shown significant potassium reabsorption [40], and this has been confirmed by puncture of terminal segments of papillary collecting ducts [47]. Only during conditions of maximal kaliuresis [39, 47] or during potassium adaptation [48] has potassium secretion been observed. It should be noted that reabsorption of potassium in the papillary collecting ducts is activated not only in animals on a low potassium diet but also in animals maintained on a low sodium intake [18]. Potassium reabsorption is accentuated in hydropenia when fluid delivery to the collecting tubules is small, and often disappears with the delivery of larger fractions of the filtrate to these terminal tubular segments [49]. It has already been pointed out that the most likely source of the potassium that enters the descending limb of Henle's loop is the medullary collecting duct, indicating the possibility of potassium recycling between collecting ducts and the loop of Henle.
II. Cellular Mechanisms of Potassium Transport
( d) Collecting Tubule and Collecting Duct Studies on collecting tubules and collecting ducts are based on either perfusion of isolated segments of cortical collecting tubules in vitro [37, 38], retrograde microcatheterization [39, 40] or puncture of the terminal segments of the papillary collecting ducts [41, 42]. Comparison of the amount of potassium reaching collecting ducts at the level of the papillary base is often found to be greater than the amount at the late distal tubular site [43]. The most likely explanation of this finding is the possibility of additional potassium secretion along the collecting tubule. This conforms with the known fact that the capacity of the renal tubules to secrete potassium extends beyond the distal tubule to the cortical collecting tubule. Isolated rabbit collecting tubules secrete potassium normally against steep concentration gradients,
The precise definition of the transport mechanisms for potassium depends upon definition of the electrochemical driving forces across both luminal and peritubular membranes of the transporting cells. Such information has begun to emerge from recent studies using potassium-sensitive microelectrodes that can be used for impalement of single proximal and distal tubule cells. These studies are of importance because it is the activity of potassium ions, not its concentration that determines the electrochemical driving forces acting on potassium ions as they move across individual cell barriers. A problem which still awaits incisive analysis is the extent to which potassium ions move through or between cells. While evidence supproting transcellular components of potassium movement is available, particularly at the level of the distal tubule and collecting ducts, less is known about the possibility that significant potassium movement may also take place via a paracellular transport route.
1004
G. Giebisch: Renal Tubular Control of Potassium Transport
Etectncat gradient, mV Proxlmat p o r t i o n Earty + 5 9 ~ Late ÷48 -'"-"
K*
/
Chemicat grad,ent, mV
Etectrochemfcat gradient, mV
*68
. . . 0.
Brush b o r d e r
\
BB
BB PT --0--
+
~
Pentubutar
E -18 ... L -29
-77
-79
PT
°--I
-11
Direction o f net t r a n s p o r t
Fig. 2. Electrochemical differences of potassium ions across the brush border (BB) and peritubular (PT) ce11 membrane of proximal tubule cells. Electrical and chemical potential differences are also shown. E=early portion of proximal tubule, L=late portion of proximal tubule. Cell interior is chosen as 0 mV reference [52]
(a) Epithelium of the Proximal Tubule The proximal convoluted and straight tubules are responsible for reabsorption of a large fraction of the filtered potassium. During this process, only small, if any, transepithelial potassium gradients are established. Proximal nephron segments have a low transepithelial resistance, and normally maintain an electrical potential difference with the lumen slightly negative at the beginning to slightly positive at the end of the proximal convoluted tubule. Small lumen positive potentials may also be expected along the straight portion of the proximal tubule [50]. Evidence in favor of participation of a component of active potassium transport is based on several observations. First, tubule fluid/plasma potassium concentration ratios may fall to values less than unity, and hence active transport must be involved in these tubule segments having a lumen negative potential. Secondly, administration of acetazolamide, known to inhibit hydrogen ion transport across the proximal tubule [14] and which as been shown to prevent both the increase in chloride concentration [51] as well as the development of lumen-positive potentials [50], does not inhibit potassium reabsorption and the establishment of luminal potassium/plasma concentration ratios of less than unity [14]. Evidence based on measurements with ion-sensitive microelectrodes has added further to our understanding of potassium transport across the proximal tubular epithelium. An important finding is that potassium activity ratios across both the luminal and peritubular membrane of proximal cells are larger than the measured electrical potential difference [52, 53, 54]. Both decrease significantly after perfusion
of the proximal tubule [52, 53] or of the peritubular capillaries [52, 53] with ouabain-containing solutions. Figure 2 provides a summary of electrochemical gradients across the cell membranes of proximal tubules. It is apparent that the electrical potential difference, particularly across the brush border membrane, is significantly smaller than the opposing chemical activity difference. The sum of the two opposing electrochemical potential gradients results in a net force of 7 to 18 mV which has to be overcome for potassium translocation from the lumen into the peritubular fluid compartment. Clearly, this inference depends upon the assumption that transepithelial potassium movement across the proximal tubule (i.e., reabsorption) includes a transcellular route. Experiments using potassium-sensitive microelectrodes have confirmed the notion that active potassium transport is present in the peritubular cell membrane of proximal tubule cells. This finding is consistent with the large body of evidence supporting an active sodium-potassium exchange pump located at this site. The observation that the difference between the membrane potential and the potassium equilibrium potential becomes smaller after administration of ouabain [52, 53], is consistent with a ouabainsensitive active potassium pump within the peritubular cell membrane. Whereas the intracellular potassium activity is sharply reduced after inhibition of peritubular Na+K÷exchange, they are found to be well maintained in several conditions in which net sodium transport is sharply curtailed [53]. Representative examples are the replacement of chloride by a less permeant ion species (cyclamate) or the reduction of luminal (and peritubular) sodium concentration. Both situations
G. Giebisch: Renal Tubular Control of Potassium Transport Lumen
Cell
Blood +
f /
K
T
1005 Lumen
~
Cell
Blood +
Na +
K
A. Neutral exchange pump
"-.~ Na
Na
B, Rheogenic pump 3 I-
--
2 0 mV
~
~
!
I
70 mV
Fig. 3. Cell schema demonstrating essential components of tubular potassium and sodium transport. Left. Late distal tubular cell. Inset A and B demonstrate vertical or electrogenic(rheogenic) mode of action of peritubular sodium-potassium exchange pump. Right. Cortical collectingtubule cell with active secretorypotassium pump in the luminal cell membrane [3, 8, 11]
are characterized by a sharp reduction of net sodium transport across the proximal tubular epithelium. Cellular potassium activities are well maintained under such conditions, a finding that is of great interest in view of the coupling between active sodium extrusion and potassium uptake across the peritubular cell membrane. Cell potassium activities seem therefore rather independent of net sodium transport under these conditions. Further studies will be necessary to evaluate whether sodium extrusion and potassium uptake can be " u n c o u p l e d " or whether there exists a relatonship between peritubular potassium permeability and the rate of sodium-potassium exchange. If the potassium permeability were to change directly or indirectly as a function of peritubular Na+-K+ex change, cellular potassium activities could be maintained at normal values even at sharply reduced peritubular pump activity. Variations in extracellular pH and bicarbonate levels have also significant effects upon the peritubular membrane potential and cellular potassium activities [53, 55]. Acidification of the extracellular medium (low [HCO 3 -], normal pCO2) reduces the peritubular cell potential as welt as cell potassium activities, whereas an increase in [ttCO3-] at normal pCO2 levels leads to both hyperpolarization of the cell potential and a significant increase in potassium activity. These observations on cellular potassium activities have also led to the conclusion that the ratio of potassium activity/chemically measured potassium is lower than the similar values of Ringer's solutions. High concentrations of cellular multivalent anions could reduce the activity coefficient by electrostatic interaction. A similar reduction of the cytoplasmic
potassium activity coefficient has also been reported in a number of other epithelia.
Distal Tubule and Collecting Tubule Figure 3 presents a summary of the individual transport mechanisms that account for the characteristic behavior of secretory cells at the distal tubular (left panel) and collecting tubule (right panel) level. (For a detailed analysis, see [3-6, 8, I0]). General aspects of the distal tubular transport system are: (1) An active potassium uptake mechanism residing within the peritubular cell membrane. Again, direct measurements of potassium activity in single distal tubule cells have shown that the electrical potential difference (lumen negative) cannot account for the high cellular activity levels [56]. Active potassium uptake is associated with active sodium extrusion, proceeding either by one-to-one coupling (inset A), i.e. electroneutrally, or by cation exchange at a ratio greater than one (electrogenic or rheogenic mode of action, inset B). Active sodium-potassium exchange depends upon cardiac glycoside-sensitive Na+-K +ATP-ase located in the basolateral membrane [28, 29, 57]. It already has been pointed out that, morphologcally, the distal tubule is a heterogenous structure. Potassium secretion is restricted to the second half of this tubular segment, that segment characterized by prevalence of principal (light) and intercalated (dark) cells. Some correlation between fine structure of tubule cells in the distal tubule and collecting tubule and potassium balance exists. Principal cells show marked changes in morphology (increase in baso-
1006 lateral surface area) when potassium secretion is stimulated [58, 59] whereas morphological changes following dietary potassium curtailment (increase in luminal surface) are found only in dark cells [60]. It is also of interest that the enhanced secretion of potassium ions following chronic elevation of dietary potassium intake is followed by a sharp increase in ATP-ase activity of distal tubule and collecting segments [28, 59]. These considerations place great importance upon the cellular potassium activity as a key regulatory factor in determining the rate of potassium secretion since the rate of potassium translocation across the luminal cell membrane depends on the cell potassium activity. Acute and chronic potassium administration, metabolic alkalosis and adrenal steroids have been shown to stimulate peritubular potassium uptake thus elevating cellular potassium activity [31, 6I, 62]. On the other hand, peritubular potassium uptake declines in metabolic acidosis and during hypokalemia. Hence, the peritubular uptake of potassium is a major mechanism that responds to most of the stimuli known to regulate, ultimately, potassium excretion (changes in potassium balance, in adrenal cortical activity, in acid-base balance). (2) The luminal cell membrane is characterized ty the following transport properties: (a) The electrical potential difference is lower than that across the peritubular membrane and, as a consequence, passive movement of potassium ions proceeds preferentially from cell interior into the lumen of distal tubules. The importance of electrical potential changes upon distal tubular potassium transport is underscored by the observation that imposition of changes in transepithelial potential affects transepithelial potassium movement: potassium secretion rises as the luminal potential becomes more negative [63]. One of the kaliopenic diuretics, amiloride, can be shown to suppress potassium secretion by reduction of the transepithelial potential difference, the latter being related to sodium entry across the luminal cell membrane [63]. (b) The luminal cell membrane is the site of sodium entry into distal tubule cells. Increased fluid and sodium delivery into the distal tubule may accelerate potassium uptake across the peritubular cell membrane by augmentation of sodium entry across the luminal membrane. This may provide an explanation for the well-established fact that potassium secretion rises dramatically whenever fluid delivery into the distal tubule is enhanced [34, 35]. It is of interest that saturation of transepithelial potassium secretion can only be demonstrated when luminal flow rate is constant and peritubular potassium levels are elevated [64]. No saturation is evident when flow rate along the distal tubule increases. Hence, distal tubular
G. Giebisch: Renal Tubular Control of Potassium Transport potassium secretion is frequently flow-limited and rises often with delivery of fluid into the distal tubule (osmotic diuresis, extracellular volume expansion, administration of " l o o p " diuretics) [6]. Two mechanisms could account for the flow-dependent enhancement of potassium secretion: increased delivery of sodium into the distal tubule is often followed by increased sodium reabsorption and stimulation of peritubular sodium potassium exchange. It is also possible that, during increased flow rate, potassium concentrations fall in the lumen, thus steepening the transmembrane gradient of potassium ions across the luminal cell membrane and thereby increasing the driving force for passive egress of potassium ions from cell to tubular lumen [65]. (c) As can be seen from inspection of Fig. 3, movement of potassium from distal tubule cells to lumen is opposed by the ability of the luminal membrane to reabsorb potassium ions. This activity is most easily demonstrated when potassium secretion along the distal tubule is suppressed or even converted to net reabsorption, for example, during dietary potassium restriction [17]. Recent experiments in which the potassium activity of distal tubule cells were measured either when those cells were reabsorbing or secreting potassium, have shown that the activity of the pumpleak system in the luminal cell membrane may undergo dramatic changes [56]. While the activity of the luminal potassium pump is augmented in the state of maximal potassium conservation and thus mimicks the behavior of proximal, potassium reabsorbing cells as depicted in Fig. I, luminal pump activity declines whenever potassium secretion is stimulated. In the latter situation, potassium ions approach equilibrium across the luminal cell membrane and the difference between potassium equilibrium potential and electrical potential difference disappears. Stated differently, with increase in the rate of potassium secretion, potassium ions cease to be pumped out of the tubular lumen and the force opposing passive potassium transfer into the lumen, is sharply diminished. We should note that we know little about the electrogenic nature of luminal potassium uptake, i.e., whether potassium reabsorption is proceeding by exchange or cotransport with other ion species. The potassium concentration achieved in the distal tubule, does not exceed that to be expected from electrochemical equilibrium across the luminal cell membrane. However, in cortical collecting tubules perfused in vitro, the concentrations of potassium in the lumen may exceed those to be expected from passive driving forces, i.e., the electrical or chemical potential gradients. Such observations have led to the conclusion that active potassium secretion occurs across the tubular membrane, most likely in some
G. Giebisch: Renal Tubular Control of Potassium Transport
way linked to the reabsorption of sodium ions [44]. Potassium secretion is sensitive to mineralocorticoid stimulation [46] but declines with luminal acidification [45]. Potassium secretion along collecting tubules is less flow-dependent than in the distal tubule; as a consequence, the relative contribution of the collecting ducts to urinary potassium secretion declines with increased flow rate. Similar to the distal tubule, however, the cortical collecting tubule potassium transport system adapts to chronic high potassium intake by increasing its ATP-ase content [29] and peritubular surface area [58]. Although there are some significant differences with respect of the functional behavior of the distal tubular and collecting tubule epithelium, both sites are responding in a similar manner to a wide variety of metabolic stimuli. It is this unique responsiveness to changes in potassium and acid-base balance which makes the distal nephron the key control site of renal potassium excretion. References 1. Berliner, R.W.: Renal mechanism for potassium excretion. Harvey Lect. 55 141 (1961) 2. Brenner, B.M., Berliner, R.W.: Transport of potassium. In: Orloff, J., and Berliner, R.W. (eds.) Handbook of physiology, Sect. 8 : Renal physiology, p. 497. Washington, D.C. : American Physiological Society 1973 3. Giebisch, G.: Renal potassium excretion. In: Rouiller, C. and Muller, A.F. (eds.) The kidney. Morphology, biochemistry, physiology. Vol. 3, p. 329. New York; Academic Press, Inc. 1971 4. Giebisch, G., Windhager, E.E.: Electrolyte transport across renal tubular membranes. In: Orloff, J. and Berliner R.W. (eds.) Handbook of physiology, Sect. 8, Renal physiology, p. 315. Washington, D.C.: American Physiological Society 1973 5. Giebisch, G. : Some reflections on the mechanism of renal tubular potassium transport. Yale J. Biol. Med. 48, 315 (1975) 6. Giebisch, G.: Effects of diuretics on renal transport of potassium. In: Methods in pharmacology, Vol. 4, p. 121. New York: Plenum Press 1977 7. Giebisch, G., Stanton, B. : Potassium transport in the nephron. Am. Rev. Physiol. 41, 241 (1979) 8. Giebisch, G.: Renal potassium transport. In: Giebisch, G., Tosteson, D., and Ussing, H.H. (eds.) Transport across biological membranes, Vol. IV A. Berlin-Heidelberg-New York: Springer 1979 9. Grantham, J.J.: Renal transport and excretion of potassium. In: Brenner, B.M., and Rector, F.C. (eds), The kidney, p. 299. London: W.B. Saunders 1976 I0. Wright, F.S.: Potassium transport by the renal tubule. In: Thurau, K. (ed.), MTP International Review of Science, Series I, Vol. 6, Kidney and urinary tract physiology, p. 79. London: Bntterworth 1974 11. Wright, F.S.: Sites and mechanisms of potassium transport along the renal tubule. Kidney Int. 11, 415 (1977) 12. Wright, F.S., Giebisch, G. : Renal potassium transport: contributions of individual nephron segments and populations. Am. J. Physiol. 235, F515 (1978) 13. Schultze, R.G.: Recent advances in the physiology and pathophysiology of potassium excretion. Arch. Intern. Med. 113, 885 (1973)
1007 14. Beck, L.H., Senesky, D., Goldberg, M.: Sodium-independent active potassium reabsorption in proximal tubule of the dog. J. Clin. Invest. 52, 2641 (1973) 15. Diezi, J., Michoud, P., Grandchamp, A., Giebisch, G. : Effects of nephrectomy on renal salt and water transport in the remaining kidney. Kidney Int. 10, 450 (1976a) 16. Tune, B.M., Burg, M.B. : Glucose transport by proximal renal tubules. Am. J. Physiol. 221, 580 (1971) 17. Malnic, C., Klose, R.M., Giebisch, G.: Micropuncture study of renal potassium excretion in the rat. Am. J. Physiol. 206, 647 (1964) 18. Malnic, G., Klose, R.M., Giebisch, G.: Micropuncture study of distal tubular potassium and sodium transport in rat nephron. Am. J. Physiol. 211, 529 (1966) 19. Jamison, R.L., Lacy, F.B., Penell, J.P., Sanjana, V.M.: Potassium secretion by the descending limb of pars recta of the juxtamednllary nephron in vivo. Kidney Int. 9, 323 (1976) 20. Battilana, C.A., Dobyan, D.C., Lacy, F.B., Bhattacharya, J., Johnston, P.A., Jamison, R.L.: Effect of chronic potassium loading on potassium secretion by the pars recta or descending limb of the juxtamedullary nephron in the rat. J. Clin. Invest. 62, 1093 (1978) 21. Dobyan, D.C., Lacy, F.B., Jamison, R.L. : Suppression of potassium recycling in the renal medulla by short term potassium deprivation. Kidney Int., in press (1979) 22. Duarte, C.G., Chom6ty, F., Giebisch, G.: Effect of amiloride, ouabaim and furosemide on distal tubular function in the rat. Am. J. Physiol. 321, 91 (1971) 23. Stoner, L.C., Burg, M.B., Orloff, J.: Ion transport in cortical collecting tubule: effect of amiloride. Am. J. Physiol. 227, 453 (1974) 24. Bargmann, W.: Niere und ableitende Harnwege. Handbuch der mikroskopischen Anatomie des Menschen,VII 5. BerlinHeidelberg-New York: Springer 1978 25. Kriz, W., Kaissling, B., Pszolla, M. : Morphological characterization of the cells in Henle's loop and the distal tubule. In: Vogel, H.G., and Ullrich, K.J. (eds.), New aspects of renal function. Workshop Conference, Hoechst. Vol. 6, p. 67. Amsterdam-Oxford: Excerpta Medica 26. Crayen, M., Thoenes, W.: Architektur und cytologische Characterisierung des distalen Tubulus der Rattenniere. Fortschr. Zool. 23, 270 (1975) 27. Morel, F., Charbades, D., Imbert, M.: Functional segmentation of the rabbit distal tubule by microdetermination of hormone-dependent adenylate cyclase activity. Kidney Int. 9, 264 (1976) 28. Schmidt, U., Habricht, A. : Localization and function of Na-KATPase activity in various structures of the nephron. In: Bolis, L., Hoffman, J.F., and Leaf, A. (eds.), Membranes and disease, p. 311. New York: Raven Press (1976) 29. Katz, A.F., Doucet, A., Morel, F.: Na+-K+-ATPase activity along the rabbit, rat and mouse nephron. Am. J. Physiol. (in press.) 30. Malnic, G., Mello-Aires, M. de, Giebisch, G. : Potassium transport across renal distal tubules during acid-base disturbances. Am. J. Physiol. 211, 1192 (1971) 31. Hierholzer, K., Wiederholt, M. : Some aspects of distal tubular solute and water transport. Kidney Int. 9, 198 (1976) 32. Hierholzer, K., Lange, S.: The effects of adrenal steroids on renal function. In: Thurau, K. (ed.), MTP International Review of Science, Series I, Vol. 6, Kidney and urinary tract physiology, p. 273. London: Butterworth & Co. : Baltimore : University Park Press 1974 33. Wright, F., Strieder, N., Fowler, N., Giebisch, G. : Potassium secretion by distal tubule after potassium adaptation. Am. J. Physiol. 221, 437 (1971) 34. Khuri, R.N., Wiederholt, M., Strieder, N., Giebisch, G.: Ef-
1008 fects of flow rate and potassium intake on distal tubular potassium transfer. Am. J. Physiol. 228, 1249 (1975) 35. Kunau, R.T., Webb, H.C., Borman, S.C.: Characteristics of the relationship between the flow rate of tubular fluid and potassium transport in the distal tubule of the rat. J. Clin. Invest. 54, 1488 (1974) 36. Kunau, R.T., Whinnery, M.A.: Potassium transfer in distal tubule of normal and remnant kidneys. Am. J. Physiol, 235, F186 (1978) 37. Burg, M.B., Orloff, J.: Perfusion of siolated renal tubules, In: Orloff, J., and Berliner, R.W. (eds.)? Handbook of physiology. Amer. Physiological Society, Washington, D.C. Vol. 8, p. 145. Baltimore: Williams andWilkins Co. 1973 38. Chonko, A., Grantham, J.J.: The use of the isolated tubule preparation for the investigation of diuretics. In: MartinezMaldonado, M. (ed.), Methods in pharmacology, Voi. 4A, p. 47. New York-London: Plenum Press, 1976 39. Hierholzer, K. : Secretion of potassium and acidification in coliecting ducts of mammalian kidney. Am. J. Physiol, 201, 3t8 (1961) 40. Sonnenberg, H., Wilson, D.R, : The role of the medullary collecting ducts in postobstructive diuresis. J. Clin. Invest. 57, 1564 (1976) 41. DeRouffignac, C., Morel, F. : Micropuncture study of water, electrolytes, and urea movements along the loops of Henle in Psammomys. J. Clin. Invest. 48, 474 (1969) 42. Jamison, R.L. : Micropuncture study of superficial and justamedullary nephrons in the rat. Am. J. Physiol. 218, 46 (1970) 43. Reineck, H.J., Osgood, R.W., Stein, J.H. : Net potassium addition beyond the superficial distal tubule of the rat. Am. J, Physiol. 235, F104 (1979) 44. Grantham, J.J., Burg, M.B., Orloff, J. : The nature of transtubular Na and K transport in isolated rabbit collecting tubules. J. Clin. Invest. 49, 18t5 (1970) 45. Boudry, J.F., Stoner, L.C., Burg, M.B.: The effect of lumen pH on potassium transport in renal cortical collecting tubules. Am. J. Physiol. 230, 239 (1976) 46. O'Neil, R.G., Helman, S.I.: Transport characteristics of renal collecting tubules : influences of DOCA and diet. Am. J. Physiol. 233, F544 (1977) 47. Diezi, J., Michoud, P., Aceves, J., Giebisch, G. : Micropuncture study of electrolyte transport across papillary collecting duct of the rat. Am. J, Physiol. 224, 623 (1973) 48. Schon, D.A., Hayslett, J.P. : Potassium adaptation in medullary collecting ducts. Kidney Int. 14, 780(A) (1978) 49. Reineck, H.J., Osgood, R.W., Ferris, T.F., Stein, J.H. : Potassium transport in the distal tubule and collecting duct of the rat. Am. J. Physiol. 229, 1403 (1975) 50. Boulpaep, E.L.: Recent advances in electrophysiology of the nephron. Annu. Rev. Physiol. 38, 20 (1976a)
G, Giebisch: Renal Tubular Control of Potassium Transport 51. Kunau, R,T, : The influence of the carbonic anhydrase inhibitor benzolamide (CL-11,366) on the reabsorption of chloride, sodium and bicarbonate in the proximal tubule of the rat. J. Clin. Invest. 51, 294 (1972) 52. Fujimoto, M., Kubota, T., Kotera, K. : Electrochemical profile of K and C1 ions across the proximal tubule of bullfrog kidneys. Centr, Nephrol. 6, 114 (1977) 53. Kubota, T., Biagi, B., Giebisch, G.: Potassium activity measurements in single proximal tubule cells of Necturus. In: Boulpaep, E. (ed.), Current topics in membranes and transport. New York: Academic Press 1979 (in press) 54. Edelman, A., Curci, S., Samarzija, I., Fr6mter, E. : Determination of intracellular K + activity in rat kidney proximal tubular cells, Pflfigers Arch. 378~ 37 (1979) 55. Fr6mter, E., Sato, K., Giessner, K. : Electrical studies on the mechanism of H +/HCO3 transport across rat kidney proximal tubule. VI. Internat. Congr. of Nephrotogy (Proc,). GiovanettL S., Bonomini, V., and D'Amico, A., (eds.), p. 108. Basel: Karger 1976 56. Kubota, T., Giebisch, G. : Potassium activity measurements in single distal tubule ceils of Amphiuma. J. Membr. Biol, (to be submitted) 57. Silva, P., Hayslett, J.P., Epstein, F.H. : The role of Na-Kactivated adenosine triphosphate in potassium adaptation, J. Clin. Invest. 52, (11), 2665 (1973) 58. Wade, J.B., O'Neil, R.G., Pryor, J i . , Boulpaep, E.L. : Modulation of cell membrane area in renal collecting tubules by corticosteroid hormones. J, Cell Biol. 81 (in press) (1979) 59. Stanton, B., Giebisch, G. : Changes of distal tubular fine structure in potassium adaptation and potassium depletion. (Manuscript in preparation) 60. Stetson, D.L., Wade, J.B., Giebisch, G. : Morphological alterations in the rat medullary collecting duct following K depletion. Kidney Int. (1979)(submitted) 61. Wiederholt, M,, Sullivan, W.J., Giebisch, G., Solomon, A.K., Curran, P.F. : Transport of potassium and sodium across single distal tubules of Amphiuma. J. Gen. Physiol. 57, 495 (t971) 62. deMello-Aires, M,, Giebisch, G., Malnic, A., Curren, P.F.: Kinetics of potassium transport across single distal tubules of tad kidney. J. Physiol, (Loud.) 232, 47 (1973) 63, Garcia, E,, Malnic, G., Giebisch, G.: Effect of changes in electrical potential difference upon distal tubular potassium concentrations, Kidney Int. 10, 584 (1976) 64. Stanton, B., Gebisch, G. : In vivo microperfusion study of potassium secretion by the distal convoluted tubule during acute K infusion. Fed. Proc. 37, 728 (1978) 65, Good, D.W., Wright, F.S.: Luminal influences on potassium secretion: sodium concentration and fluid flow rate. Am. J. Physiol. 236, F192 (1979)
Klin. Wochenschr. 57, 1001-1008 (1979)
© Springer-Verlag 1979
Renal Tubular Control of Potassium Transport* G. Giebisch Department of Physiology, Yale University School of Medicine, New Haven, Connecticut, 06510, USA
Key words- Renal K transport - Tubular heterogeneity - Micropuncture Intracellular K activity Acid base balance - Adrenal steroids - Diuretics Homeostasis. This review will address problems related to (1) potassium transport sites within the nephron and, (2) to some of the control mechanisms modifying net transport of this ion species. Attention will be directed towards definition of the electrochemical driving forces across the luminal and peritubular cell membranes of those tubule cells which control potassium translocation. Furthermore, an effort will be made to pinpoint those transport parameters which are involved in the modulation of potassium excretion.
proximal tubule reabsorbs 60-70% of the filtered potassium. Proximal tubular potassium concentrations are close to that of plasma potassium concentrations. Normally, the rate of reabsorption of potassium along the proximal convoluted tubule is tightly coupled to that of sodium and water, although after acetazolamide [14] and after unilateral nephrectomy [15] proximal tubular sodium reabsorption is depressed whereas that of potassium continues at a fairly undiminished rate. This demonstrates that, potentially at least, proximal tubular potassium transfer may be regulated independently of sodium and of fluid transport. Studies on proximal straight tubules by perfusion of isolated segments in vitro have shown that net reabsorption takes place, although at a significantly lower rate than across the proximal convoluted tubule [16]
I. Tubular Sites of Potassium Transport
A large body of experimental evidence supports the view that most of the filtered potassium is reabsorbed in the proximal tubule and the loop of Henle. It is the distal tubule, the collecting tubule and the papillary collecting duct which determine and control the rate of urinary potassium excretion. Renal control of potassium excretion consists of either reabsorption of that amount of potassium that escapes proximal tubular reabsorption, or of net secretion, in response to specific stimuli, of variable amounts of potassium across the distal nephron. The topic of renal transport and control of potassium has been extensively reviewed [1-13].
(a) Proximal Tubule Micropuncture studies have firmly established that the accessible portion of the superficial mammalian * Studies carried out in the author's laboratory were supported by N.I.H. Grant PHS-AM-17433
(b) Loop of Henle Some recent evidence has evolved that suggests that the loops of Henle of superficial and juxtamedullary nephrons may exhibit somewhat different potassium transport patterns. Figure 1 provides a summary of what is presently known about the progression of potassium transport along cortical and juxtamedullary nephrons [12]. Micropuncture experiments carried out on superficial mammalian tubules have clearly shown that only some 5-15 % of the amount of potassium filtered reach the early distal tubule. Importantly, this fraction of potassium is not substantially altered even when the final excretion rate varies dramatically, even over as large as a two hundred fold range [17, 18]. The amount of potassium reaching the early distal tubule is usually less than that present at the end of the late proximal tubule, thus demonstrating that net reabsorption of potassium continues beyond the proxi-
1002
G. Giebisch: Renal Tubular Control of Potassium Transport
0 - ~50%
©
CORTEX
(?%)
j
f
J
MEDULLA
©
5-t50%
mal convoluted tubule along some section of the loop of Henle. The situation pertaining to the loop of Henle of juxtamedullary nephrons is less clear. Micropuncture of tubular fluid at the tip of the renal papilla in rat kidneys has shown a sharp increase in potassium concentration as fluid passes from the end of the proximal convoluted tubule to the hairpin turn of the loop of Henle [19-21]. The amount of potassium at the hairpin turn of juxtamedullary nephrons may exceed even the quantity of potassium filtered, and has been shown to be strongly dependent on the state of potassium secretion along the distal tubule and collecting tubules. Thus, acute and chronic potassium loading increase the amount of potassium at the end of the descending limb whereas administration of amiloride, a diuretic that blocks potassium secretion across the distal tubule [22] and the cortical collecting tubule [23], sharply curtails the amount of potassium at the tip of Henle's loops of juxtamedullary nephrons. The very significant entry of potassium into the descending limb ofjuxtamedultary nephrons raises the possibility that this process of potassium secretion may contribute to urinary potassium excretion. Because it is presently impossible to determine whether (1) potassium similarly enters the descending limb of superficial nephrons or (2) potassium that enters
Fig. 1. Schematic representation of tubular structures showing outer cortical and inner juxtamedullary nephrons: proximal tubule, thin and thick limbs of Henle's loop, distal tubule and collecting tubules. Numbers indicate, where known, fractional delivery rates of potassium (in % of filtered load). Arrows show direction of net potassium transport: reabsorption or secretion. Solid arrows indicate that experimental measurements have been made; open arrows indicate possibilities not yet supported by strong evidence [12]
the descending limb of deep nephrons is lost from those deep nephrons before the fluid reaches the distal tubule, it is difficult to assign a physiological rote to the process of potassium secretion into the loop of Henle of juxtameduIlary nephrons. Two possibilities must be considered. On the one hand, it is possible that the ascending limbs of juxtamedullary nephrons increase their reabsorptive rate as potassium secretion is augmented, as can be shown to be the case when superficial loops of Henle are presented with an elevated potassium load. If this were the case, the addition of potassium to the loop of juxtamedullary nephrons would not contribute much if at all to the final rate of urinary potassium excretion. Alternatively, as a significantly greater amount of potassium ions enters the descending limb of juxtamedullary nephrons, due to their deeper penetration to the papilla, than enters the descending limb of superficial nephrons, it is possible that the fraction of potassium that is delivered to the early distal tubule of juxtamedullary nephrons is also proportionally larger. Conceivably, such potassium delivery could contribute to urinary excretion of potassium [12].
(c) Distal Tubule Morphologically and functionally, the distal tubule is a heterogenous structure and different cell types
G. Giebisch: Renal Tubular Control of Potassium Transport have been described [11, 24, 25]. Morphological separation is based on differences in appearance of segments dissected in vitro [27] and on differences in ultrastructure by electron microscopy [24-26]. Significant differences in biochemical properties have also been reported [28, 29]. Typical for the distal tubule, which most inclusively applies to the nephron starting after the end of the ascending limb and ends with the junction of two distal tubules, - is a gradual cytological change in which cells typical of the cortical collecting duct gradually replace distal tubule cells. Under almost all experimental conditions it can be shown that the distal tubule contributes a major fraction of potassium to the urine and that this potassium transport operation is dramatically affected by a variety of stimuli. Thus, the increase in potassium concentration along the distal tubule varies greatly and has been shown to depend critically upon the metabolic situation. Changes in acid base balance [30], the effects of some diuretics [6], adrenal steroids [31, 32], changes in dietary potassium intake [17, 33], stimulation of potassium excretion by the delivery of increased amounts of fluid and sodium [34, 35] and adaptive changes following loss of renal mass [15, 36] have been shown to modulate the amount of potassium secretion in the distal tubule. Most of the functional adjustments of distal tubular potassium transport consist in varying the rate of tubule potassium secretion. However, complete suppression of potassium secretion in the distal tubule as well as net potassium reabsorption have also been observed, and underscore the great functional flexibility of the distal tubule with respect to potassium transport.
1003 particularly when perfused at a low rate with solutions having high sodium concentration [44]. Amiloride [23] and acidification [45] of the luminal contents inhibit potassium secretion whereas collecting tubules from rabbits maintained on DOCA show a progressive increase in their ability to secrete potassium [46]. Whereas potassium secretion is most frequently observed along cortical collecting tubules, there is a marked tendency of papillary collecting tubules to show potassium reabsorption. Studies using microcatheterization of single papillary collecting tubules have often shown significant potassium reabsorption [40], and this has been confirmed by puncture of terminal segments of papillary collecting ducts [47]. Only during conditions of maximal kaliuresis [39, 47] or during potassium adaptation [48] has potassium secretion been observed. It should be noted that reabsorption of potassium in the papillary collecting ducts is activated not only in animals on a low potassium diet but also in animals maintained on a low sodium intake [18]. Potassium reabsorption is accentuated in hydropenia when fluid delivery to the collecting tubules is small, and often disappears with the delivery of larger fractions of the filtrate to these terminal tubular segments [49]. It has already been pointed out that the most likely source of the potassium that enters the descending limb of Henle's loop is the medullary collecting duct, indicating the possibility of potassium recycling between collecting ducts and the loop of Henle.
II. Cellular Mechanisms of Potassium Transport
( d) Collecting Tubule and Collecting Duct Studies on collecting tubules and collecting ducts are based on either perfusion of isolated segments of cortical collecting tubules in vitro [37, 38], retrograde microcatheterization [39, 40] or puncture of the terminal segments of the papillary collecting ducts [41, 42]. Comparison of the amount of potassium reaching collecting ducts at the level of the papillary base is often found to be greater than the amount at the late distal tubular site [43]. The most likely explanation of this finding is the possibility of additional potassium secretion along the collecting tubule. This conforms with the known fact that the capacity of the renal tubules to secrete potassium extends beyond the distal tubule to the cortical collecting tubule. Isolated rabbit collecting tubules secrete potassium normally against steep concentration gradients,
The precise definition of the transport mechanisms for potassium depends upon definition of the electrochemical driving forces across both luminal and peritubular membranes of the transporting cells. Such information has begun to emerge from recent studies using potassium-sensitive microelectrodes that can be used for impalement of single proximal and distal tubule cells. These studies are of importance because it is the activity of potassium ions, not its concentration that determines the electrochemical driving forces acting on potassium ions as they move across individual cell barriers. A problem which still awaits incisive analysis is the extent to which potassium ions move through or between cells. While evidence supproting transcellular components of potassium movement is available, particularly at the level of the distal tubule and collecting ducts, less is known about the possibility that significant potassium movement may also take place via a paracellular transport route.
1004
G. Giebisch: Renal Tubular Control of Potassium Transport
Etectncat gradient, mV Proxlmat p o r t i o n Earty + 5 9 ~ Late ÷48 -'"-"
K*
/
Chemicat grad,ent, mV
Etectrochemfcat gradient, mV
*68
. . . 0.
Brush b o r d e r
\
BB
BB PT --0--
+
~
Pentubutar
E -18 ... L -29
-77
-79
PT
°--I
-11
Direction o f net t r a n s p o r t
Fig. 2. Electrochemical differences of potassium ions across the brush border (BB) and peritubular (PT) ce11 membrane of proximal tubule cells. Electrical and chemical potential differences are also shown. E=early portion of proximal tubule, L=late portion of proximal tubule. Cell interior is chosen as 0 mV reference [52]
(a) Epithelium of the Proximal Tubule The proximal convoluted and straight tubules are responsible for reabsorption of a large fraction of the filtered potassium. During this process, only small, if any, transepithelial potassium gradients are established. Proximal nephron segments have a low transepithelial resistance, and normally maintain an electrical potential difference with the lumen slightly negative at the beginning to slightly positive at the end of the proximal convoluted tubule. Small lumen positive potentials may also be expected along the straight portion of the proximal tubule [50]. Evidence in favor of participation of a component of active potassium transport is based on several observations. First, tubule fluid/plasma potassium concentration ratios may fall to values less than unity, and hence active transport must be involved in these tubule segments having a lumen negative potential. Secondly, administration of acetazolamide, known to inhibit hydrogen ion transport across the proximal tubule [14] and which as been shown to prevent both the increase in chloride concentration [51] as well as the development of lumen-positive potentials [50], does not inhibit potassium reabsorption and the establishment of luminal potassium/plasma concentration ratios of less than unity [14]. Evidence based on measurements with ion-sensitive microelectrodes has added further to our understanding of potassium transport across the proximal tubular epithelium. An important finding is that potassium activity ratios across both the luminal and peritubular membrane of proximal cells are larger than the measured electrical potential difference [52, 53, 54]. Both decrease significantly after perfusion
of the proximal tubule [52, 53] or of the peritubular capillaries [52, 53] with ouabain-containing solutions. Figure 2 provides a summary of electrochemical gradients across the cell membranes of proximal tubules. It is apparent that the electrical potential difference, particularly across the brush border membrane, is significantly smaller than the opposing chemical activity difference. The sum of the two opposing electrochemical potential gradients results in a net force of 7 to 18 mV which has to be overcome for potassium translocation from the lumen into the peritubular fluid compartment. Clearly, this inference depends upon the assumption that transepithelial potassium movement across the proximal tubule (i.e., reabsorption) includes a transcellular route. Experiments using potassium-sensitive microelectrodes have confirmed the notion that active potassium transport is present in the peritubular cell membrane of proximal tubule cells. This finding is consistent with the large body of evidence supporting an active sodium-potassium exchange pump located at this site. The observation that the difference between the membrane potential and the potassium equilibrium potential becomes smaller after administration of ouabain [52, 53], is consistent with a ouabainsensitive active potassium pump within the peritubular cell membrane. Whereas the intracellular potassium activity is sharply reduced after inhibition of peritubular Na+K÷exchange, they are found to be well maintained in several conditions in which net sodium transport is sharply curtailed [53]. Representative examples are the replacement of chloride by a less permeant ion species (cyclamate) or the reduction of luminal (and peritubular) sodium concentration. Both situations
G. Giebisch: Renal Tubular Control of Potassium Transport Lumen
Cell
Blood +
f /
K
T
1005 Lumen
~
Cell
Blood +
Na +
K
A. Neutral exchange pump
"-.~ Na
Na
B, Rheogenic pump 3 I-
--
2 0 mV
~
~
!
I
70 mV
Fig. 3. Cell schema demonstrating essential components of tubular potassium and sodium transport. Left. Late distal tubular cell. Inset A and B demonstrate vertical or electrogenic(rheogenic) mode of action of peritubular sodium-potassium exchange pump. Right. Cortical collectingtubule cell with active secretorypotassium pump in the luminal cell membrane [3, 8, 11]
are characterized by a sharp reduction of net sodium transport across the proximal tubular epithelium. Cellular potassium activities are well maintained under such conditions, a finding that is of great interest in view of the coupling between active sodium extrusion and potassium uptake across the peritubular cell membrane. Cell potassium activities seem therefore rather independent of net sodium transport under these conditions. Further studies will be necessary to evaluate whether sodium extrusion and potassium uptake can be " u n c o u p l e d " or whether there exists a relatonship between peritubular potassium permeability and the rate of sodium-potassium exchange. If the potassium permeability were to change directly or indirectly as a function of peritubular Na+-K+ex change, cellular potassium activities could be maintained at normal values even at sharply reduced peritubular pump activity. Variations in extracellular pH and bicarbonate levels have also significant effects upon the peritubular membrane potential and cellular potassium activities [53, 55]. Acidification of the extracellular medium (low [HCO 3 -], normal pCO2) reduces the peritubular cell potential as welt as cell potassium activities, whereas an increase in [ttCO3-] at normal pCO2 levels leads to both hyperpolarization of the cell potential and a significant increase in potassium activity. These observations on cellular potassium activities have also led to the conclusion that the ratio of potassium activity/chemically measured potassium is lower than the similar values of Ringer's solutions. High concentrations of cellular multivalent anions could reduce the activity coefficient by electrostatic interaction. A similar reduction of the cytoplasmic
potassium activity coefficient has also been reported in a number of other epithelia.
Distal Tubule and Collecting Tubule Figure 3 presents a summary of the individual transport mechanisms that account for the characteristic behavior of secretory cells at the distal tubular (left panel) and collecting tubule (right panel) level. (For a detailed analysis, see [3-6, 8, I0]). General aspects of the distal tubular transport system are: (1) An active potassium uptake mechanism residing within the peritubular cell membrane. Again, direct measurements of potassium activity in single distal tubule cells have shown that the electrical potential difference (lumen negative) cannot account for the high cellular activity levels [56]. Active potassium uptake is associated with active sodium extrusion, proceeding either by one-to-one coupling (inset A), i.e. electroneutrally, or by cation exchange at a ratio greater than one (electrogenic or rheogenic mode of action, inset B). Active sodium-potassium exchange depends upon cardiac glycoside-sensitive Na+-K +ATP-ase located in the basolateral membrane [28, 29, 57]. It already has been pointed out that, morphologcally, the distal tubule is a heterogenous structure. Potassium secretion is restricted to the second half of this tubular segment, that segment characterized by prevalence of principal (light) and intercalated (dark) cells. Some correlation between fine structure of tubule cells in the distal tubule and collecting tubule and potassium balance exists. Principal cells show marked changes in morphology (increase in baso-
1006 lateral surface area) when potassium secretion is stimulated [58, 59] whereas morphological changes following dietary potassium curtailment (increase in luminal surface) are found only in dark cells [60]. It is also of interest that the enhanced secretion of potassium ions following chronic elevation of dietary potassium intake is followed by a sharp increase in ATP-ase activity of distal tubule and collecting segments [28, 59]. These considerations place great importance upon the cellular potassium activity as a key regulatory factor in determining the rate of potassium secretion since the rate of potassium translocation across the luminal cell membrane depends on the cell potassium activity. Acute and chronic potassium administration, metabolic alkalosis and adrenal steroids have been shown to stimulate peritubular potassium uptake thus elevating cellular potassium activity [31, 6I, 62]. On the other hand, peritubular potassium uptake declines in metabolic acidosis and during hypokalemia. Hence, the peritubular uptake of potassium is a major mechanism that responds to most of the stimuli known to regulate, ultimately, potassium excretion (changes in potassium balance, in adrenal cortical activity, in acid-base balance). (2) The luminal cell membrane is characterized ty the following transport properties: (a) The electrical potential difference is lower than that across the peritubular membrane and, as a consequence, passive movement of potassium ions proceeds preferentially from cell interior into the lumen of distal tubules. The importance of electrical potential changes upon distal tubular potassium transport is underscored by the observation that imposition of changes in transepithelial potential affects transepithelial potassium movement: potassium secretion rises as the luminal potential becomes more negative [63]. One of the kaliopenic diuretics, amiloride, can be shown to suppress potassium secretion by reduction of the transepithelial potential difference, the latter being related to sodium entry across the luminal cell membrane [63]. (b) The luminal cell membrane is the site of sodium entry into distal tubule cells. Increased fluid and sodium delivery into the distal tubule may accelerate potassium uptake across the peritubular cell membrane by augmentation of sodium entry across the luminal membrane. This may provide an explanation for the well-established fact that potassium secretion rises dramatically whenever fluid delivery into the distal tubule is enhanced [34, 35]. It is of interest that saturation of transepithelial potassium secretion can only be demonstrated when luminal flow rate is constant and peritubular potassium levels are elevated [64]. No saturation is evident when flow rate along the distal tubule increases. Hence, distal tubular
G. Giebisch: Renal Tubular Control of Potassium Transport potassium secretion is frequently flow-limited and rises often with delivery of fluid into the distal tubule (osmotic diuresis, extracellular volume expansion, administration of " l o o p " diuretics) [6]. Two mechanisms could account for the flow-dependent enhancement of potassium secretion: increased delivery of sodium into the distal tubule is often followed by increased sodium reabsorption and stimulation of peritubular sodium potassium exchange. It is also possible that, during increased flow rate, potassium concentrations fall in the lumen, thus steepening the transmembrane gradient of potassium ions across the luminal cell membrane and thereby increasing the driving force for passive egress of potassium ions from cell to tubular lumen [65]. (c) As can be seen from inspection of Fig. 3, movement of potassium from distal tubule cells to lumen is opposed by the ability of the luminal membrane to reabsorb potassium ions. This activity is most easily demonstrated when potassium secretion along the distal tubule is suppressed or even converted to net reabsorption, for example, during dietary potassium restriction [17]. Recent experiments in which the potassium activity of distal tubule cells were measured either when those cells were reabsorbing or secreting potassium, have shown that the activity of the pumpleak system in the luminal cell membrane may undergo dramatic changes [56]. While the activity of the luminal potassium pump is augmented in the state of maximal potassium conservation and thus mimicks the behavior of proximal, potassium reabsorbing cells as depicted in Fig. I, luminal pump activity declines whenever potassium secretion is stimulated. In the latter situation, potassium ions approach equilibrium across the luminal cell membrane and the difference between potassium equilibrium potential and electrical potential difference disappears. Stated differently, with increase in the rate of potassium secretion, potassium ions cease to be pumped out of the tubular lumen and the force opposing passive potassium transfer into the lumen, is sharply diminished. We should note that we know little about the electrogenic nature of luminal potassium uptake, i.e., whether potassium reabsorption is proceeding by exchange or cotransport with other ion species. The potassium concentration achieved in the distal tubule, does not exceed that to be expected from electrochemical equilibrium across the luminal cell membrane. However, in cortical collecting tubules perfused in vitro, the concentrations of potassium in the lumen may exceed those to be expected from passive driving forces, i.e., the electrical or chemical potential gradients. Such observations have led to the conclusion that active potassium secretion occurs across the tubular membrane, most likely in some
G. Giebisch: Renal Tubular Control of Potassium Transport
way linked to the reabsorption of sodium ions [44]. Potassium secretion is sensitive to mineralocorticoid stimulation [46] but declines with luminal acidification [45]. Potassium secretion along collecting tubules is less flow-dependent than in the distal tubule; as a consequence, the relative contribution of the collecting ducts to urinary potassium secretion declines with increased flow rate. Similar to the distal tubule, however, the cortical collecting tubule potassium transport system adapts to chronic high potassium intake by increasing its ATP-ase content [29] and peritubular surface area [58]. Although there are some significant differences with respect of the functional behavior of the distal tubular and collecting tubule epithelium, both sites are responding in a similar manner to a wide variety of metabolic stimuli. It is this unique responsiveness to changes in potassium and acid-base balance which makes the distal nephron the key control site of renal potassium excretion. References 1. Berliner, R.W.: Renal mechanism for potassium excretion. Harvey Lect. 55 141 (1961) 2. Brenner, B.M., Berliner, R.W.: Transport of potassium. In: Orloff, J., and Berliner, R.W. (eds.) Handbook of physiology, Sect. 8 : Renal physiology, p. 497. Washington, D.C. : American Physiological Society 1973 3. Giebisch, G.: Renal potassium excretion. In: Rouiller, C. and Muller, A.F. (eds.) The kidney. Morphology, biochemistry, physiology. Vol. 3, p. 329. New York; Academic Press, Inc. 1971 4. Giebisch, G., Windhager, E.E.: Electrolyte transport across renal tubular membranes. In: Orloff, J. and Berliner R.W. (eds.) Handbook of physiology, Sect. 8, Renal physiology, p. 315. Washington, D.C.: American Physiological Society 1973 5. Giebisch, G. : Some reflections on the mechanism of renal tubular potassium transport. Yale J. Biol. Med. 48, 315 (1975) 6. Giebisch, G.: Effects of diuretics on renal transport of potassium. In: Methods in pharmacology, Vol. 4, p. 121. New York: Plenum Press 1977 7. Giebisch, G., Stanton, B. : Potassium transport in the nephron. Am. Rev. Physiol. 41, 241 (1979) 8. Giebisch, G.: Renal potassium transport. In: Giebisch, G., Tosteson, D., and Ussing, H.H. (eds.) Transport across biological membranes, Vol. IV A. Berlin-Heidelberg-New York: Springer 1979 9. Grantham, J.J.: Renal transport and excretion of potassium. In: Brenner, B.M., and Rector, F.C. (eds), The kidney, p. 299. London: W.B. Saunders 1976 I0. Wright, F.S.: Potassium transport by the renal tubule. In: Thurau, K. (ed.), MTP International Review of Science, Series I, Vol. 6, Kidney and urinary tract physiology, p. 79. London: Bntterworth 1974 11. Wright, F.S.: Sites and mechanisms of potassium transport along the renal tubule. Kidney Int. 11, 415 (1977) 12. Wright, F.S., Giebisch, G. : Renal potassium transport: contributions of individual nephron segments and populations. Am. J. Physiol. 235, F515 (1978) 13. Schultze, R.G.: Recent advances in the physiology and pathophysiology of potassium excretion. Arch. Intern. Med. 113, 885 (1973)
1007 14. Beck, L.H., Senesky, D., Goldberg, M.: Sodium-independent active potassium reabsorption in proximal tubule of the dog. J. Clin. Invest. 52, 2641 (1973) 15. Diezi, J., Michoud, P., Grandchamp, A., Giebisch, G. : Effects of nephrectomy on renal salt and water transport in the remaining kidney. Kidney Int. 10, 450 (1976a) 16. Tune, B.M., Burg, M.B. : Glucose transport by proximal renal tubules. Am. J. Physiol. 221, 580 (1971) 17. Malnic, C., Klose, R.M., Giebisch, G.: Micropuncture study of renal potassium excretion in the rat. Am. J. Physiol. 206, 647 (1964) 18. Malnic, G., Klose, R.M., Giebisch, G.: Micropuncture study of distal tubular potassium and sodium transport in rat nephron. Am. J. Physiol. 211, 529 (1966) 19. Jamison, R.L., Lacy, F.B., Penell, J.P., Sanjana, V.M.: Potassium secretion by the descending limb of pars recta of the juxtamednllary nephron in vivo. Kidney Int. 9, 323 (1976) 20. Battilana, C.A., Dobyan, D.C., Lacy, F.B., Bhattacharya, J., Johnston, P.A., Jamison, R.L.: Effect of chronic potassium loading on potassium secretion by the pars recta or descending limb of the juxtamedullary nephron in the rat. J. Clin. Invest. 62, 1093 (1978) 21. Dobyan, D.C., Lacy, F.B., Jamison, R.L. : Suppression of potassium recycling in the renal medulla by short term potassium deprivation. Kidney Int., in press (1979) 22. Duarte, C.G., Chom6ty, F., Giebisch, G.: Effect of amiloride, ouabaim and furosemide on distal tubular function in the rat. Am. J. Physiol. 321, 91 (1971) 23. Stoner, L.C., Burg, M.B., Orloff, J.: Ion transport in cortical collecting tubule: effect of amiloride. Am. J. Physiol. 227, 453 (1974) 24. Bargmann, W.: Niere und ableitende Harnwege. Handbuch der mikroskopischen Anatomie des Menschen,VII 5. BerlinHeidelberg-New York: Springer 1978 25. Kriz, W., Kaissling, B., Pszolla, M. : Morphological characterization of the cells in Henle's loop and the distal tubule. In: Vogel, H.G., and Ullrich, K.J. (eds.), New aspects of renal function. Workshop Conference, Hoechst. Vol. 6, p. 67. Amsterdam-Oxford: Excerpta Medica 26. Crayen, M., Thoenes, W.: Architektur und cytologische Characterisierung des distalen Tubulus der Rattenniere. Fortschr. Zool. 23, 270 (1975) 27. Morel, F., Charbades, D., Imbert, M.: Functional segmentation of the rabbit distal tubule by microdetermination of hormone-dependent adenylate cyclase activity. Kidney Int. 9, 264 (1976) 28. Schmidt, U., Habricht, A. : Localization and function of Na-KATPase activity in various structures of the nephron. In: Bolis, L., Hoffman, J.F., and Leaf, A. (eds.), Membranes and disease, p. 311. New York: Raven Press (1976) 29. Katz, A.F., Doucet, A., Morel, F.: Na+-K+-ATPase activity along the rabbit, rat and mouse nephron. Am. J. Physiol. (in press.) 30. Malnic, G., Mello-Aires, M. de, Giebisch, G. : Potassium transport across renal distal tubules during acid-base disturbances. Am. J. Physiol. 211, 1192 (1971) 31. Hierholzer, K., Wiederholt, M. : Some aspects of distal tubular solute and water transport. Kidney Int. 9, 198 (1976) 32. Hierholzer, K., Lange, S.: The effects of adrenal steroids on renal function. In: Thurau, K. (ed.), MTP International Review of Science, Series I, Vol. 6, Kidney and urinary tract physiology, p. 273. London: Butterworth & Co. : Baltimore : University Park Press 1974 33. Wright, F., Strieder, N., Fowler, N., Giebisch, G. : Potassium secretion by distal tubule after potassium adaptation. Am. J. Physiol. 221, 437 (1971) 34. Khuri, R.N., Wiederholt, M., Strieder, N., Giebisch, G.: Ef-
1008 fects of flow rate and potassium intake on distal tubular potassium transfer. Am. J. Physiol. 228, 1249 (1975) 35. Kunau, R.T., Webb, H.C., Borman, S.C.: Characteristics of the relationship between the flow rate of tubular fluid and potassium transport in the distal tubule of the rat. J. Clin. Invest. 54, 1488 (1974) 36. Kunau, R.T., Whinnery, M.A.: Potassium transfer in distal tubule of normal and remnant kidneys. Am. J. Physiol, 235, F186 (1978) 37. Burg, M.B., Orloff, J.: Perfusion of siolated renal tubules, In: Orloff, J., and Berliner, R.W. (eds.)? Handbook of physiology. Amer. Physiological Society, Washington, D.C. Vol. 8, p. 145. Baltimore: Williams andWilkins Co. 1973 38. Chonko, A., Grantham, J.J.: The use of the isolated tubule preparation for the investigation of diuretics. In: MartinezMaldonado, M. (ed.), Methods in pharmacology, Voi. 4A, p. 47. New York-London: Plenum Press, 1976 39. Hierholzer, K. : Secretion of potassium and acidification in coliecting ducts of mammalian kidney. Am. J. Physiol, 201, 3t8 (1961) 40. Sonnenberg, H., Wilson, D.R, : The role of the medullary collecting ducts in postobstructive diuresis. J. Clin. Invest. 57, 1564 (1976) 41. DeRouffignac, C., Morel, F. : Micropuncture study of water, electrolytes, and urea movements along the loops of Henle in Psammomys. J. Clin. Invest. 48, 474 (1969) 42. Jamison, R.L. : Micropuncture study of superficial and justamedullary nephrons in the rat. Am. J. Physiol. 218, 46 (1970) 43. Reineck, H.J., Osgood, R.W., Stein, J.H. : Net potassium addition beyond the superficial distal tubule of the rat. Am. J, Physiol. 235, F104 (1979) 44. Grantham, J.J., Burg, M.B., Orloff, J. : The nature of transtubular Na and K transport in isolated rabbit collecting tubules. J. Clin. Invest. 49, 18t5 (1970) 45. Boudry, J.F., Stoner, L.C., Burg, M.B.: The effect of lumen pH on potassium transport in renal cortical collecting tubules. Am. J. Physiol. 230, 239 (1976) 46. O'Neil, R.G., Helman, S.I.: Transport characteristics of renal collecting tubules : influences of DOCA and diet. Am. J. Physiol. 233, F544 (1977) 47. Diezi, J., Michoud, P., Aceves, J., Giebisch, G. : Micropuncture study of electrolyte transport across papillary collecting duct of the rat. Am. J, Physiol. 224, 623 (1973) 48. Schon, D.A., Hayslett, J.P. : Potassium adaptation in medullary collecting ducts. Kidney Int. 14, 780(A) (1978) 49. Reineck, H.J., Osgood, R.W., Ferris, T.F., Stein, J.H. : Potassium transport in the distal tubule and collecting duct of the rat. Am. J. Physiol. 229, 1403 (1975) 50. Boulpaep, E.L.: Recent advances in electrophysiology of the nephron. Annu. Rev. Physiol. 38, 20 (1976a)
G, Giebisch: Renal Tubular Control of Potassium Transport 51. Kunau, R,T, : The influence of the carbonic anhydrase inhibitor benzolamide (CL-11,366) on the reabsorption of chloride, sodium and bicarbonate in the proximal tubule of the rat. J. Clin. Invest. 51, 294 (1972) 52. Fujimoto, M., Kubota, T., Kotera, K. : Electrochemical profile of K and C1 ions across the proximal tubule of bullfrog kidneys. Centr, Nephrol. 6, 114 (1977) 53. Kubota, T., Biagi, B., Giebisch, G.: Potassium activity measurements in single proximal tubule cells of Necturus. In: Boulpaep, E. (ed.), Current topics in membranes and transport. New York: Academic Press 1979 (in press) 54. Edelman, A., Curci, S., Samarzija, I., Fr6mter, E. : Determination of intracellular K + activity in rat kidney proximal tubular cells, Pflfigers Arch. 378~ 37 (1979) 55. Fr6mter, E., Sato, K., Giessner, K. : Electrical studies on the mechanism of H +/HCO3 transport across rat kidney proximal tubule. VI. Internat. Congr. of Nephrotogy (Proc,). GiovanettL S., Bonomini, V., and D'Amico, A., (eds.), p. 108. Basel: Karger 1976 56. Kubota, T., Giebisch, G. : Potassium activity measurements in single distal tubule ceils of Amphiuma. J. Membr. Biol, (to be submitted) 57. Silva, P., Hayslett, J.P., Epstein, F.H. : The role of Na-Kactivated adenosine triphosphate in potassium adaptation, J. Clin. Invest. 52, (11), 2665 (1973) 58. Wade, J.B., O'Neil, R.G., Pryor, J i . , Boulpaep, E.L. : Modulation of cell membrane area in renal collecting tubules by corticosteroid hormones. J, Cell Biol. 81 (in press) (1979) 59. Stanton, B., Giebisch, G. : Changes of distal tubular fine structure in potassium adaptation and potassium depletion. (Manuscript in preparation) 60. Stetson, D.L., Wade, J.B., Giebisch, G. : Morphological alterations in the rat medullary collecting duct following K depletion. Kidney Int. (1979)(submitted) 61. Wiederholt, M,, Sullivan, W.J., Giebisch, G., Solomon, A.K., Curran, P.F. : Transport of potassium and sodium across single distal tubules of Amphiuma. J. Gen. Physiol. 57, 495 (t971) 62. deMello-Aires, M,, Giebisch, G., Malnic, A., Curren, P.F.: Kinetics of potassium transport across single distal tubules of tad kidney. J. Physiol, (Loud.) 232, 47 (1973) 63, Garcia, E,, Malnic, G., Giebisch, G.: Effect of changes in electrical potential difference upon distal tubular potassium concentrations, Kidney Int. 10, 584 (1976) 64. Stanton, B., Gebisch, G. : In vivo microperfusion study of potassium secretion by the distal convoluted tubule during acute K infusion. Fed. Proc. 37, 728 (1978) 65, Good, D.W., Wright, F.S.: Luminal influences on potassium secretion: sodium concentration and fluid flow rate. Am. J. Physiol. 236, F192 (1979)
