(t min) and the sodium-depleted (tj min) diets was faster than that of

Journal of Physiology (1990), 424, pp. 317-327 317 With 3 figures Printed in Great Britain THE EFFECT OF DIETARY SODIUM AND POTASSIUM INTAKE ON POTASSIUM SECRETION AND KINETICS IN RAT DISTAL COLON BY C. J. EDMONDS AND C. L. WILLIS From the Endocrinology Research Group, Clinical Research Centre, Watford Road, Harrow, Middlesex HA1 3UJ (Received 22 November 1989) SUMMARY 1. Potassium secretion by the distal colon was examined in relation to the secretion of chloride and absorption of sodium and to the epithelial turnover of 86Rb+ (as an analogue of potassium) in vivo in rats fed a standard, a potassium-rich or a sodium-depleted diet. 2. An acute intravenous potassium load stimulated potassium secretion two- to threefold. In rats fed the standard diet, sodium secretion was also increased but no significant change in the lumen-to-plasma sodium flux was detected. The potassium and sodium secretions were accompanied by increased chloride secretion which occurred even when the intravenous load contained no chloride. In rats fed the potassium-rich diet, there was a small increase in sodium absorption and a less marked increase of chloride secretion. In the sodium-depleted rats, however, about 70-80% of the increased potassium secretion was balanced by increased sodium absorption. 3. The epithelial turnover rate of 86Rb+ in the rats fed the potassium-rich (t1 12-14 min) and the sodium-depleted (tj 11-13 min) diets was faster than that of those of the standard diet (t1 22-27 min). 4. The epithelial potassium content was 304 + 9 nmol (mg dry weight)-' and was not significantly altered despite considerable variations in potassium secretion rate induced by dietary changes and acute potassium loading. 5. It is concluded that epithelial potassium turnover rate is increased during chronic states of increased potassium secretion and that the rise in potassium secretion is consistent with increased activity of the basolateral Na+-K+ pump. Whether the increased potassium secretion is associated with chloride secretion or with increased sodium absorption appears to depend on the absence or presence of the amiloride-sensitive sodium pathway in the apical membrane. INTRODUCTION Potassium secretion by the distal colon is considerably increased when rats are fed a diet restricted in sodium or rich in potassium (Edmonds, 1967; Fisher, Binder & Hayslett, 1976). The electrical polarization of colonic epithelium favours accumulation of potassium in the lumen but the amount of potassium secretion often exceeds that expected from the transepithelial electrochemical gradient indicating MS 7488

318 C. J. EDMONDS AND C. L. WILLIS that the secretion process cannot simply be one of passive diffusion (Edmonds, 1967; Smith & McCabe, 1984). Comparison of the other changes in colonic ionic transport during dietary adaptation shows important differences between sodium depletion and potassium excess. In sodium-depleted rats, sodium absorption and the transepithelial electrical potential difference (PD) are substantially increased when compared with rats taking a normal diet. But in rats fed a potassium-rich diet, sodium absorption is unchanged and, providing dietary potassium is not in gross excess, the PD is also unchanged (Fisher et al. 1976; Budinger, Foster, Hayslett & Binder, 1986; Edmonds & Willis, 1988a). A further difference is in the effect of amiloride at low concentration (10-4 M or less in the lumen) which abolishes the PD by completely blocking active sodium absorption in sodium-depleted rats but which only partially reduces the PD in the rats fed a potassium-rich diet (Will, Lebowitz & Hopfer, 1980; Edmonds & Willis, 1988a). There is a substantial body of evidence showing that the adrenal secretion of aldosterone plays a major part in these epithelial adaptations (Edmonds & Marriott, 1967; Bastl, Binder & Hayslett, 1980; Foster, Jones, Hayslett & Binder, 1985; Martin, Oszi, Brocca, Arrizurietta & Hayslett, 1986). Recent studies indicate, moreover, that the difference between the effects found in low-sodium and high-potassium dietary states reflects differences in the rate of aldosterone secretion which is increased more by sodium depletion than by a potassium-rich diet (Edmonds & Willis, 1988b). Sodium, chloride and bicarbonate ions are also transported by colonic epithelium (for review, see Wrong, Edmonds & Chadwick, 1981) and it would be anticipated that the transepithelial movement of at least some of these other ions must be affected by the large increase of potassium secretion provoked by acute administration of potassium loads. One of the principal objects of the present work was, therefore, to determine what alterations in the transport of these other ions occurred. Moreover, it seems likely that the secreted potassium is largely derived from the abundant potassium contained in the epithelial cells (Wills & Biagi, 1982). Previous kinetic studies indicated that the secreted potassium came from a potassium pool with a relatively fast turnover, a finding consistent with the existence of an epithelial cell secreting mechanism (Edmonds & Smith, 1979). Using 86 Rb+ as an analogue of potassium we have, therefore, in the present work also examined the origin of the secreted potassium, particularly in relation to the variable rates of potassium secretion. METHODS Male albino rats weighing 300-350 g were used. They were fed a diet of standard pellets, or of similar pellets enriched with potassium or, when sodium was restricted, a diet based on rice. The potassium-rich pellets were prepared by soaking standard pellets in 0-3 M-KCl and then drying. They contained about 0 7 mmol g-1 of potassium compared with 0-2 mmol g-l in the standard pellets. The rats were fed these for 7 days before the experiment. The rats ate these diets in similar amounts, having a daily intake of about 7 g (100 g body weight)-'. They had free access to drinking water and access to food up to the morning of the colon perfusion experiments. In the case of sodium-depleted rats, a small amount of body sodium was removed 5 days before the experiment by intraperitoneal injection of 5 ml (100 g body weight)-' of 5% (w/v) glucose solution. This was removed 2 h later through a large-bore needle with the animals briefly anaesthetized with methoxyfluorane. Subsequently they were fed the low-sodium rice diet for 5 days before the experiment.

POTASSIUM SECRETION IN RAT DISTAL COLON For the measurement of absorption and secretion in the colon, the rats were anaesthetized with sodium pentobarbitone, 6 mg (100 g body weight)-' injected intraperitoneally well away from the distal colon. A segment of about 3 cm of distal colon was rinsed clean and cannulated as previously described (Edmonds & Mackenzie, 1984). A Polythene cannula was inserted into the external jugular vein to allow the adminstration of the intravenous potassium load. In the experiments in which sodium fluxes and chloride secretion were measured, 05 ml of a solution of sodium isethionate (50 mmol 1-1) and mannitol (200 mmol 1-1) containing 22Na+, 4 Bq (/Zmol sodium)-' (Radiochemical Centre, Amersham, Bucks.) was instilled into the empty lumen. After 10 min exposure, the solution was completely rinsed out and collected. Sodium absorption, unidirectional fluxes and transcellular (active) sodium fluxes were estimated from these observations as previously described (Edmonds & Mackenzie, 1984). The intravenous potassium load was given as KCl (120 mmol 1-1) with KHCO, (30 mmol 1-1) by constant infusion pump at a rate of 270 jul min-' for the first 10 min and subsequently at 190 jd min'. In all instances the 10 min measurements were made in duplicate, those during the potassium load infusion being over the period 10-30 min after commencing the infusion. Over the limited time of the experiments, no apparent adverse effects were observed with this rate of potassium administration. Venous blood samples were collected from the inferior vena cava immediately at the end of each experiment but before the infusion was stopped. The transepithelial PD was recorded throughout using a high-input impedance millivoltmeter, calomel half-cells and agar-saline bridges connected to the lumen and to the serosal surface of the segment of colon. For the experiments in which 86Rb+ was used, about 0 5 ml of a solution composed of NaCl (50 mmol 1-1), KCl (5 mmol 1-1) and mannitol (190 mmol 1-1) and containing 86RbCl, 200 kbq (umol potassium)-' (Radiochemical Centre, Amersham, Bucks) was introduced into the empty cannulated segment of colon. The solution was rapidly replaced every 10 min to ensure that little change in its composition occurred during the total 30 min period that the 86Rb+-containing solution was in the lumen. The solution was then quickly rinsed out with mannitol (300 mmol 1-l). This initial procedure was to introduce 86Rb+ into the epithelial cells as a marker of cellular potassium. In each experimental group the rats were in two sets. In one set (TO) the segment of colon was removed immediately after the 86Rb+-containing solution had been washed out at the end of the 30 min loading period. In the second set (T21), the colon segment was perfused with a solution of NaCl (50 mmol 1-) and mannitol (200 mmol 1-l) at 37 0C at 1 ml min' using a Watson-Marlow H. R. Flow Inducer and consecutive 3 min collections of the effluent made for 21 min. The segment of colon was then immediately removed. Blood samples were taken from the inferior vena cava and scrapings of the epithelium taken from the colon (Edmonds & Mackenzie, 1984). Sodium and potassium were measured by flame photometry, chloride by potentiometric titration and 86Rb+ and 22Na+ by y-counting. Before scraping the epithelium, the length of the segment of colon was measured on a standard glass tube and the flux results are expressed per square centimetre of area of the mucosa based on the relationship of 2-1 cm2 as equivalent to one centimetre of measured gut length. The results are given as means + S.E.M. and statistical significance was examined by Student's t test. 319 RESULTS Sodium fluxes and chloride secretion Chloride secretion rate could be measured more accurately when the luminal solution was initially chloride free. In preliminary experiments, we established that the use of chloride-free solutions did not affect the rate of potassium secretion when compared with that observed when NaCl (50 mmol 1-1) was in the lumen. Accordingly in the present experiments the solution used in the lumen for measurement of sodium fluxes and chloride and potassium secretion contained sodium isethionate (50 mmol 1-1) and mannitol (200 mmol 1-1). In three groups of rats which had been fed the standard diet, the potassium-rich diet or the low-sodium diet, sodium fluxes were measured before and during an acute

320 C. J. EDMONDS AND C. L. WILLIS intravenous potassium infusion. During the pre-infusion period, both the control and the rats fed the potassium-rich diet had similar transepithelial PD and sodium flux rates (Fig. 1). In the sodium-depleted rats the PD was elevated (P < 0 005) as was the lumen-to-plasma flux Jms (P < 0-05) due largely to the transcellular > 80] a 40- i co 0] x 120_.E 80 0'I 0 4C E 40- ii i Standard Potassium- Sodium-depleted diet rich diet diet Fig. 1. Effect ofintravenous infusion ofthe potassium load on the transepithelial sodium flux from lumen to plasma (Jm.) and PD in groups of five rats fed the standard, the potassiumrich or the sodium-depleted diet. The first bar of each group is based on duplicate measurements before the infusion; the adjacent bar shows the results for the period 10-30 min after the infusion commenced. The hatched areas indicate the estimated transcellular (active) sodium flux (J&, ms) component Jc ms which was almost doubled. During infusion of the potassium load, no significant change in the PD or in the lumen-to-plasma sodium flux rates was observed in the rats fed the standard diet (Fig. 1). In the rats fed the potassium-rich diet, there was a small increase in these sodium fluxes (P < 0 05, paired t test). In the sodium-depleted rats on the other hand, the PD fell during the potassium infusion (P < 0-05, paired t test) while both Jms and Jc ms were increased (P < 0-01, paired t test). The basal, pre-infusion potassium secretion rate was greater in the rats fed the potassium-rich or sodium-depleted diet (22 + 5 and 27 + 6 nmol min- cm-2 respectively) than in those fed the standard diet (6 + 2 nmol min- cm-2). In all groups the secretion rate increased by two- to threefold during the potassium infusion, a change comparable to that observed previously (Edmonds & Willis, 1988a). The particular interest of these experiments, however, was the effect of the acute potassium load on chloride secretion. During the acute potassium infusion, chloride secretion increased significantly in all groups (Fig. 2). With those fed the standard diet, the increase in chloride secretion considerably exceeded that of potassium secretion, the difference being accounted for by a rise in sodium secretion. In the rats

POTASSIUM SECRETION IN RAT DISTAL COLON on the potassium-rich diet, the acute potassium infusion produced some sodium absorption (P < 0-02, comparison with rats on standard diet). Chloride secretion was less increased in these animals but the difference from rats on the standard diet was not significant. In the case of the sodium-depleted rats, the acute potassium infusion +30-321 E,-0 1 E +10- -5 Cu -0-10 x c 30 20 0-20- 97 Cu-30Q.C) -40 -Standard diet Potassiumrich diet Sodiumdepleted Standard diet: chloridediet free infusion Fig. 2. The change in net sodium (filled bar), potassium (open) and chloride (hatched) transepithelial movements induced by intravenous infusion of the potassium load in groups of five rats fed the standard, potassium-rich or sodium-depleted diet. The fourth group were infused with a chloride-free potassium gluconate-khco3 solution. The increase of absorption is indicated as positive and of secretion as negative values. produced an increase of chloride secretion that was even smaller (P < 0-05 compared with rats on the standard diet) and substantially less (P < 0-001) than the increase of potassium secretion. In the rats, therefore, the potassium secretion stimulated by the acute potassium load was, to the extent of 70-80 %, balanced by increased sodium absorption. In the above experiments, the potassium load infused was accompanied by a substantial chloride load. To see whether the chloride load itself may have stimulated the rise of chloride secretion, which was particularly evident in the rats on the standard diet, we carried out further experiments in which a similar potassium load was accompanied by gluconate instead of chloride (Fig. 2). Chloride secretion was still increased and the increase was not significantly different from that observed when potassium chloride was infused. However, the plasma chloride concentration was, to a small extent but significantly (P < 0-01), reduced by the chloride-free infusion (to 92 + 1-8 mmol 1-1 compared with 103 + 2-1 mmol 1-1 in the plasma of four uninfused rats). Kinetics of epithelial potassium using 86Rb+ Rubidium-86 has been widely used as an analogue of potassium and it was shown previously that 86Rb+ behaved similarly to potassium in the distal colon (Edmonds & Willis, 1988a). In the present experiments, we used 86Rb+ to examine epithelial secretion under various experimental conditions. The procedure involved initial 11 PHY 424

322 C. J. EDMONDS AND C. L. WILLIS loading of the epithelial cells for 30 min with 86Rb+. For each experimental condition, in one set of rats (TO), the amount of 86Rb+ accumulating in the cells during this period was determined by taking epithelial scrapings immediately after the radioisotope-containing solution had been thoroughly rinsed out of the lumen. In another set of rats (T21), following removal of the radioisotope solution, the lumen was perfused for the subsequent 21 min (referred to as the outflow phase), and collected at 3 min intervals. Immediately on completing this perfusion epithelial scrapings were taken. TABLE 1. Epithelial 86Rb+/K+ ratio following a 30 min period with a 86Rb+-containing solution in the lumen (TO) and after a subsequent 21 min perfusion of the lumen (T21) Epithelial 86Rb+/K+ ratio (c.p.m. nmol-1) Epithelial Secreted Diet TO T21 ti (min) th (min) Standard 225+2-2 (8) 13-4+1-1 (8) 27 22 Potassium rich 19-7+2-1 (4) 6-2+1P0 (4) 12 14 Potassium rich 3-1+0-9 (4) 8 12 +i.v. potassium load Sodium depleted 11-1±+08 (4) 2-8+09 (4) 11 13 Duplicate specimens of epithelial scrapings were taken for each rat; the number of rats for each value is in parentheses. The data were normalized on the basis of a luminal 86Rb+/K+ ratio of 155 c.p.m. nmol-'. The epithelial th is based on the assumption of drainage from a single epithelial pool and the secreted t1 is derived from the data shown in Fig. 3. Comparison of the 86Rb+/K+ ratio present at the end of the 56Rb+-loading period (derived from TO set) shows that, whereas those of the control rats and those fed the potassium-rich diet were similar (Table 1), that of the sodium-depleted rats was significantly (P < 001) lower. Following the 21 min perfusion of the lumen, the outflow of 86Rb+ resulted in a fall of epithelial 86Rb+ in all the groups studied. Now, however, the ratio observed in the epithelial scrapings of the rats taking the potassium-rich diet was significantly (P < 0-01) less than that of the control group on the standard diet reflecting an enhanced rate of epithelial turnover of 86Rb+ in the potassium-fed animals. Although in the case of the sodium-depleted rats the initial amount of 86Rb+ in the epithelium was lower than in the other groups, there was also an enhanced rate of loss of 86Rb+ compared with those on the standard diet. The 86Rb+/K+ ratio in the epithelial secretion fell over the 21 min period during which the effluent was collected (Fig. 3). The observed rates of fall agree reasonably well with those expected from the change of epithelial 86Rb+ content assuming the outflux was from a single pool in the epithelium (Table 1). The fall of 86Rb+/K+ ratio in the epithelial secretion was almost twice as fast in the sodium-depleted rats and rats fed the potassium-rich diet as in those fed the standard diet. The effect of an intravenous potassium load, of sodium removal and of tetraethylammonium chloride The effect of an intravenous infusion of a potassium load was also examined (Table 1; Fig. 3). Potassium secretion rate was increased during the infusion and this

POTASSIUM SECRETION IN RAT DISTAL COLON 323 accounted for the reduced 86Rb+/K+ ratio in the epithelial secretion. The amount of 86Rb+ remaining in the epithelium at the end of the 21 min perfusion was significantly (P < 005) reduced when compared with that of the uninfused potassium-fed rats. 20-0 E 10- C.2 5- EN.0 co 1- I IA 0 5 10 15 20 Time from removal of luminal 86Rb+ (min) Fig. 3. The fall of specific activity (86Rb+/K+ ratio) in the epithelial secretion in groups of rats variously treated. *, rats (8) on the standard diet; 0, rats (4) on the potassiumrich diet;., rats (4) on the potassium-rich diet and given the intravenous potassium load; *, rats (4) on the sodium-depleted diet. TABLE 2. Epithelial potassium content and plasma potassium concentration in the various groups of rats from which the data of Table 1 and Fig. 3 were obtained Epithelial Plasma potassium (nmol potassium Diet (mg dry weight)-') (nmol 1-1) Standard (16) 304+9 4 0+0 11 Potassium rich (8) 287 + 13 5-4 + 0 40 Potassium rich 293+10 7-7 +0-29 with i.v. potassium (4) Sodium depleted (8) 279+18 4-4+0039 Duplicate samples in each with the number of rats in parentheses. Since 86Rb+ outflow rate reflected potassium turnover rate in the epithelial cells which depended on the operation of the Na+-K+ pump in the basolateral membranes, then the 86Rb+ outflow might depend, at least in part, on the transcellular sodium flux derived from absorbing sodium from the lumen. Restriction of sodium entry to the epithelial cells by removal of sodium from the luminal solution would then, by reducing sodium available to the Na+-K+ pump, be expected to reduce the 86Rb+ 11-2

324 C. J. EDMONDS AND C. L. WILLIS turnover rate. This was examined in experiments on rats taking a potassium-rich diet in which choline chloride (50 mmol l-l) was substituted for sodium chloride in the perfusion solution, the mannitol content being unchanged. Potassium secretion rate was reduced by about 20 %, averaging 17 + 2 (6) nmol min-' cm-2 but the 86Rb+ turnover rate (t, of 12 and 10-8 min based on secretion and on epithelial scrapings respectively) was not significantly different from that observed when sodium was present in the lumen. Reducing potassium secretion alone might also be expected to affect potassium turnover in the epithelium. When the potassium channel blocker tetraethylammonium chloride 30 mmol 1-1 was added to the perfusion solution (the mannitol content being appropriately reduced), potassium secretion rate was almost halved averaging 12 + 2 (6) nmol min-' cm-2 but again no significant change in 86Rb+ turnover (ti of 15-8 and 13-6 min, based on secretion and on epithelial scrapings respectively) was observed. Epithelial potassium content, transepithelial PD and potassium secretion rate As noted above, potassium secretion rates were, by comparison with the rats fed the standard diet, increased considerably in those fed a potassium-rich diet or one which had been sodium depleted while the intravenous potassium load more than doubled the secretion rate. Despite these substantial changes of potassium secretion, measurement of the potassium content of the epithelial scrapings (Table 2) revealed no change of epithelial potassium content. Potassium content of the epithelium at 280 + 12 (4) nmol (mg dry weight)-' was also similar for the rats in which the lumen had contained the sodium-free choline solution. Plasma potassium was similar in all the groups averaging 4-6 +04 mmol 1-1 but was elevated by the intravenous potassium load averaging 7-7 + 0-3 mmol 1-1 by the end of the infusion. Plasma chloride concentration averaged 98 + 3 mmol 1-1 and was not significantly altered by the infusion of the potassium load. DISCUSSION The high rate of potassium secretion in the distal colon of rats which have been fed a potassium-rich or sodium-depleted diet or which are being given an acutely administered potassium load, appears to depend on the high concentration of potassium in the epithelial cells together with a variable potassium pathway in the apical plasma membrane of these cells (Wills & Biagi, 1982). Recent studies identifying a variable apical potassium conductance are consistent with this view (Wills, 1985; Sandle, Foster, Lewis, Binder & Hayslett, 1985). Changes in the transmembrane and transepithelial electrical gradients may also be important, particularly in sodium-depleted animals in which a relatively high transepithelial PD develops with the lumen negatively charged, a change which would augment the flux of potassium through the paracellular pathway. Aldosterone plays a major part in this epithelial adaptation (Foster, Jones, Hayslett & Binder, 1985; Martin et al. 1986) potassium adaptation being achieved at a relatively low level of aldosterone stimulation, one at which there is little effect on the sodium transport pathway (Edmonds & Willis, 1988b). The results of the present experiments showed that both in the rats fed the standard diet and the potassium-rich diet, the substantial increase of potassium

POTASSIUM SECRETION IN RAT DISTAL COLON secretion provoked by the acute potassium load, was accompanied by an increase of chloride secretion. In the rats on the potassium-rich diet, the acute potassium load appeared in addition to stimulate some increase of sodium absorption. Finally, in the sodium-depleted rats, the acute potassium load provoked less increase of chloride secretion but a substantial rise in sodium absorption and in the transcellular sodium flux from lumen to plasma. In attempting to interpret these findings we need to recall that the apical membrane sodium pathways of the colonic epithelial cells of sodiumdepleted rats are amiloride sensitive, (that is, having sodium pathways blocked by a low concentration, 10-4 M or less, of amiloride) so belonging to the diffusion type of sodium channel (Will et al. 1980; Benos, 1982). In the control rats on the standard diet, on the other hand, the apical sodium pathways are of a different type being almost exclusively amiloride insensitive at this low amiloride concentration. The apical membrane in the rats adapted to the potassium-rich diet does undergo some modification as regards the sodium channels showing partial amiloride sensitivity presumably due to a limited degree of aldosterone stimulation (Edmonds & Willis, 1988a). These previous observations taken with the present results suggest that stimulation of potassium secretion by the acutely administered potassium load produces an increase of sodium transcellular absorption through the amiloridesensitive sodium diffusion channels but does not affect absorption when this is limited to the amiloride-insensitive pathway. The interaction of sodium and potassium movements may depend on changes in the apical transmembrane electrical PD induced by the increased flux of potassium from cell to lumen. The fall of transepithelial PD that was observed in the sodium-depleted rats during acute potassium loading is consistent with this possibility. In all the rats but particularly those fed the standard diet, an increase of chloride secretion occurred during the acute potassium infusion and this was so even when the potassium infusion was chloride free. Ordinarily in the distal colon, chloride appears to move from plasma to lumen by way of the paracellular route (Edmonds & Smith, 1979) and an increase of chloride permeability of the paracellular pathway could account for the observed rise of chloride secretion. That sodium movement into the colonic lumen was also increased in the rats on the normal diet lends some support to the possibility that the paracellular pathway had become more 'leaky ' during the infusion, to both sodium and chloride. An alternative possibility is that chloride was secreted by the colonic epithelial cells and this is well recognized in some circumstances (Frizzell, 1976; Frizzell, Koch & Schultz, 1976; Browning, Hardeastle, Hardeastle & Sandford, 1977) but, in general, the accompanying cation was sodium and the transepithelial PD was increased. Recent in vitro studies of rabbit distal colon have shown, however, that active secretion of potassium can occur independently of sodium absorption and be associated with increased chloride secretion (Halm & Frizzell, 1986). Moreover tumours derived from the epithelium appear sometimes to behave as though they have developed this capacity (Schnitka, Friedman, Kidd & Mackenzie, 1961; Shields, 1966). These considerations suggest that the rise of chloride secretion that we observed during acute potassium loading depended on increased chloride secretion through the cellular pathway but the present results do not establish this unequivocally. The findings of the experiments with 86RbV were consistent with the view that much of the secreted potassium was derived from the epithelial cells. The results of 325

326 C. J. EDMONDS AND C. L. WILLIS previous experiments (Edmonds & Smith, 1979) suggested that only part of the potassium pool of the epithelium was involved in the secretary process. This is presumably the potassium of those epithelial cells concerned in ionic transport. The turnover rate of this cellular potassium (as indicated by the 86Rb+ kinetics) was relatively fast with ti of 21-27 min in the rats fed the standard diet. Both potassium feeding and sodium restriction, procedures which considerably enhanced the potassium secretion rate, were also associated with an increased rate of 86Rb+ turnover. Since cellular potassium content was unchanged this presumably reflected increased activity of the basolateral pumps delivering potassium into the cells, a supposition consistent with observed changes in the basolateral membrane in these states (Kashgarian, Taylor, Binder & Hayslett, 1980; Foster, Hayslett & Binder, 1984). A further small increase of86rb+ turnover rate was demonstrable during the acute potassium infusion, a finding which presumably reflected increased delivery of potassium to the cells associated with the rise of potassium secretion. The high colonic potassium secretion rate of the rats fed the potassium-rich diet was little reduced when sodium was removed from the luminal solution, a finding consistent with the notion that the sodium needed for the basolateral Na+/K+ pump during potassium secretion came largely by a basolateral Na-Cl co-transport process (Frizzell, Halm & Krasny, 1984). In conclusion, the present results support the notion that potassium secretion in the rat distal colon derives largely from the epithelial potassium, the turnover of which is accelerated in conditions associated with chronically enhanced potassium secretion, presumably as a result of increased basolateral Na+-K+ pump activity. The amount of potassium secretion, which reflects the flux from the cells into the lumen across the apical membrane, depends on the apical potassium pathway. This pathway seems to be variable being particularly increased when the rats are sodium depleted or potassium adapted, apparently through the mediation of aldosterone, and when an acute potassium load is administered, through a mechanism at present unknown. In addition, it has been shown that when amiloride-insensitive sodium pathways are present in the apical membrane, the increased potassium secretion is accompanied by increased chloride secretion. The rise of chloride secretion could not be accounted for by an increase of the electrochemical gradient since this was either unchanged or, as in those rats infused with a chloride-free potassium load, reduced. Whether it is due to a change in passive paracellular conductance or to epithelial chloride secretion is at present unresolved. When, however, amiloride-sensitive sodium diffusion pathways are present, such as predominate in sodium-depleted animals, the increased potassium secretion is associated largely with increased sodium absorption and chloride secretion is much less affected. REFERENCES BASTL, C., BINDER, H. J. & HAYSLETT, J. P. (1980). Role of glucocorticoids and aldosterone in maintenance of colonic cation transport. American Journal of Phy8iology 238, F181-186. BENOS, D. J. (1982). Amiloride: a molecular probe of sodium transport in tissues and cells. American Journal of Phy8iology 242, C131-145. BROWNING, J. G., HARDCASTLE, J., HARDCASTLE, P. T. & SANDFORD, P. A. (1977). The role of acetylcholine in the regulation of ion transport by rat colon mucosa. Journal of Phy8iology 272, 737-754.

POTASSIUM SECRETION IN RAT DISTAL COLON BUDINGER, M. E., FOSTER, E. S., HAYSLETT, J. P. & BINDER, H. J. (1986). Sodium and chloride transport in the large intestine of potassium loaded rats. American Journal of Physiology 251, G249-252. EDMONDS, C. J. (1967). Transport of potassium by the colon of normal and sodium-depleted rats. Journal of Physiology 193, 603-617. EDMONDS, C. J. & MACKENZIE, J. (1984). Amiloride sensitive and insensitive sodium pathways and the cellular sodium transport pool of colonic epithelium in rats. Journal of Physiology 346, 61-71. EDMONDS, C. J. & MARRIOTT, J. C. (1967). The effect of aldosterone and adrenalectomy on the electrical potential difference of rat colon and on the transport of sodium, potassium, chloride and bicarbonate. Journal of Endocrinology 39, 517-531. EDMONDS, C. J. & SMITH, T. (1979). Epithelial transport pathways of rat colon determined in vivo by impulse response analysis. Journal of Physiology 296, 471-485. EDMONDS, C. J. & WILLIS, C. L. (1988a). Potassium secretion by rat distal colon during acute potassium loading: effect of sodium, potassium intake and aldosterone. Journal of Physiology 401, 39-51. EDMONDS, C. J. & WILLIS, C. L. (1988b). Aldosterone in colonic potassium adaptation in rats. Journal of Endocrinology 117, 379-386. FISHER, K. A., BINDER, H. J. & HAYSLETT, J. P. (1976). Potassium secretion by colonic mucosal cells after potassium adaptation. American Journal of Physiology 231, F987-994. FOSTER, E. S., HAYSLETT, J. P. & BINDER, H. J. (1984). Mechanisms of active potassium absorption and secretion in the rat colon. American Journal ofphysiology 246, G611-617. FOSTER, E. S., JONES, W. J., HAYSLETT, J. P. & BINDER, H. J. (1985). Role of aldosterone and dietary potassium in potassium adaptation in the distal colon of the rat. Gastroenterology 88, 41-46. FRIZZELL, R. A. (1976). Active chloride secretion by rabbit colon: calcium dependent stimulation by Jonophore A23187. Journal ofmembrane Biology 35, 175-187. FRIZZELL, R. A., HALM, D. R. & KRASNY, E. J. (1984). Relationships between chloride and potassium secretion across large intestine. In Mechanisms of Intestinal Electrolyte Transport and Regulation by Calcium, ed. DONOWITZ, M. & SHARP, G. W. G. pp. 35-46. Alan R. Liss, New York. FRIZZELL, R. A., KOCH, M. J. & SCHULTZ, S. G. (1976). Ion transport by the rabbit colon. I. Active and passive components. Journal ofmembrane Biology 27, 297-316. HALM, D. R. & FRIZZELL, R. A. (1986). Active K transport across rabbit distal colon: relation to Na absorption and Cl secretion. American Journal of Physiology 251, C252-267. KASHGARIAN, M., TAYLOR, C. R., BINDER, H. J. & HAYSLETT, J. P. (1980). Amplification of cell membrane surface in potassium adaptation. Laboratory Investigation 42, 581-588. MARTIN, R. S., OSZI, P., BROCCA, S., ARRIZURIETTA, E. & HAYSLETT, J. P. (1986). Failure of potassium adaptation in vivo in the colon of aldosterone-deficient rats. Journal oflaboratory and Clinical Medicine 108, 241-245. SANDLE, G. I., FOSTER, E. S., LEWIS, S. A., BINDER, H. J & HAYSLETT, J. P. (1985). The electrical basis for enhanced potassium secretion in rat distal colon during dietary potassium loading. Pflugers Archiv 403, 433-439. SCHNITKA, T. K., FRIEDMAN, M. H. W., KIDD, E. G. & MACKENZIE, W. C. (1961). Villous tumours of the rectum and colon characterized by severe fluid and electrolyte loss. Surgery, Gynaecology and Obstetrics 112, 609-621. SHIELDS, R. (1966). Absorption and secretion of electrolytes and water by the human colon with particular reference to benign adenoma and papilloma. British Journal of Surgery 53, 893-897. SMITH, P. L. & MCCABE, R. D. (1984). Mechanism and regulation of transcellular potassium transport by the colon. American Journal of Physiology 247, G445-456. WILL, P. C., LEBOWITZ, J. L. & HOPFER, U. (1980). Induction of amiloride-sensitive sodium transport in the rat colon by mineralocorticoids. American Journal of Physiology 238, F261-268. WILLS, N. K. (1985). Apical membrane potassium and chloride permeabilities in surface cells of rabbit descending colon epithelium. Journal of Physiology 358, 433-445. WILLS, N. K. & BIAGI, B. (1982). Active potassium transport by rabbit descending colon epithelium. Journal of Membrane Biology 64, 195-203. WRONG, 0., EDMONDS, C. J. & CHADWICK, V. S. (1981). The Large Intestine, chap. 6, pp. 73-105. MTP Press Ltd, Lancaster. 327