Absorption and secretion of potassium by rabbit descending colon

Colonic Potassium Absorption and Secretion in Health and Disease

The colon has large capacities for K⁺ absorption and K⁺ secretion, but its role in maintaining K⁺ homeostasis is often overlooked. For many years, passive diffusion and/or solvent drag were thought to be the primary mechanisms for K⁺ absorption in human and animal colon. However, it is now clear that apical H⁺ ,K⁺ -ATPase, in coordination with basolateral K⁺ -Cl^- cotransport and/or K⁺ and Cl^- channels operating in parallel, mediate electroneutral K⁺ absorption in animal colon. We now know that K⁺ absorption in rat colon reflects ouabain-sensitive and ouabain-insensitive apical H⁺ ,K⁺ -ATPase activities. Ouabain-insensitive and ouabain-sensitive H⁺ ,K⁺ -ATPases are localized in surface and crypt cells, respectively. Colonic H⁺ ,K⁺ -ATPase consists of α- (HKC_α ) and β- (HKC_β ) subunits which, when coexpressed, exhibit ouabain-insensitive H⁺ ,K⁺ -ATPase activity in HEK293 cells, while HKC_α coexpressed with the gastric β-subunit exhibits ouabain-sensitive H⁺ ,K⁺ -ATPase activity in Xenopus oocytes. Aldosterone enhances apical H⁺ ,K⁺ -ATPase activity, HKC_α specific mRNA and protein expression, and K⁺ absorption. Active K⁺ secretion, on the other hand, is mediated by apical K⁺ channels operating in a coordinated way with the basolateral Na⁺ -K⁺ -2Cl^- cotransporter. Both Ca²⁺ -activated intermediate conductance K⁺ (IK) and large conductance K⁺ (BK) channels are located in the apical membrane of colonic epithelia. IK channel-mediated K⁺ efflux provides the driving force for Cl^- secretion, while BK channels mediate active (e.g., cAMP-activated) K⁺ secretion. BK channel expression and activity are increased in patients with end-stage renal disease and ulcerative colitis. This review summarizes the role of apical H⁺ ,K⁺ -ATPase in K⁺ absorption, and apical BK channel function in K⁺ secretion in health and disease. © 2018 American Physiological Society. Compr Physiol 8:1513-1536, 2018.

Colonic potassium handling

Homeostatic control of plasma K+ is a necessary physiological function. The daily dietary K+ intake of approximately 100 mmol is excreted predominantly by the distal tubules of the kidney. About 10% of the ingested K+ is excreted via the intestine. K+ handling in both organs is specifically regulated by hormones and adapts readily to changes in dietary K+ intake, aldosterone and multiple local paracrine agonists. In chronic renal insufficiency, colonic K+ secretion is greatly enhanced and becomes an important accessory K+ excretory pathway. During severe diarrheal diseases of different causes, intestinal K+ losses caused by activated ion secretion may become life threatening. This topical review provides an update of the molecular mechanisms and the regulation of mammalian colonic K+ absorption and secretion. It is motivated by recent results, which have identified the K+ secretory ion channel in the apical membrane of distal colonic enterocytes. The directed focus therefore covers the role of the apical Ca2+ and cAMP-activated BK channel (KCa1.1) as the apparently only secretory K+ channel in the distal colon.

Class I antiarrhythmics inhibit Na+ absorption and Cl− secretion in rabbit descending colon epithelium

To clarify the mechanism of the diarrhea associated with the clinical use of antiarrhythmic drugs we assessed the effects of these agents on transepithelial Na+ absorption and Cl- secretion, on basolateral K+ conductance, and on the properties of single basolateral K+ channels of rabbit colon epithelium. Quinidine and propafenone, both at 10 microM, inhibited Na+ absorption by 27 and 38% respectively, compared with 50% with 5 mM Ba2+. The other tested class I antiarrhythmics disopyramide, mexiletine, lidocaine, and flecainide decreased Na+ current by 9-13%. Procainamide and the class III antiarrhythmics N-acetylprocainamide, sotalol, ibutilide, and amiodarone were no or were very weak inhibitors of Na+ absorption. Cl- secretion, stimulated with the adenosine analogue NECA (5'-N-ethylcarboxamide-adenosine), was reduced by 54% with quinidine and by 29% with propafenone compared with 100% with Ba2+. Mexiletine, lidocaine, and flecainide inhibited Cl- secretion by 10-23%, whereas the class III antiarrhythmics were no or were weak inhibitors. Those antiarrhythmics that inhibited Na+ and Cl- transport also reduced basolateral K+ conductance, determined in amphotericin B permeabilized epithelia. The activity of the high-conductance, Ca2+-activated, voltage-dependent K+ (BK(Ca)) channel, which is primarily responsible for basolateral K+ recycling during Na+ absorption, was inhibited by 10-30 microM quinidine or propafenone in the form of a rapidly dissociating block. Mexiletine and flecainide inhibited the single channel conductance at higher concentrations; disopyramide, lidocaine, and procainamide were ineffective. In conclusion, the present evidence suggests that the diarrhea caused by class I antiarrhythmic drugs such as quinidine and propafenone is a result of a reduction in basolateral K+ conductance and inhibition of BK(Ca) channels, thereby impeding transepithelial Na+ and water absorption.

Basolateral potassium channels of rabbit colon epithelium: role in sodium absorption and chloride secretion

In order to assess the role of different classes of K(+) channels in recirculation of K(+) across the basolateral membrane of rabbit distal colon epithelium, the effects of various K(+) channel inhibitors were tested on the activity of single K(+) channels from the basolateral membrane, on macroscopic basolateral K(+) conductance, and on the rate of Na(+) absorption and Cl(-) secretion. In single-channel measurements using the lipid bilayer reconstitution system, high-conductance (236 pS), Ca(2+)-activated K(+) (BK(Ca)) channels were most frequently detected; the second most abundant channel was a low-conductance K(+) channel (31 pS) that exhibited channel rundown. In addition to Ba(2+) and charybdotoxin (ChTX), the BK(Ca) channels were inhibited by quinidine, verapamil and tetraethylammonium (TEA), the latter only when present on the side of the channel from which K(+) flow originates. Macroscopic basolateral K(+) conductance, determined in amphotericin-permeabilised epithelia, was also markedly reduced by quinidine and verapamil, TEA inhibited only from the lumen side, and serosal ChTX was without effect. The chromanol 293B and the sulphonylurea tolbutamide did not affect BK(Ca) channels and had no or only a small inhibitory effect on macroscopic basolateral K(+) conductance. Transepithelial Na(+) absorption was partly inhibited by Ba(2+), quinidine and verapamil, suggesting that BK(Ca) channels are involved in basolateral recirculation of K(+) during Na(+) absorption in rabbit colon. The BK(Ca) channel inhibitors TEA and ChTX did not reduce Na(+) absorption, probably because TEA does not enter intact cells and ChTX is 'knocked off' its extracellular binding site by K(+) outflow from the cell interior. Transepithelial Cl(-) secretion was inhibited completely by Ba(2+) and 293B, partly by quinidine but not by the other K(+) channel blockers, indicating that the small (<3 pS) K(V)LQT1 channels are responsible for basolateral K(+) exit during Cl(-) secretion. Hence different types of K(+) channels mediate basolateral K(+) exit during transepithelial Na(+) and Cl(-) transport.

beta-adrenergic agonists stimulate Na+-K+-Cl- cotransport by inducing intracellular Ca2+ liberation in crypt cells

Epinephrine and beta-adrenergic agonists (beta1 and beta2 for isoproterenol, beta1 for dobutamine, beta2 for salbutamol) stimulated K+ (or 86Rb) influx mediated by the Na+-K+-2Cl- cotransporter and the Na+-K+ pump in isolated colonic crypt cells. Preincubation with bumetanide abolished the epinephrine effect on the Na+-K+ pump, suggesting that the primary effect is on the cotransporter. Maximal effect was obtained with 1 microM epinephrine with an EC50 of 91.6 +/- 9.98 nM. Epinephrine-induced K+ transport was blocked by propranolol with an IC50 of 134 +/- 28.2 nM. alpha-Adrenergic drugs did not modify K+ transport mechanisms. Neither Ba2+ nor tetraethylammonium nor DIDS modified the adrenergic enhancement on the cotransporter. In addition, epinephrine did not affect K+ efflux. Dibutyryl cAMP did not alter K+ transport. Reduction of extracellular Ca2+ to 30 nM did not influence the response to epinephrine. However, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM abolished epinephrine-induced K+ transport. Ionomycin increased Na+-K+-2Cl- cotransport activity. Moreover, epinephrine increased intracellular Ca2+ concentration in a process inhibited by propranolol. In conclusion, epinephrine stimulated the Na+-K+-2Cl- cotransporter in a process mediated by beta1- and beta2-receptors and modulated by intracellular Ca2+ liberation.

Localization of cAMP- and aldosterone-induced K+ secretion in rat distal colon by conductance scanning

1. Aldosterone- and adrenaline-induced K+ secretion were investigated in rat late distal colon using conductance scanning and Ussing chamber techniques. K+ secretion was unmasked by the K+ channel blocker tetraethylammonium (TEA). Electrogenic Na+ absorption was inhibited by amiloride. Rb+ net fluxes consistently measured about 80% of K+ secretion estimated using change in short-circuit current (delta ISC) measurements. 2. Partial block of K+ absorption by mucosal ouabain did not change TEA-sensitive K+ secretion. Thus, K+ absorption and K+ secretion are not coupled. 3. Additivity of Rb+ fluxes as well as delta ISC caused by 3 nM aldosterone (6 h in vitro incubation) and, subsequently, adrenaline suggested additivity of aldosterone-induced and cAMP-mediated K+ secretion in the presence of amiloride. 4. Conductance scanning under control conditions revealed a small TEA-sensitive K+ conductivity in surface epithelium (0.3 +/- 0.2 mS cm-2) but not in crypts, as well as a small basal K+ secretion in surface epithelium (delta ISC = 0.3 mumol h-1 cm-2), which increased during sham incubation. 5. Aldosterone (3 nM, 6 h in vitro incubation) resulted, after correction for the basal K+ secretion, in a K+ secretion of delta ISC = 0.9 mumol h-1 cm-2. Aldosterone induced a TEA-sensitive conductivity of 1.1 +/- 0.3 mS cm-2 in surface epithelium, but not in crypts. 6. Adrenaline (5 microM) caused, in fresh tissue, a K+ secretion of delta ISC = 1.2 mumol h-1 cm-2 and equal conductivity changes in crypts (0.7 +/- 0.2 mS cm-2) and surface epithelium (0.7 +/- 0.1 mS cm-2). 7. We conclude that K+ secretion induced by aldosterone in physiological concentration is restricted to surface epithelium, whereas cAMP-mediated K+ secretion is located equally in crypts and surface epithelium.

Tetraethylammonium-sensitive apical K+ channels mediating K+ secretion by turtle colon

1. Apical membrane K+ channels in turtle colon were identified and characterized using current fluctuation analysis. 2. Under short-circuit conditions in NaCl-Ringer solution, the power density spectrum (PDS) of the short-circuit current (Isc) sometimes exhibited a clearly discernible Lorentzian component, indicating spontaneous fluctuations produced by a population of apical ion channels. The Lorentzian component had a characteristic corner frequency (fc) which averaged 10.2 +/- 0.9 Hz (mean +/- S.E.M., n = 20). 3. The power of the spontaneous fluctuations was enhanced (So increased) by manoeuvres that depolarize the apical membrane electrical potential (Va). Discernible fluctuations were enhanced or induced by raising the serosal K+ concentration ([K+]s = 50-115 mM, Na+ replacement), by clamping the transepithelial potential (Vt) to serosa-positive values, or by blocking basolateral K+ channels with Ba2+. 4. Mucosal amiloride (100 microM) attenuated the spontaneous fluctuations observed in NaCl-Ringer solution but had no effect in the presence of serosal high K+, indicating that amiloride did not block the K(+)-permeable channels but these channels resided in the same cells as the amiloride-sensitive Na+ channels. 5. Raising the mucosal K+ concentration attenuated spontaneous fluctuations. 6. In the presence of serosal high K+ and mucosal amiloride, the spontaneous fluctuations were often accompanied by a reversed Isc consistent with K+ secretion. These conditions were used to test the effects of putative channel blockers. 7. Mucosal Ba2+ and tetraethylammonium (TEA+) were effective inhibitors of the spontaneous fluctuations and the reversed Isc. At a concentration of 10 mM, TEA+ was maximally effective but the TEA+ analogues tetramethylammonium (TMA+) and tetrapropylammonium (TPrA+) were much less effective. Mucosal Rb+ or Cs+ did not inhibit at a concentration of 10 mM. 8. Mucosal lidocaine (200 microM), quinidine (200 microM), or diphenylamine-2-carboxylate (DPC, 1 mM) had little or no effect on the spontaneous fluctuations and reversed Isc. Quinine (100 microM), 4-aminopyridine (1 mM), and apamin (100 nM) were also without effect. 9. Mucosal TEA+ (10 mM) abolished the active secretory K+ flux measured in the presence of serosa-positive transepithelial potentials. 10. These experiments identified a population of TEA(+)-sensitive, apical K+ channels which mediate active K+ secretion in turtle colon. Sensitivity to external TEA+ distinguishes these channels from basolateral K+ channels in turtle colon and demonstrates similarity to apical K+ channels in mammalian colon.

Effects of a low-sodium diet on electrolyte transport in the proximal and distal colon of the guinea pig (Cavia porcellus)

1. Animals were randomly assigned to three different treatment groups. (I) Control group maintained on a standard guinea pig diet, (II) a group obtaining a low-Na diet (0.02% Na w/w) and (III) a group fed the low-Na diet and in addition being treated with K-canrenoate prior to the perfusion experiment. 2. Proximal and distal colonic segments of anesthetized guinea pigs were perfused simultaneously with Ringer's solution. 3. In the proximal colon of the control group Na and Cl were absorbed in equal quantity and K was secreted. 4. In the distal colon of the control group Na and K were absorbed and Cl was secreted. Na absorption was only approximately 1/10 of that in the proximal colon. 5. Feeding a low-Na diet for two weeks induced secondary hyperaldosteronism. Plasma aldosterone concentration increased 3.4-fold. Na and Cl absorption rose 34% and 27% above control in the proximal colon, respectively. In the distal colon Na absorption was elevated 3.7-fold above control. 6. Na absorption and transepithelial potential difference (PD) in the proximal colon were amiloride-insensitive in all groups. In the distal colon Na absorption was abolished by luminal amiloride and the transepithelial PD was reversed in polarity. 7. K-canrenoate treatment prevented the increase in Na transport in the distal colon.

The colon carcinoma cell line Caco-2 contains an H+/K(+)-ATPase that contributes to intracellular pH regulation

The presence of an H+/K(+)-ATPase and its contribution to the regulation of intracellular pH (pHi) was investigated in Caco-2 cells. The H+/K(+)-ATPase was detected immunologically using the monoclonal antibody 5-B6, which was raised against hog gastric H+/K(+)-ATPase. Cell pH was determined using the pH-sensitive dye 2',7'-bis(carboxyethyl)-carboxyfluorescein. Control pHi, measured in HCO(3-)-free medium, was 7.62 +/- 0.03 (n = 27) when cells were cultured for 14 days and decreased to 7.40 +/- 0.03 (n = 18) after 35 days in culture. Recovery of pHi following a NH+4/NH3 pulse could be reduced by either 100 microM SCH 28080 or 1 mM amiloride, or by removing extracellular Na+. The inhibitory effects of SCH 28080 and amiloride were additive, demonstrating the involvement of a gastric-like H+/K(+)-ATPase and a Na+/H+ exchanger in regulating pHi. Recovery rates at pHi 6.8 were not significantly different in cells cultured for up to 21 days, but were significantly lower in cells cultured for 28 and 35 days. This decrease in recovery rate was due to a decrease in the SCH-28080-insensitive recovery, indicating a reduction of the relative importance of Na+/H+ exchange to the recovery. Recovery of pHi was also inhibited by 1 mM N-ethylmaleimide. However, it is unlikely that N-ethyl-maleimide inhibited a vacuolar type of H+-ATPase, since bafilomycin A1 had no effect on pHi recovery. In conclusion, Caco-2 cells contain a SCH-28080-sensitive mechanism for regulating pHi, which is most conveniently studied after 28 days in culture, when the relative contribution of a Na+/H+ exchanger to pHi regulation is decreased.

Vanadium-induced Cl(-)-secretion in rabbit descending colon is mediated by prostaglandins

Vanadium in the 4+ (vanadyl-ion) and 5+ (vanadate-ion) oxidation state stimulates furosemide-sensitive electrogenic Cl- secretion in isolated epithelia of rabbit descending colon. This effect is associated with an increased release of prostaglandin E2 from the tissue. Inhibitors of phospholipase A2 or cyclooxygenase abolish both vanadium-induced release of prostaglandin E2 and Cl- secretion. Neuronal mechanisms are not likely to be involved, as tetrodotoxin does not affect the vanadate induced Cl- secretion. Although vanadate is known to inhibit Na+,K(+)-ATPase activity, no inhibition of active Na+ transport was observed in intact colonic epithelia suggesting a rapid intracellular reduction of vanadate ions to vanadyl ions which have no inhibitory effect on the Na+,K(+)-ATPase. The present findings therefore indicate that vanadate stimulated colonic Cl- secretion involves intracellular conversion of vanadate to vanadyl and release of prostaglandin E2.

Endothelin-1 stimulates chloride and potassium secretion in rabbit descending colon

The vasoactive peptide endothelin-1 (ET-1) which is present in high concentrations in the colon, causes concentration-dependent electrogenic Cl- secretion in rabbit descending colon. This effect is half-maximal at 0.11 mumol/l. Like other secretagogues, ET-1 also stimulates K+ secretion. The secretory effect of ET-1 is associated with increased release of prostaglandin E2 from the serosal surface of the mucosa. ET-1-induced Cl- secretion is completely inhibited by the loop diuretic bumetanide and by indomethacin and quinacrine, inhibitors of prostaglandin synthesis. Neuronal mechanisms do not seem to be involved, as tetrodotoxin did not affect the secretory response to ET-1 significantly. On the other hand, neither the catalytic activity nor the transport function of the Na+/K(+)-ATPase of rabbit colon epithelium is affected by endothelin-1 (ET-1) in concentrations up to 10 mumol/l. It is concluded that ET-1 causes Cl- and K+ secretion by stimulating phospholipase A2 and release of prostaglandins, whereas Na+ transport is not altered.

Mucosal ouabain and Na+ inhibit active Rb+(K+) absorption in normal and sodium-depleted rat distal colon

To determine the effect of mucosal sodium and mucosal ouabain on active Rb+(K+) absorption, unidirectional and net 86Rb+ fluxes were measured under voltage-clamp conditions in the distal colon of normal and sodium-depleted rats. The role of mucosal sodium (independent of serosal sodium) was evaluated in a model of Rb+(K+) absorption in which serosal ouabain markedly enhanced active Rb+(K+) absorption. In normal rats, mucosal sodium was a competitive inhibitor of Rb+(K+) absorption, and Rb+(K+) absorption consisted of a mucosal sodium-sensitive component and a mucosal sodium-insensitive component. Further, mucosal ouabain almost completely inhibited the mucosal sodium-insensitive component but did not affect the mucosal sodium-sensitive component. In sodium-depleted rats, both mucosal sodium-sensitive and mucosal sodium-insensitive fractions of Rb+(K+) absorption were also identified. Aldosterone markedly stimulated the mucosal sodium-sensitive component (1.68 +/- 0.15 vs. 0.60 +/- 0.10 muEq.h-1.cm-2) but not the sodium-insensitive component (0.88 +/- 0.09 vs. 0.64 +/- 0.06 muEq.h-1.cm-2) component of Rb+(K+) absorption; however, in contrast to normal animals, mucosal sodium in sodium-depleted animals was a noncompetitive inhibitor of Rb+(K+) absorption. The mucosal sodium-insensitive component of Rb+(K+) absorption in sodium-depleted animals was substantially inhibited by mucosal ouabain, but the mucosal sodium-sensitive component, unlike that in normal animals, was partially inhibited by mucosal ouabain. These studies indicate that the characteristics of the Rb+(K+) absorptive process in sodium-depleted animals differ significantly from those present in normal animals, suggesting that aldosterone induces an Rb+(K+) absorptive mechanism not present in normal animals.

Influence of ouabain of electrolyte and water transport in the rabbit ileum and colon in vitro and in vivo

We studied the influence of the Na(+)-K+ pump inhibitor ouabain on water and electrolyte transport in the rabbit distal ileum and proximal colon in vitro and in vivo. Under in vitro conditions, ouabain markedly reduced the absorption of water, sodium, chloride and bicarbonate, and reduced potassium secretion in both the ileum and the colon. In vivo results were similar but less marked than those obtained in vitro, with significant differences only at the level of the distal ileum.

Rabbit distal colon epithelium: III. Ca2(+)-activated K+ channels in basolateral plasma membrane vesicles of surface and crypt cells

In the mammalian distal colon, the surface epithelium is responsible for electrolyte absorption, while the crypts are the site of secretion. This study examines the properties of electrical potential-driven 86Rb+ fluxes through K+ channels in basolateral membrane vesicles of surface and crypt cells of the rabbit distal colon epithelium. We show that Ba2(+)-sensitive, Ca2(+)-activated K+ channels are present in both surface and crypt cell derived vesicles with half-maximal activation at 5 x 10(-7) M free Ca2+. This suggests an important role of cytoplasmic Ca2+ in the regulation of the bidirectional ion fluxes in the colon epithelium. The properties of K+ channels in the surface cell membrane fraction differ from those of the channels in the crypt cell derived membranes. The peptide toxin apamin inhibits Ca2(+)-activated K+ channels exclusively in surface cell vesicles, while charybdotoxin inhibits predominantly in the crypt cell membrane fraction. Titrations with H+ and tetraethylammonium show that both high- and low-sensitive 86Rb+ flux components are present in surface cell vesicles, while the high-sensitive component is absent in the crypt cell membrane fraction. The Ba2(+)-sensitive, Ca2(+)-activated K+ channels can be solubilized in CHAPS and reconstituted into phospholipid vesicles. This is an essential step for further characterization of channel properties and for identification of the channel proteins in purification procedures.

Active potassium absorption in rat distal colon

1. Active potassium (K+) absorption in rat distal colon was investigated by measuring mucosal-to-serosal (JK, ms) and serosal-to-mucosal (JK, sm) 42K+ fluxes (mu equiv h-1 cm-2) across isolated stripped mucosa under short-circuit conditions in normal and dietary Na-depleted animals. As previously demonstrated, removal of Na+ from both mucosal and serosal solutions bathing the normal colon slightly increased net K+ absorption as a result of inhibition of JK, sm without affecting JK, ms, while in the Na-depleted group net K+ secretion (-0.54 +/- 0.11) was converted to a marked net K+ absorption (1.68 +/- 0.30, P less than 0.001). 2. In both groups of animals in Na(+)-free Ringer solution, JK, ms exhibited saturable and linear components, while JK, sm was a linear function of [K+]. Estimated affinity constants (mM) for saturable net K+ absorption were similar in normal (0.52 +/- 0.12) and Na-depleted (0.67 +/- 0.11) animals; however, there was a greater than 3-fold increase in the saturable flux (Jmax) from 0.54 +/- 0.04 in the normal colon to 1.78 +/- 0.08 mu equiv h-1 cm-2 in Na-depleted animals. 3. Mucosal orthovanadate (100 microM) inhibited JK, ms in both normal (control, 0.66 +/- 0.05 vs. orthovanadate, 0.36 +/- 0.03 mu equiv h-1 cm-2, P less than 0.001) and Na-depleted animals (control 1.20 +/- 0.13 vs. orthovanadate 0.77 +/- 0.07 mu equiv h-1 cm-2, P less than 0.01) without affecting JK, sm or the short-circuit current. In the Na-depleted group mucosal omeprazole or SCH28080 (100 microM), inhibitors of gastric K(+)-H(+)-ATPase, insignificantly or slightly reduced (by 10%) JK, ms respectively; in contrast, mucosal ouabain (1 mM) markedly inhibited JK, ms (control, 1.61 +/- 0.16 vs. ouabain, 0.83 +/- 0.98 mu equiv h-1 cm-2, P less than 0.001). 4. Mucosal Na+ appeared to be a competitor of K+ uptake across the apical membrane. 5. These results indicate that dietary Na-depletion increases electroneutral K+ absorption by increasing its transport capacity and suggest that the mechanism of this active K+ absorption process may involve an apical K(+)-ATPase with properties that are unlike the gastric K(+)-H(+)-ATPase but similar, in part, to Na(+)-K(+)-ATPase.

Factors affecting the potassium concentration at the mucosal surface of the proximal and the distal colon of guinea pig

K+ concentrations were measured in vitro with K+ sensitive microelectrodes in the microclimate at the luminal cell surface of the colon of guinea pigs. The serosal K+ concentration was mostly 5.4 mmol/1, the mucosal K+ concentrations were changed (0, 5, 50, or 70 mmol/l). Under control conditions K+ concentrations in the microclimate of the proximal colon were also low (6-9 mmol/l) and rather independent from K+ concentrations in the bulk luminal solution. In the distal colon K+ concentrations in the microclimate increased from 3.7 mmol/l when no K+ was in the luminal solution, up to 22 mmol/l when the mucosal K+ concentrations was 70 mmol/l. Attempts to decrease K+ conductance of the apical membrane with Ba++, to impair K+ transport with ouabain and to increase the paracellular shunt with deoxycholic acid did not affect K+ concentrations in the microclimate of the proximal colon but decreased K+ concentrations in the distal colon. When valinomycin or triaminopyrimidine were added to the mucosal solution at high K+ concentrations in the luminal solutions the K+ concentration in the microclimate was raised. At low luminal K+ concentrations valinomycin had no effect, triaminopyrimidine significantly diminished K+ concentrations at the cell surface. Regional differences in paracellular shunt conductance and in the preepithelial diffusion barrier are thought to be responsible for the observed differences between the proximal and the distal colon. Obviously, however, further unknown mechanisms have to be involved.

Intestinal transport of potassium. Effects of changing apical and basolateral influx of sodium in the isolated mucosa of the hen (Gallus domesticus) colon

1. Net potassium secretion (JKnet) by the sodium-depleted hen's colon (low sodium colon) is 0.17 +/- 0.07 mumol/cm2.hr. Amiloride or ouabain eliminates short circuit currents (Isc) of 16-20 mumol/cm2.hr without affecting JKsm. 2. In the NaCl-loaded hen's colon (high sodium colon) stimulating Isc by (a) glucose, (b) amino acids, and inhibiting with (c) ouabain changes JKnet from 0.08 +/- 0.04 to (a) 0.42 +/- 0.07, to (b) 0.60 +/- 0.07 to (c) 0.13 +/- 0.13 mumol/cm2.hr. 3. In both colonic types theophylline increases and bumetanide decreases JKnet by 1 mumol/cm2.hr. 4.

CONCLUSION

Apical membranes of sodium-absorbing and chloride-secreting cells of the high sodium colon are potassium permeable. In the low sodium colon sodium-absorbing cells have potassium-impermeable apical membranes.

Rabbit distal colon epithelium: II. Characterization of (Na+,K+,Cl-)-cotransport and [3H]-bumetanide binding

Loop diuretic-sensitive (Na+,K+,Cl-)-cotransport activity was found to be present in basolateral membrane vesicles of surface and crypt cells of rabbit distal colon epithelium. The presence of gradients of all three ions was essential for optimal transport activity. (Na+,K+) gradient-driven 36Cl fluxes were half-maximally inhibited by 0.14 microM bumetanide and 44 microM furosemide. While 86Rb uptake rates showed hyperbolic dependencies on Na+ and K+ concentrations with Hill coefficients of 0.8 and 0.9, respectively, uptakes were sigmoidally related to the Cl concentration, Hill coefficient 1.8, indicating a 1 Na+:1 K+:2 Cl stoichiometry of ion transport. The interaction of putative (Na+,K+,Cl-)-cotransport proteins with loop diuretics was studied from equilibrium-binding experiments using [3H]-bumetanide. The requirement for the simultaneous presence of Na+,K+, and Cl-, saturability, reversibility, and specificity for diuretics suggest specific binding to the (Na+,K+,Cl-)-cotransporter. [3H]-bumetanide recognizes a minimum of two classes of diuretic receptor sites, high-affinity (KD1 = 0.13 microM; Bmax1 = 6.4 pmol/mg of protein) and low-affinity (KD2 = 34 microM; Bmax2 = 153 pmol/mg of protein) sites. The specific binding to the high-affinity receptor was found to be linearly competitive with Cl- (Ki = 60 mM), whereas low-affinity sites seem to be unaffected by Cl-. We have shown that only high-affinity [3H]-bumetanide binding correlates with transport inhibition raising questions on the physiological significance of diuretic receptor site heterogeneity observed in rabbit distal colon epithelium.

Ca2+ and cAMP activate K+ channels in the basolateral membrane of crypt cells isolated from rabbit distal colon

Using patch-clamp techniques, we have studied Ca2+-activated K+ channels in the basolateral membrane of freshly isolated epithelial cells from rabbit distal colon. Epithelial cell clusters were obtained from distal colon by gentle mechanical disruption of isolated crypts. Gigaohm seals were obtained on the basolateral surface of the cell clusters. At the resting potential (approximately -45 mV), with NaCl Ringer's bathing the cell, the predominant channels had a conductance of 131 +/- 25 pS. Channel activity depended on voltage as depolarization of the membrane increased the open probability. In excised inside-out patches, channels were found to be selective for K+ over Na+. Channel activity correlated directly with bath Ca2+ concentration in the excised patches. Channel currents were blocked by 5 mM TEA+ and 1 mM Ba2+. In cell-attached patches, after addition of the Ca2+ ionophore A23187, which increases intracellular Ca2+, open probability was markedly increased. Channel activity was also regulated by cAMP as addition of 1 mM dibutyryl-cAMP in the bath solution in cell-attached patches increased channel open probability over 20-fold. Channels that had been activated by cAMP were further activated by Ca2+. We conclude that the basolateral membrane of epithelial cells from descending colon contains a class of potassium channels, which are regulated by intracellular Ca2+ and cAMP.

Characterization of aldosterone-induced potassium secretion in rat distal colon

The role of apical and basolateral membranes in aldosterone-induced active potassium (K) secretion in rat distal colon was investigated by measuring mucosal-to-serosal (Jms) and serosal-to-mucosal (Jsm) 42K fluxes (mueq.h-1.cm-2) across isolated stripped mucosa under short-circuit conditions in normal and secondary-hyperaldosterone animals. In normal colons mucosal tetraethylammonium (TEA; 30 mM) or barium (Ba; 5 mM), but not cesium (Cs; 15 mM), reduced Jsm without affecting Jms. In aldosterone animals (a) net K secretion (-0.54 +/- 0.11) was converted to net K absorption (0.63 +/- 0.15) by mucosal TEA, which produced a marked reduction in Jsm (0.82 +/- 0.07) and an increase in Jms (0.35 +/- 0.07). In contrast mucosal Ba resulted in a relatively smaller reduction in JK(sm) without altering JK(ms), whereas mucosal Cs was ineffective; (b) serosal bumetanide or the removal of serosal Na or Cl markedly inhibited JK(sm and abolished net K secretion; and (c) serosal ouabain (1 mM) produced qualitatively similar effects to those of serosal bumetanide. These results demonstrate that (a) normal rat distal colon contains apical TEA- and Ba-sensitive K channels; (b) aldosterone induces TEA-sensitive and Ba-sensitive apical K channels; (c) aldosterone-induced K secretion requires both the Na,K-pump and Na-K-2Cl cotransport for K uptake across the basolateral membrane; and (d) alteration of any of these processes results in inhibition of aldosterone-induced active K secretion simultaneously with stimulation of K absorption.

Potassium secretion by neonatal rat distal colon

Electrogenic K+ secretion across the distal colon of young rats was investigated by measuring the sensitivity of the short-circuit current to Ba2+ added to the mucosal side of the tissue. Ba2+-sensitive short-circuit current (IBasc) was high during the suckling and weaning periods but very low in adult animals. Increasing the mucosal K+ concentration was accompanied by the inhibition of the serosa-to-mucosa IBasc and the induction of the mucosa-to-serosa IBasc. The IBasc was decreased by serosal omission of either Na+ or Cl- as well as by serosal addition of furosemide or ouabain. Mucosal omission of Na+ did not change IBasc. By increasing the plasma level of aldosterone (low-sodium diet) IBasc rose by 95% whereas treatment decreasing this level (high-sodium diet) reduced IBasc by 76%. Bilateral adrenalectomy lowered IBasc by 59% and treatment of adrenalectomized rats with deoxycorticosterone acetate prevented the reduction of IBasc. Tetraethylammonium and quinidine had similar effects on Isc as Ba2+. These data are consistent with the presence of a high level of K+ secretion in the distal colon of neonatal rats. This secretory pathway is electrogenic and independent of Na+ absorption. It appears to be mediated by the Na-K-ATPase as well as a furosemide-sensitive Na-Cl or Na-Cl-K cotransport on the basolateral side and by Ba2+-sensitive K+ conductive pathways on the mucosal side. The results suggest that this K+ secretion can be regulated by mineralocorticoids. The mineralocorticoids are necessary for "stimulated" K+ secretion but they are not essential for maintaining "basal" K+ secretion.