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.

The gastric acid secretion of juvenile Sparus aurata was characterized in Ussing chambers; secretion rates were determined by a pH-stat method at pH5.50 and bioelectrical parameters were measured in current-clamped tissues. The basal secretion equaled to 535±87nmol·cm(-2)·h(-1). Serosal carbachol 100μM produced an increase (ΔJH(+)) of 725±133nmol·cm(-2)·h(-1) from basal secretion, this effect being inhibited by mucosal omeprazole 100μM. Basal secretion was also sensitive to the combination of serosal forskolin (FK) 10μM+serosal isobutylmethylxanthine (IBMX) 100μM (ΔJH(+)=793±239nmol·cm(-2)·h(-1)); this effect was insensitive to mucosal omeprazole 100mM but inhibited by mucosal bafilomycin A1 100nM. The effect of carbachol proceeded within a few minutes (<10min), whereas the effect of FK+IBMX was gradual, taking 40min to reach the maximum. The addition of mucosal gadolinium (Gd(3+)) 100μM, a potent calcium-sensing receptor (CaR) agonist, stimulated the basal secretion (ΔJH(+)=340±81nmol·cm(-2)·h(-1)). The present results indicate that the acid secretion mechanism in the sea bream stomach is regulated by muscarinic and CaR-like receptors, cAMP is implicated in the signal transduction, and at least two proton pumps, a HK-ATPase and a V-ATPase contribute to acid secretion.

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

K(+) recycling at the apical membrane of gastric parietal cells is a prerequisite for gastric acid secretion. Two K(+) channels are currently being considered for this function, namely KCNQ1 and inwardly rectifying K(+) channels (Kir). This study addresses the subcellular localization, trafficking, and potential functional significance of KCNQ1 and Kir4.1 channels during stimulated acid secretion.

The effect of pharmacologic KCNQ1 blockade on acid secretion was studied in cultured rat and rabbit parietal cells and in isolated mouse gastric mucosa. The subcellular localization of KCNQ1 and Kir4.1 was determined in highly purified membrane fractions by Western blot analysis as well as in fixed and living cells by confocal microscopy.

In cultured parietal cells and in isolated gastric mucosa, a robust acid secretory response was seen after complete pharmacologic blockade of KCNQ1. Both biochemical and morphologic data demonstrate that Kir4.1 and KCNQ1 colocalize with the H(+)/K(+)-ATPase but do so in different tubulovesicular pools. All Kir4.1 translocates to the apical membrane after stimulation in contrast to only a fraction of KCNQ1, which mostly remains cytoplasmic.

Acid secretion can be stimulated after complete pharmacologic blockade of KCNQ1 activity, suggesting that additional apical K(+) channels regulate gastric acid secretion. The close association of Kir4.1 channels with H(+)/K(+)-ATPase in the resting and stimulated membrane suggests a possible role for Kir4.1 channels during the acid secretory cycle.

We have previously shown that stimulation of acid secretion in parietal cells causes rapid initial cell shrinkage, followed by Na(+)/H(+) exchange-mediated regulatory volume increase (RVI). The factors leading to the initial cell shrinkage are unknown. We therefore monitored volume changes in cultured rabbit parietal cells by confocal measurement of the cytoplasmic calcein concentration. Although blocking the presumably apically located K(+) channel KCNQ1 with chromanol 293b reduced both the forskolin- and carbachol-induced cell shrinkage, inhibition of Ca(2+)-sensitive K(+) channels with charybdotoxin strongly inhibited the cell volume decrease after carbachol, but not after forskolin stimulation. The cell shrinkage induced by both secretagogues was partially inhibited by blocking H(+)-K(+)-ATPase with SCH28080 and completely absent after incubation with NPPB, which inhibits parietal cell anion conductances involved in acid secretion. The subsequent RVI was strongly inhibited with the Na(+)/H(+) exchanger 1 (NHE1)-specific concentration of HOE642 and completely by 500 muM dimethyl-amiloride (DMA), which also inhibits NHE4. None of the above substances induced volume changes under baseline conditions. Our results indicate that cell volume decrease associated with acid secretion is dependent on the activation of K(+) and Cl(-) channels by the respective secretagogues. K(+), Cl(-), and water secretion into the secretory canaliculi is thus one likely mechanism of stimulation-associated cell shrinkage in cultured parietal cells. The observed RVI is predominantly mediated by NHE1.

Gastric acid secretion is a complex process that requires hormonal, neuronal, or calcium-sensing receptor activation for insertion of pumps into the apical surface of the parietal cell. Activation of any or all these pathways causes the parietal cell to secrete concentrated acid with a pH at or close to 1. This acidic fluid combines with enzymes that are secreted from neighbouring chief cells and passes out of the gland up through a mucous gel layer covering the surface of the stomach producing a final intragastric pH of less than 4 during the active phase of acid secretion. Defects in either the mucosal barrier or in the regulatory mechanisms that modulate the secretory pathways will result in erosion of the barrier and ulcerations of the stomach or esophagus. The entire process of acid secretion relies on activation of the catalytic cycle of the gastric H+,K+-ATPase, resulting in the secretion of acid into the parietal cell canaliculus, with K+ being the important and rate-limiting ion in this activation process. In addition to K+ as a rate limiter for acid production, Cl- secretion via an apical channel must also occur. In this review we present a discussion of the mechanics of acid secretion and a discussion of recently identified transporter proteins and receptors. Included is a discussion of some of the recent candidates for the apical K' recycling channel, as well as two recently identified apical proteins (NHE-3, PAT-1), and the newly characterized calcium-sensing receptor (CaSR). We hope that this review will give additional insight into the complex process of acid secretion.

Our objective was to identify and localize a K+ channel involved in gastric HCl secretion at the parietal cell secretory membrane and to characterize and compare the functional properties of native and recombinant gastric K+ channels. RT-PCR showed that mRNA for Kir2.1 was abundant in rabbit gastric mucosa with lesser amounts of Kir4.1 and Kir7.1, relative to beta-actin. Kir2.1 mRNA was localized to parietal cells of rabbit gastric glands by in situ RT-PCR. Resting and stimulated gastric vesicles contained Kir2.1 by Western blot analysis at approximately 50 kDa as observed with in vitro translation. Immunoconfocal microscopy showed that Kir2.1 was present in parietal cells, where it colocalized with H+ -K+ -ATPase and ClC-2 Cl- channels. Function of native K+ channels in rabbit resting and stimulated gastric mucosal vesicles was studied by reconstitution into planar lipid bilayers. Native gastric K+ channels exhibited a linear current-voltage relationship and a single-channel slope conductance of approximately 11 pS in 400 mM K2SO4. Channel open probability (Po) in stimulated vesicles was high, and that of resting vesicles was low. Reduction of extracellular pH plus PKA treatment increased resting channel Po to approximately 0.5 as measured in stimulated vesicles. Full-length rabbit Kir2.1 was cloned. When stably expressed in Chinese hamster ovary (CHO) cells, it was activated by reduced extracellular pH and forskolin/IBMX with no effects observed in nontransfected CHO cells. Cation selectivity was K+ = Rb+ >> Na+ = Cs+ = Li+ = NMDG+. These findings strongly suggest that the Kir2.1 K+ channel may be involved in regulated gastric acid secretion at the parietal cell secretory membrane.

Hydrochloric acid is produced in parietal cells of the gastric glands by an H(+)/K(+)-ATPase. This proton pump couples the outwards movement of H(+) to the inwards movement of K(+) thus requiring the presence of luminal K(+) to operate. To maintain the activity of the pump, K(+) recirculates over the apical membrane via conductive pathways, the molecular nature of which has been identified in the past few years. This review gives a short overview about the recent advances in the understanding of the role of K(+) channels in the process of parietal cell H(+) secretion and focuses on the identification of KCNQ1/KCNE2 K(+) channel as the molecular correlate of the parietal cell apical potassium conductance.

We recently proposed that extracellular Ca(2+) ions participate in a novel form of intercellular communication involving the extracellular Ca(2+)-sensing receptor (CaR). Here, using Ca(2+)-selective microelectrodes, we directly measured the profile of agonist-induced [Ca(2+)]ext changes in restricted domains near the basolateral or luminal membranes of polarized gastric acid-secreting cells. The Ca(2+)-mobilizing agonist carbachol elicited a transient, La(3+)-sensitive decrease in basolateral [Ca(2+)] (average approximately 250 microM, but as large as 530 microM). Conversely, carbachol evoked an HgCl2-sensitive increase in [Ca(2+)] (average approximately 400 microM, but as large as 520 microM) in the lumen of single gastric glands. Both responses were significantly reduced by pre-treatment with sarco-endoplasmic reticulum Ca(2+) ATPase (SERCA) pump inhibitors or with the intracellular Ca(2+) chelator BAPTA-AM. Immunofluorescence experiments demonstrated an asymmetric localization of plasma membrane Ca(2+) ATPase (PMCA), which appeared to be partially co-localized with CaR and the gastric H(+)/K(+)-ATPase in the apical membrane of the acid-secreting cells. Our data indicate that agonist stimulation results in local fluctuations in [Ca(2+)]ext that would be sufficient to modulate the activity of the CaR on neighboring cells.

We describe a method to isolate epithelial cells from gallbladders of Necturus maculosus with preserved structural and functional polarity. Isolation was carried out with a mixture of collagenase and protease, with only a brief exposure to a divalent-cation-free medium. About 40% of the isolated epithelial cells had a "figure-eight" shape and retained metabolic and cell membrane integrity. Figure-eight cells display features consistent with preserved polarity for several hours, including the following: 1) the "apical" and "basolateral" membrane domains were differentially labeled by a hydrophobic fluorescent dye; 2) freeze fracture electron microscopy verified two plasma membrane domains differing in the presence of microvilli and folds and separated by tight junctions; 3) proteins such as ZO-1, NHE3, and Na(+)-K(+)-ATPase remained localized in the junctional, apical, and basolateral regions, respectively; 4) after apical surface exposure to wheat germ agglutinin, the label remained in the apical membrane after cell isolation; and 5) patch-clamp experiments demonstrated polarized expression of K+ channels. Polarity was rapidly lost after removal of extracellular Ca2+, exposure to trypsin, or ATP depletion. Therefore, this preparation allows for structural and functional studies of epithelial transport in single cells retaining the essential features present in the assembled epithelium.

Following the technical approach described in the preceding publication we have investigated if, and how, stimulation of gastric HCl secretion affects the basolateral ion transport properties of oxyntopeptic cells of Rana catesbeiana stomach. To this end microdissected gastric glands were punctured with conventional or H(+)-sensitive glass microelectrodes and the effects of changing bath ion concentrations on the cell membrane potential (Vb) and cell pH (pHi) were determined. Except for a transient alkalinization, histamine (0.5 mmol/l) did not significantly affect Vb or pHi. The latter averaged 7.18 +/- 0.03 (mean +/- SEM, n = 5) under resting conditions (0.1 mmol/l cimetidine) and 7.21 +/- 0.07 (n = 5) in the presence of histamine. In addition, neither the initial velocity nor the final steady-state value of the cell alkalinization following a 10:1 reduction of bath Cl- concentration changed in the presence of histamine, and the same holds true for the cell acidification following a 10:1 reduction of bath HCO3- concentration. These observations indicate that the basolateral Cl-/HCO3- exchanger was not stimulated by histamine, and that no other base transporters were activated. By contrast, the Vb response to elevation of bath K+ concentration decreased, and so did the initial depolarizing Vb response to bath Cl- substitution, while the secondary hyperpolarizing response increased. The latter observations are compatible with the notion that stimulation by histamine reduced a pH-insensitive part of the basolateral K+ conductance and reduced also the basolateral Cl- conductance.

1. Necturus gastric mucosa secretes Cl- actively across the gastric glands which are composed almost entirely of acid- and enzyme-secreting oxynticopeptic cells. Single channel studies on Necturus oxynticopeptic cells have shown that the basolateral membrane possesses multiple K(+)-selective channels but no observable Cl- channels while the apical membrane has Cl- channels but no observable K+ channels. To relate these channel properties to the conductance of the whole cell we have investigated the macroscopic membrane currents with conventional whole-cell patch-clamp techniques. 2. When bathed in amphibian Ringer solution, gastric oxynticopeptic cells had a membrane resistance of 47.8 +/- 2.8 M omega and a membrane capacitance of 75.5 +/- 2.7 pF (n = 82). This gave a specific membrane resistance of 3260 +/- 160 omega cm2 (n = 82). Reversal potentials of the oxynticopeptic cells were -13.8 +/- 1.2 mV (n = 45) for an intracellular Cl- concentration ([Cl-]i) of 42 mM and were significantly more negative -24.4 +/- 3.1 mV (n = 31, P < 0.001) for [Cl-]i = 22 mM. 3. In the absence of ATP in the pipette solution, there was an 80% reduction of the whole-cell current with a typical half-time (t1/2) of 5 min. The run-down was not observed when the pipette solution contained 4 mM ATP. 4. A slow and voltage-independent inhibition of 80% of the whole-cell currents occurred after addition of NPPB (35 microM). Ba2+ (10 mM) produced a reversible inhibition of 20% of the total current. Together, 35 microM NPPB and 10 mM Ba2+ eliminated 95% of the whole-cell currents. These data suggest that in the resting oxynticopeptic cells Cl- carried the major fraction of the current while K+ ions carried only a small fraction. 5. Total replacement of Cl- in the pipette and bath solution by gluconate- increased the membrane resistance to 751 +/- 104 M omega (n = 53) and shifted the reversal potential to -38.1 +/- 2.8 mV (n = 53). 6. Increasing the bath K+ concentration from 6 to 91 mM activated a current which had a high selectivity for K+ over choline+, Li+, Na+, Rb+ and Cs+ and was independent of Cl-. The activation of this K+ current (IK*) by high external K+ was not seen with ATP-free pipette solution. 7. Ba2+ or Cs+ had a voltage-dependent blocking effect of this inward K+ current. Ouabain (1 mM) or SCH 28080 (200 microM), specific inhibitors of the Na+,K(+)-ATPase and H+,K(+)-ATPase, had no effect.(ABSTRACT TRUNCATED AT 400 WORDS)