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

Lack of Kcnn4 improves mucociliary clearance in muco-obstructive lung disease

Airway mucociliary clearance (MCC) is the main mechanism of lung defense keeping airways free of infection and mucus obstruction. Airway surface liquid volume, ciliary beating, and mucus are central for proper MCC and critically regulated by sodium absorption and anion secretion. Impaired MCC is a key feature of muco-obstructive diseases. The calcium-activated potassium channel KCa.3.1, encoded by Kcnn4, participates in ion secretion, and studies showed that its activation increases Na⁺ absorption in airway epithelia, suggesting that KCa3.1-induced hyperpolarization was sufficient to drive Na⁺ absorption. However, its role in airway epithelium is not fully understood. We aimed to elucidate the role of KCa3.1 in MCC using a genetically engineered mouse. KCa3.1 inhibition reduced Na⁺ absorption in mouse and human airway epithelium. Furthermore, the genetic deletion of Kcnn4 enhanced cilia beating frequency and MCC ex vivo and in vivo. Kcnn4 silencing in the Scnn1b-transgenic mouse (Scnn1b^tg/+), a model of muco-obstructive lung disease triggered by increased epithelial Na⁺ absorption, improved MCC, reduced Na⁺ absorption, and did not change the amount of mucus but did reduce mucus adhesion, neutrophil infiltration, and emphysema. Our data support that KCa3.1 inhibition attenuated muco-obstructive disease in the Scnn1b^tg/+ mice. K⁺ channel modulation may be a therapeutic strategy to treat muco-obstructive lung diseases.

Silencing the calcium-activated potassium channel KCa.3.1 improves mucociliary clearance in muco-obstructive lung disease by decreasing sodium absorption in the airways.

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.

Regulation of Intestinal Glucose Absorption by Ion Channels and Transporters

The absorption of glucose is electrogenic in the small intestinal epithelium. The major route for the transport of dietary glucose from intestinal lumen into enterocytes is the Na⁺/glucose cotransporter (SGLT1), although glucose transporter type 2 (GLUT2) may also play a role. The membrane potential of small intestinal epithelial cells (IEC) is important to regulate the activity of SGLT1. The maintenance of membrane potential mainly depends on the activities of cation channels and transporters. While the importance of SGLT1 in glucose absorption has been systemically studied in detail, little is currently known about the regulation of SGLT1 activity by cation channels and transporters. A growing line of evidence suggests that cytosolic calcium ([Ca²⁺]_cyt) can regulate the absorption of glucose by adjusting GLUT2 and SGLT1. Moreover, the absorption of glucose and homeostasis of Ca²⁺ in IEC are regulated by cation channels and transporters, such as Ca²⁺ channels, K⁺ channels, Na⁺/Ca²⁺ exchangers, and Na⁺/H⁺ exchangers. In this review, we consider the involvement of these cation channels and transporters in the regulation of glucose uptake in the small intestine. Modulation of them may be a potential strategy for the management of obesity and diabetes.

The Epac1 signaling pathway regulates Cl- secretion via modulation of apical KCNN4c channels in diarrhea

The apical membrane of intestinal epithelia expresses intermediate conductance K(+) channel (KCNN4), which provides the driving force for Cl(-) secretion. However, its role in diarrhea and regulation by Epac1 is unknown. Previously we have established that Epac1 upon binding of cAMP activates a PKA-independent mechanism of Cl(-) secretion via stimulation of Rap2-phospholipase Cε-[Ca(2+)]i signaling. Here we report that Epac1 regulates surface expression of KCNN4c channel through its downstream Rap1A-RhoA-Rho-associated kinase (ROCK) signaling pathway for sustained Cl(-) secretion. Depletion of Epac1 protein and apical addition of TRAM-34, a specific KCNN4 inhibitor, significantly abolished cAMP-stimulated Cl(-) secretion and apical K(+) conductance (IK(ap)) in T84WT cells. The current-voltage relationship of basolaterally permeabilized monolayers treated with Epac1 agonist 8-(4-chlorophenylthio)-2'-O- methyladenosine 3',5'-cyclic monophosphate showed the presence of an inwardly rectifying and TRAM-34-sensitive K(+) channel in T84WT cells that was absent in Epac1KDT84 cells. Reconstructed confocal images in Epac1KDT84 cells revealed redistribution of KCNN4c proteins into subapical intracellular compartment, and a biotinylation assay showed ∼83% lower surface expression of KCNN4c proteins compared with T84WT cells. Further investigation revealed that an Epac1 agonist activates Rap1 to facilitate IK(ap). Both RhoA inhibitor (GGTI298) and ROCK inhibitor (H1152) significantly reduced cAMP agonist-stimulated IK(ap), whereas the latter additionally reduced colocalization of KCNN4c with the apical membrane marker wheat germ agglutinin in T84WT cells. In vivo mouse ileal loop experiments showed reduced fluid accumulation by TRAM-34, GGTI298, or H1152 when injected together with cholera toxin into the loop. We conclude that Rap1A-dependent signaling of Epac1 involving RhoA-ROCK is an important regulator of intestinal fluid transport via modulation of apical KCNN4c channels, a finding with potential therapeutic value in diarrheal diseases.

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

Estradiol rapidly induces the translocation and activation of the intermediate conductance calcium activated potassium channel in human eccrine sweat gland cells

BACKGROUND AND AIMS

Steroid hormones target K+ channels as a means of regulating electrolyte and fluid transport. In this study, ion transporter targets of Estradiol (E2) were investigated in the human eccrine sweat gland cell line NCL-SG3.

RESULTS

Whole cell patch-clamp studies revealed E2 (10 nM) rapidly activates a whole cell K+ conductance, which is abolished by clotrimazole (30 microM), an inhibitor of the intermediate conductance calcium activated K+ channel (IKCa). The estrogen receptor (ER) antagonist ICI 182, 780 had no effect on this E2 activated K+ conductance, suggesting an estrogen receptor independent mechanism of activation. Confocal microscopy studies revealed under basal conditions that the IKCa channel is located within the cell cytoplasm and in the presence of E2, rapidly translocates to both the apical and basolateral membrane. In the presence of E2, tyrosine phosphorylation of calmodulin, which is known to regulate trafficking of the IKCa channel, is increased, and treatment of cells with the calmodulin inhibitor trifluoperazine (TFP) prevents the E2-induced translocation.

CONCLUSIONS

Estradiol rapidly regulates a K+ conductance through the IKCa channel in an estrogen receptor independent manner. E2 stimulates the translocation of IKCa to the cell membrane in a calmodulin dependent manner, representing a novel paradigm of estrogen action in sweat gland epithelial cells.

Role of cholinergic-activated KCa1.1 (BK), KCa3.1 (SK4) and KV7.1 (KCNQ1) channels in mouse colonic Cl- secretion

AIM

Colonic crypts are the site of Cl- secretion. Basolateral K+ channels provide the driving force for luminal cystic fibrosis transmembrane regulator-mediated Cl- exit. Relevant colonic epithelial K+ channels are the intermediate conductance Ca2+-activated K(Ca)3.1 (SK4) channel and the cAMP-activated K(V)7.1 (KCNQ1) channel. In addition, big conductance Ca2+-activated K(Ca)1.1 (BK) channels may play a role in Ca2+-activated Cl- secretion. Here we use K(Ca)1.1 and K(Ca)3.1 knock-out mice, and the K(V)7.1 channel inhibitor 293B (10 microm) to investigate the role of K(Ca)1.1, K(Ca)3.1 and K(V)7.1 channels in cholinergic-stimulated Cl- secretion.

METHODS

A Ussing chamber was used to quantify agonist-stimulated increases in short circuit current (Isc) in distal colon. Chloride secretion was activated by bl. forskolin (FSK, 2 microm) followed by bl. carbachol (CCH, 100 microm). Luminal Ba2+ (5 mm) was used to inhibit K(Ca)1.1 channels.

RESULTS

K(Ca)1.1 WT and KO mice displayed identical FSK and CCH-stimulated Isc changes, indicating that K(Ca)1.1 channels are not involved in FSK- and cholinergic-stimulated Cl- secretion. CCH-stimulated DeltaIsc was significantly reduced in K(Ca)3.1 KO mice, underscoring the known relevance of this channel in the activation of Cl- secretion by an intracellular Ca2+ increasing agonist. The residual CCH effect observed in K(Ca)3.1 KO mice suggests that yet another K+ channel is driving the CCH-stimulated Cl- secretion. In the presence of the specific K(V)7.1 channel blocker 293B, the residual CCH effect was abolished.

CONCLUSIONS

This demonstrates that both K(Ca)3.1 and K(V)7.1 channels are activated by cholinergic agonists and drive Cl- secretion. In contrast, K(Ca)1.1 channels are not involved in stimulated electrogenic Cl- secretion.

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.

Molecular diversity and function of voltage-gated (Kv) potassium channels in epithelial cells

Voltage-gated K+ channels belonging to Kv1-9 subfamilies are widely expressed in excitable cells where they play an essential role in membrane hyperpolarization during an action potential and in the propagation of action potentials along the plasma membrane. Early patch clamp studies on epithelial cells revealed the presence of K+ currents with biophysical and pharmacologic properties characteristic of Kv channels expressed in excitable cells. More recently, molecular approaches including PCR and the availability of more selective antibodies directed against Kv alpha and auxiliary subunits, have demonstrated that epithelial cells from various organ systems, express a remarkable diversity Kv channel subunits. Unlike neurons and myocytes however, epithelial cells do not typically generate action potentials or exhibit dynamic changes in membrane potential necessary for activation of Kv alpha subunits. Moreover, the fact that many Kv channels expressed in epithelial cells exhibit inactivation suggest that their activities are relatively transient, making it difficult to ascribe a functional role for these channels in transepithelial electrolyte or nutrient transport. Other proposed functions have included (i) cell migration and wound healing, (ii) cell proliferation and cancer, (iii) apoptosis and (iv) O2 sensing. Certain Kv channels, particularly Kv1 and Kv2 subfamily members, have been shown to be involved in the proliferation of prostate, colon, lung and breast carcinomas. In some instances, a significant increase in Kv channel expression has been correlated with tumorogenesis suggesting the possibility of using these proteins as markers for transformation and perhaps reducing the rate of tumor growth by selectively inhibiting their functional activity.

Cl- secretory effects of EBIO in the rabbit conjunctival epithelium

Experiments were conducted to determine whether the Cl- secretagogue, 1-ethyl-2-benzimidazolinone (EBIO), stimulates Cl- transport in the rabbit conjunctival epithelium. For this study, epithelia were isolated in an Ussing-type chamber under short-circuit conditions. The effects of EBIO on the short-circuit current (I(sc)) and transepithelial resistance (R(t)) were measured under physiological conditions, as well as in experiments with altered electrolyte concentrations. Addition of 0.5 mM EBIO to the apical bath stimulated the control I(sc) by 64% and reduced R(t) by 21% (P < 0.05; paired data). Under Cl(-)-free conditions, I(sc) stimulation using EBIO was markedly attenuated. In the presence of an apical-to-basolateral K+ gradient and permeabilization of the apical membrane, the majority of the I(sc) reflected the transcellular movement of K+ via basolateral K+ channels. Under these conditions, EBIO in combination with A23187 elicited nearly instantaneous 60-90% increases in I(sc) that were sensitive to the calmodulin antagonist calmidazolium and the K+ channel blocker tetraethyl ammonium. In the presence of an apical-to-basolateral Cl- gradient and nystatin permeabilization of the basolateral aspect, EBIO increased the Cl(-)-dependent I(sc), an effect prevented by the channel blocker glibenclamide (0.3 mM). The latter compound also was used to determine the proportion of EBIO-evoked unidirectional 36Cl- fluxes in the presence of the Cl- gradient that traversed the epithelium transcellularly. Overall, EBIO activated apical Cl- channels and basolateral K+ channels (presumably those that are Ca2+ dependent), thereby suggesting that this compound, or related derivatives, may be suitable as topical agents to stimulate fluid transport across the tissue in individuals with lacrimal gland deficiencies.

Metabolic regulation of potassium channels

Potassium (K+) channels exist in all three domains of organisms: eubacteria, archaebacteria, and eukaryotes. In higher animals, these membrane proteins participate in a multitude of critical physiological processes, including food and fluid intake, locomotion, stress response, and cognitive functions. Metabolic regulatory factors such as O2, CO2/pH, redox equivalents, glucose/ATP/ADP, hormones, eicosanoids, cell volume, and electrolytes regulate a diverse group of K+ channels to maintain homeostasis.

Ammonium effects on colonic Cl- secretion: anomalous mole fraction behavior

A significant amount of ammonium (NH4+) is absorbed by the colon. The nature of NH4+ effects on transport and NH4+ transport itself in colonic epithelium is poorly understood. The goal of this study was to elucidate the effects of NH4+ on cAMP-stimulated Cl- secretion in the colonic cell line T84. In HEPES-buffered solutions, application of basolateral NH4+ resulted in a reduced level of Cl- secretory current. The effect of NH4+ appears to occur by at least three mechanisms: 1) basolateral membrane depolarization, 2) a competitive effect with K+, and 3) a long-term (>20 min) increase in transepithelial resistance (TER). The competitive effect with K+ exhibits anomalous mole fraction behavior. Transepithelial current relative to that in 10 mM basolateral K+ was inhibited 15% by 10 mM NH4+ alone and by 30% with a mixture of 2 mM K+ and 8 mM NH4+. A mole fraction mix of 2 mM K+:8 mM NH4+ produced a greater inhibition of basolateral membrane K+ current than pure K+ or NH4+ alone. Similar anomalous behavior was also observed for inhibition of bumetanide-sensitive 36Cl- uptake, e.g., Na+-K+-2Cl- -cotransporter (NKCC-1). No anomalous effect was observed on Na+-K+-ATPase current. Both NKCC-1 and Na+-K+-ATPase activity were elevated in 10 mM NH4+ with respect to 10 mM K+. The effect on TER did not exhibit anomalous mole fraction behavior. The overall effect of basolateral NH4+ on cAMP-stimulated transport is dependent on the [K+]o /[NH4+]o ratio at the basolateral membrane, where o is outside of the cell.

Effects of ammonium on ion channels and transporters in colonic secretory cells

Basolateral ammonium produces an inhibition of Cl- secretion the magnitude of which is dependent on the NH4+ to K+ concentration ratio. Inhibition is maximal at a mole fraction ratio of 0.25 K+ to NH4+. This anomalous mole fraction effect is due to effects on the basolateral K+ channel as well as Na(+)-K(+)-2Cl- cotransporter. However, only Cl- loading, not K+ loading, appears affected in an anomalous mole fraction manner. Transepithelial current is only slightly inhibited relative to equilmolar K+ by NH4+. As in other systems, both Na(+)-K(+)-ATPase and Na(+)-K(+)-2Cl- can act in Na(+)-NH4(+)-ATPase and Na(+)-NH4(+)-2Cl- transport modes. NH4+ conducts through most K+ channels and thus likely through the apical K+ channel present in native crypt cells. This suggests that, similar to the kidney, colonic secretory cells have the capacity to secrete NH4+ when in a K(+)-secreting mode with elevated basolateral NH4+ levels.