Cyclic AMP-dependent regulation of K+ transport in the rat distal colon

Potassium channels in intestinal epithelial cells and their pharmacological modulation: a systematic review

Several potassium channels (KCs) have been described throughout the gastrointestinal tract. Notwithstanding, their contribution to both physiologic and pathophysiologic conditions, as inflammatory bowel disease (IBD), remains underexplored. Therefore, we aim to systematically review, for the first time, the evidence on the characteristics and modulation of KCs in intestinal epithelial cells (IECs). PubMed, Scopus, and Web of Science were searched to identify studies focusing on KCs and their modulation in IECs. The included studies were assessed using a reporting inclusiveness checklist. From the 745 identified records, 73 met the inclusion criteria; their reporting inclusiveness was moderate-high. Some studies described the physiological role of KCs, while others explored their importance in pathological settings. Globally, in IBD animal models, apical K_Ca1.1 channels, responsible for luminal secretion, were upregulated. In human colonocytes, basolateral K_Ca3.1 channels were downregulated. The pharmacological inhibition of K_2P and K_v influenced intestinal barrier function, promoting inflammation. Evidence suggests a strong association between KCs expression and secretory mechanisms in human and animal IECs. Further research is warranted to explore the usefulness of KC pharmacological modulation as a therapeutic target.

Cyclic AMP-Dependent Regulation of Kv7 Voltage-Gated Potassium Channels

Voltage-gated Kv7 potassium channels, encoded by KCNQ genes, have major physiological impacts cardiac myocytes, neurons, epithelial cells, and smooth muscle cells. Cyclic adenosine monophosphate (cAMP), a well-known intracellular secondary messenger, can activate numerous downstream effector proteins, generating downstream signaling pathways that regulate many functions in cells. A role for cAMP in ion channel regulation has been established, and recent findings show that cAMP signaling plays a role in Kv7 channel regulation. Although cAMP signaling is recognized to regulate Kv7 channels, the precise molecular mechanism behind the cAMP-dependent regulation of Kv7 channels is complex. This review will summarize recent research findings that support the mechanisms of cAMP-dependent regulation of Kv7 channels.

K2P TASK-2 and KCNQ1-KCNE3 K+ channels are major players contributing to intestinal anion and fluid secretion

KEY POINTS

K⁺ channels are important in intestinal epithelium as they ensure the ionic homeostasis and electrical potential of epithelial cells during anion and fluid secretion. Intestinal epithelium cAMP-activated anion secretion depends on the activity of the (also cAMP dependent) KCNQ1-KCNE3 K⁺ channel, but the secretory process survives after genetic inactivation of the K⁺ channel in the mouse. Here we use double mutant mice to investigate which alternative K⁺ channels come into action to compensate for the absence of KCNQ1-KCNE3 K⁺ channels. Our data establish that whilst Ca²⁺ -activated K_Ca 3.1 channels are not involved, K_2P two-pore domain TASK-2 K⁺ channels are major players providing an alternative conductance to sustain the intestinal secretory process. Work with double mutant mice lacking both TASK-2 and KCNQ1-KCNE3 channels nevertheless points to yet-unidentified K⁺ channels that contribute to the robustness of the cAMP-activated anion secretion process.

ABSTRACT

Anion and fluid secretion across the intestinal epithelium, a process altered in cystic fibrosis and secretory diarrhoea, is mediated by cAMP-activated CFTR Cl^- channels and requires the simultaneous activity of basolateral K⁺ channels to maintain cellular ionic homeostasis and membrane potential. This function is fulfilled by the cAMP-activated K⁺ channel formed by the association of pore-forming KCNQ1 with its obligatory KCNE3 β-subunit. Studies using mice show sizeable cAMP-activated intestinal anion secretion in the absence of either KCNQ1 or KCNE3 suggesting that an alternative K⁺ conductance must compensate for the loss of KCNQ1-KCNE3 activity. We used double mutant mouse and pharmacological approaches to identify such a conductance. Ca²⁺ -dependent anion secretion can also be supported by Ca²⁺ -dependent K_Ca 3.1 channels after independent CFTR activation, but cAMP-dependent anion secretion is not further decreased in the combined absence of K_Ca 3.1 and KCNQ1-KCNE3 K⁺ channel activity. We show that the K_2P K⁺ channel TASK-2 is expressed in the epithelium of the small and large intestine. Tetrapentylammonium, a TASK-2 inhibitor, abolishes anion secretory current remaining in the absence of KCNQ1-KCNE3 activity. A double mutant mouse lacking both KCNQ1-KCNE3 and TASK-2 showed a much reduced cAMP-mediated anion secretion compared to that observed in the single KCNQ1-KCNE3 deficient mouse. We conclude that KCNQ1-KCNE3 and TASK-2 play major roles in the intestinal anion and fluid secretory phenotype. The persistence of an, admittedly reduced, secretory activity in the absence of these two conductances suggests that further additional K⁺ channel(s) as yet unidentified contribute to the robustness of the intestinal anion secretory process.

Potassium channels in pancreatic duct epithelial cells: their role, function and pathophysiological relevance

Pancreatic ductal epithelial cells play a fundamental role in HCO3 (-) secretion, a process which is essential for maintaining the integrity of the pancreas. Although several studies have implicated impaired HCO3 (-) and fluid secretion as a triggering factor in the development of pancreatitis, the mechanism and regulation of HCO3 (-) secretion is still not completely understood. To date, most studies on the ion transporters that orchestrate ductal HCO3 (-) secretion have focussed on the role of Cl(-)/HCO3 (-) exchangers and Cl(-) channels, whereas much less is known about the role of K(+) channels. However, there is growing evidence that many types of K(+) channels are present in ductal cells where they have an essential role in establishing and maintaining the electrochemical driving force for anion secretion. For this reason, strategies that increase K(+) channel function may help to restore impaired HCO3 (-) and fluid secretion, such as in pancreatitis, and therefore provide novel directions for future pancreatic therapy. In this review, our aims are to summarize the types of K(+) channels found in pancreatic ductal cells and to discuss their individual roles in ductal HCO3 (-) secretion. We will also describe how K(+) channels are involved in pathophysiological conditions and discuss how they could act as new molecular targets for the development of therapeutic approaches to treat pancreatic diseases.

Disruption of the K+ channel beta-subunit KCNE3 reveals an important role in intestinal and tracheal Cl- transport

The KCNE3 beta-subunit constitutively opens outwardly rectifying KCNQ1 (Kv7.1) K(+) channels by abolishing their voltage-dependent gating. The resulting KCNQ1/KCNE3 heteromers display enhanced sensitivity to K(+) channel inhibitors like chromanol 293B. KCNE3 was also suggested to modify biophysical properties of several other K(+) channels, and a mutation in KCNE3 was proposed to underlie forms of human periodic paralysis. To investigate physiological roles of KCNE3, we now disrupted its gene in mice. kcne3(-/-) mice were viable and fertile and displayed neither periodic paralysis nor other obvious skeletal muscle abnormalities. KCNQ1/KCNE3 heteromers are present in basolateral membranes of intestinal and tracheal epithelial cells where they might facilitate transepithelial Cl(-) secretion through basolateral recycling of K(+) ions and by increasing the electrochemical driving force for apical Cl(-) exit. Indeed, cAMP-stimulated electrogenic Cl(-) secretion across tracheal and intestinal epithelia was drastically reduced in kcne3(-/-) mice. Because the abundance and subcellular localization of KCNQ1 was unchanged in kcne3(-/-) mice, the modification of biophysical properties of KCNQ1 by KCNE3 is essential for its role in intestinal and tracheal transport. Further, these results suggest KCNE3 as a potential modifier gene in cystic fibrosis.

Characteristics of Cl- uptake in rat alveolar type I cells

Although Cl- transport in fetal lung is important for fluid secretion and normal lung development, the role of Cl- transport in adult lung is not well understood. In physiological studies, the cystic fibrosis transmembrane regulator (CFTR) plays a role in fluid absorption in the distal air spaces of adult lung, and alveolar type II cells cultured for 5 days have the capacity to transport Cl-. Although both alveolar type I and type II cells express CFTR, it has previously not been known whether type I cells transport Cl-. We studied Cl- uptake in isolated type I cells directly, using either radioisotopic tracers or halide-sensitive fluorescent indicators. By both methods, type I cells take up Cl-. In the presence of beta-adrenergic agonist stimulation, Cl- uptake can be inhibited by CFTR antagonists. Type I cells express both the Cl-/HCO3- anion exchanger AE2 and the voltage-gated Cl- channels CLC5 and CLC2. Inhibitors of AE2 also block Cl- uptake in type I cells. Together, these results demonstrate that type I cells are capable of Cl- uptake and suggest that the effects seen in whole lung studies establishing the importance of Cl- movement in alveolar fluid clearance may be, in part, the result of Cl- transport across type I cells.

Regulation of K+ transport in the rat distal colon via angiotensin II subtype receptors and K+ -pathways

The role of angiotensin subtype-1 (AT1) and -2 (AT2) receptors in mediating the effects of angiotensin II (ANG II) on several K+ transporters was studied in rat distal colon using an Ussing chamber. Angiotensin II induced K+ secretion at two different doses. Secretion occurred at 10-(8) and 10-(4) M, as a result of an increase in serosal-to-mucosal flux (Js-m). The ANG II-induced stimulation of Js-m at a low dose (10-(8) M) was abolished by PD123319 while losartan did not alter the low-dose ANG II-dependent increase in Js-m. In contrast, the increase in Js-m induced by a high-dose of ANG II (10-(4) M) was blocked by losartan, whereas PD123319 partially reduced the stimulatory effect. In the presence of both blockers, high-dose ANG II induced an inhibition of basal Js-m. Low-dose ANG II activated the barium-sensitive K+ channels, whereas the Na+, K+, 2Cl- cotransporter and the Na+, K+ -ATPase pump were unchanged. At the high dose, ANG II activated the barium-sensitive K+ channels and the Na+, K+, 2Cl- cotransporter and inhibited the Na+, K+ -ATPase pump. These data indicate that ANG II stimulates serosal-to-mucosal K+ flux in the rat distal colon at high and low doses via different receptors and K+ transporters.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

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.

Abolition of Ca2+-mediated intestinal anion secretion and increased stool dehydration in mice lacking the intermediate conductance Ca2+-dependent K+ channel Kcnn4

Intestinal fluid secretion is driven by apical membrane, cystic fibrosis transmembrane conductance regulator (CFTR)-mediated efflux of Cl- that is concentrated in cells by basolateral Na(+)-K(+)-2Cl- cotransporters (NKCC1). An absolute requirement for Cl- efflux is the parallel activation of K(+) channels which maintain a membrane potential that sustains apical anion secretion. Both cAMP and Ca(2+) are intracellular signals for intestinal Cl- secretion. The K(+) channel involved in cAMP-dependent secretion has been identified as the KCNQ1-KCNE3 complex, but the identity of the K(+) channel driving Ca(2+)-activated Cl- secretion is controversial. We have now used a Kcnn4 null mouse to show that the intermediate conductance IK1 K(+) channel is necessary and sufficient to support Ca(2+)-dependent Cl- secretion in large and small intestine. Ussing chambers were used to monitor transepithelial potential, resistance and equivalent short-circuit current in colon and jejunum from control and Kcnn4 null mice. Na(+), K(+) and water content of stools was also measured. Distal colon and small intestinal epithelia from Kcnn4 null mice had normal cAMP-dependent Cl- secretory responses. In contrast, they completely lacked Cl- secretion in response to Ca(2+)-mobilizing agonists. Ca(2+)-activated electrogenic K(+) secretion was increased in colon epithelium of mice deficient in the IK1 channel. Na(+) and water content of stools was diminished in IK1-null animals. The use of Kcnn4 null mice has allowed us to demonstrate that IK1 K(+) channels are solely responsible for driving intestinal Ca(2+)-activated Cl- secretion. The absence of this channel leads to a marked reduction in water content in the stools, probably as a consequence of decreased electrolyte and water secretion.

Histamine-induced ion secretion across rat distal colon: involvement of histamine H1 and H2 receptors

The aim of the present study was to investigate the effect of histamine, a product of e.g. mast cells, on short-circuit current (I(sc)) across rat distal colon. Histamine concentration-dependently stimulated an increase in I(sc), which often was preceded by a transient negative current. Neither a release of neurotransmitters nor a release of prostaglandins contributed to the histamine response. The histamine-induced increase in I(sc) was blocked by the histamine H(1) antagonist, pyrilamine, but was resistant against the histamine H(2) antagonist, cimetidine. Conversely, the histamine H(1) agonist, TMPH (2-(3-trifluoromethylphenyl)histamine), exclusively evoked an increase in I(sc), whereas the histamine H(2) agonist, amthamine, evoked only a decrease in I(sc) suggesting that stimulation of different types of histamine receptors is responsible for the two phases of the response evoked by native histamine. Histamine induces the opening of glibenclamide-sensitive Cl(-) channels and of charybdotoxin-sensitive K(+) channels in the apical membrane as demonstrated by experiments at basolaterally depolarized epithelia. A further action site is the basolateral membrane, because histamine stimulates a charybdotoxin- and tetrapentylammonium-sensitive K(+) conductance in this membrane as observed in tissues, in which the apical membrane was permeabilized with an ionophore, nystatin. The increase in I(sc) evoked by histamine was blocked after depletion of intracellular Ca(2+) stores with cyclopiazonic acid and after blockade of inositol 1,4,5-trisphosphate (IP(3)) receptors, suggesting a release of stored Ca(2+). This was confirmed by the observation that the histamine H(1) agonist TMPH induced an increase in the fura-2 ratio signal of epithelial cells within isolated colonic crypts. Consequently, the mediator histamine seems to stimulate both histamine H(1) and H(2) receptors, from which the former seems to be prominently involved in the induction of epithelial chloride secretion.

The KCNQ1 potassium channel: from gene to physiological function

The voltage-gated KCNQ1 (KvLQT1, Kv7.1) potassium channel plays a crucial role in shaping the cardiac action potential as well as in controlling the water and salt homeostasis in several epithelial tissues. KCNQ1 channels in these tissues are tightly regulated by auxiliary proteins and accessory factors, capable of modulating the properties of the channel complexes. This paper reviews the current knowledge about the KCNQ1 channel with a major focus on interacting proteins and physiological functions.

Effects of I(Ks) channel inhibitors in insulin-secreting INS-1 cells

Potassium channels regulate insulin secretion. The closure of K(ATP) channels leads to membrane depolarisation, which triggers Ca(2+) influx and stimulates insulin secretion. The subsequent activation of K(+) channels terminates secretion. We examined whether KCNQ1 channels are expressed in pancreatic beta-cells and analysed their functional role. Using RT/PCR cellular mRNA of KCNQ1 but not of KCNE1 channels was detected in INS-1 cells. Effects of two sulfonamide analogues, 293B and HMR1556, inhibitors of KCNQ1 channels, were examined on voltage-activated outwardly rectifying K(+) currents using the patch-clamp method. It was found that 293B inhibited 60% of whole-cell outward currents induced by voltage pulses from -70 to +50 mV with a concentration for half-maximal inhibition (IC(50)) of 37 microM. The other sulfonamide analogue HMR1556 inhibited 48% of the outward current with an IC(50) of 7 microM. The chromanol 293B had no effect on tolbutamide-sensitive K(ATP) channels. Action potentials induced by current injections were broadened and after-repolarisation was attenuated by 293B. Insulin secretion in the presence but not in the absence of tolbutamide was significantly increased by 293B. These results suggest that 293B- and HMR1556-sensitive channels, probably in concert with other voltage-activated K(+) channels, influence action potential duration and frequency and thus insulin secretion.

Pathways for K+ Efflux in Isolated Surface and Crypt Colonic Cells. Activation by Calcium

K+ -conductive pathways were evaluated in isolated surface and crypt colonic cells, by measuring (86)Rb efflux. In crypt cells, basal K+ efflux (rate constant: 0.24 +/- 0.044 min(-1), span: 24 +/- 1.3%) was inhibited by 30 mM TEA and 5 mM Ba2+ in an additive way, suggesting the existence of two different conductive pathways. Basal efflux was insensitive to apamin, iberiotoxin, charybdotoxin and clotrimazole. Ionomycin (5 microM) stimulated K+ efflux, increasing the rate constant to 0.65 +/- 0.007 min(-1) and the span to 83 +/- 3.2%. Ionomycin-induced K+ efflux was inhibited by clotrimazole (IC(50) of 25 +/- 0.4 microM) and charybdotoxin (IC(50) of 65 +/- 5.0 nM) and was insensitive to TEA, Ba2+, apamin and iberiotoxin, suggesting that this conductive pathway is related to the Ca2+-activated intermediate-conductance K+ channels (IK(ca)). Absence of extracellular Ca2+ did neither affect basal nor ionomycin-induced K+ efflux. However, intracellular Ca2+ depletion totally inhibited the ionomycin-induced K+ efflux, indicating that the activation of these K+ channels mainly depends on intracellular calcium liberation. K+ efflux was stimulated by intracellular Ca(2+) with an EC(50) of 1.1 +/- 0.04 microM. In surface cells, K+ efflux (rate constant: 0.17 +/- 0.027 min(-1); span: 25 +/- 3.4%) was insensitive to TEA and Ba2+. However, ionomycin induced K+ efflux with characteristics identical to that observed in crypt cells. In conclusion, both surface and crypt cells present IK(Ca) channels but only crypt cells have TEA- and Ba2+-sensitive conductive pathways, which would determine their participation in colonic K+ secretion.

Zinc inhibits cAMP-stimulated Cl secretion via basolateral K-channel blockade in rat ileum

Zn, an essential micronutrient and second most abundant trace element in cell and tissues, reduces stool output when administered to children with acute diarrhea. The mechanism by which Zn improves diarrhea is not known but could result from stimulating Na absorption and/or inhibiting anion secretion. The aim of this study was to investigate the direct effect of Zn on intestinal epithelial ion absorption and secretion. Rat ileum was partially stripped of serosal and muscle layers, and the mucosa was mounted in lucite chambers. Potential difference and short-circuit current were measured by conventional current-voltage clamp method. 86Rb efflux and uptake were assessed for serosal K channel and Na-K-2Cl cotransport activity, respectively. Efflux experiments were performed in isolated cells preloaded with 86Rb in the presence of ouabain and bumetanide, whereas uptake experiments were performed in low-Cl isotonic buffer containing Ba and ouabain. Neither mucosal nor serosal Zn affected glucose-stimulated Na absorption. In contrast, forskolin-induced Cl secretion was markedly reduced by serosal but not mucosal addition of Zn. Zn also substantially reversed the increase in Cl secretion induced by 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) with half-maximal inhibitory concentration of 0.43 mM. In contrast, serosal Zn did not alter Cl secretion stimulated by carbachol, a Ca-dependent agonist. Zn inhibited 8-BrcAMP-stimulated 86Rb efflux but not carbachol-stimulated 86Rb efflux. Zn had no effect on bumetanide-sensitive 86Rb uptake, Na-K-ATPase, or CFTR. We conclude from these studies that Zn inhibits cAMP-induced Cl secretion by blocking basolateral membrane K channels.

Cyclic AMP-dependent Cl- secretion induced by thromboxane A2 in isolated human colon

Increased release of thromboxane A(2) (TXA(2)) has been shown to be involved in inflammatory bowel diseases. In the present study, we have investigated the effect of a stable TXA(2) analogue (STA(2)) on the electrical parameters in isolated human colonic mucosa. In the human mucosa set between Ussing chambers, STA(2) stimulated Cl- secretion in a concentration-dependent manner with an EC(50) of 0.06 microm. The STA(2)-induced Cl- secretion was significantly inhibited by ONO-3708 (10 microm), a specific TXA(2) receptor antagonist. The effect of STA(2) (0.3 microm) was independent of the colonic segment from which the tissue was obtained, from caecum to rectum. Chromanol 293B, an inhibitor of the cAMP-dependent KvLQT1 channel, attenuated the STA(2)-induced Cl- secretion in the human colonic mucosa (IC(50) value 1.18 microm). We found that KvLQT1 mRNA and protein were expressed in all the tested segments of the human colon. The STA(2)-induced Cl- secretion was significantly inhibited by 8-bromo-2'-monobutyryladenosine-3',5'-cyclic monophosphorothioate (50 microm), a membrane-permeant cAMP antagonist. STA(2) (0.3 microm) significantly increased the intracellular cAMP levels and the short-circuit current via TXA(2) receptor in a human colonic cell line. These results suggest that the TXA(2)-induced Cl- secretion in the colon is mediated via the cAMP pathway in addition to the Ca(2+)-calmodulin pathway which was previously reported.

Regulation of intestinal secretion involved in the interaction between neurotransmitters and prostaglandin E2

In this short review, it will be described that neurotransmitter-induced secretion in the intestine may be influenced by the tissue level of prostaglandin E2 (PGE2). In the normal condition, vasoactive intestinal polypeptide (VIP) and acetylcholine (ACh) are the predominant neurotransmitters of secretomotor neurones. VIP and ACh activate distinct second messenger systems in epithelial cells, i.e. adenosine 3', 5'-cyclic monophosphate (cAMP) and calcium ion (Ca2+), respectively. An increase in intracellular cAMP induces a small amount of chloride (Cl-) secretion in epithelial cells, while simultaneous increases in intracellular Ca2+ and cAMP greatly enhances the cAMP-induced Cl- secretion. When the concentration of prostaglandins reaches a high level in the intestinal tissue substance P, which is a neurotransmitter of sensory neurones, can also induce a massive Cl- secretion by cross-potentiation of cAMP and Ca2+ in epithelial cells. In conclusion, it is considered that the concentration of tissue PGE2 may indicate tissue alert level, and when this level elevates, PGE2 enhances ACh and SP-induced Cl- secretion, thus mediating massive fluid secretion for host defence.

Active K+ secretion through multiple KCa-type channels and regulation by IKCa channels in rat proximal colon

Colonic K+ secretion stimulated by cholinergic agents requires activation of muscarinic receptors and the release of intracellular Ca2+. However, the precise mechanisms by which this rise in Ca2+ leads to K+ efflux across the apical membrane are poorly understood. In the present study, Northern blot analysis of rat proximal colon revealed the presence of transcripts encoding rSK2 [small conductance (SK)], rSK4 [intermediate conductance (IK)], and rSlo [large conductance (BK)] Ca2+-activated K+ channels. In dietary K+-depleted animals, only rSK4 mRNA was reduced in the colon. On the basis of this observation, a cDNA encoding the K+ channel rSK4 was cloned from a rat colonic cDNA library. Transfection of this cDNA into Chinese hamster ovary (CHO) cells led to the expression of Ca2+-activated K+ channels that were blocked by the IK channel inhibitor clotrimazole (CLT). Confocal immunofluorescence confirmed the presence of IK channels in proximal colonic crypts, and Western blotting demonstrated that IK protein sorted to both the apical and basolateral surfaces of colonic epithelia. In addition, transcellular active K+ secretion was studied on epithelial strips of rat proximal colon using unidirectional 86Rb+ fluxes. The addition of thapsigargin or carbachol to the serosal surface enhanced net 86Rb+ secretion. The mucosal addition of CLT completely inhibited carbachol-induced net 86Rb+ secretion. In contrast, only partial inhibition was observed with the BK and SK channel inhibitors, iberiotoxin and apamin, respectively. Finally, in parallel with the reduction in SK4 message observed in animals deprived of dietary K+, carbachol-induced 86Rb+ secretion was abolished in dietary K+-depleted animals. These results suggest that the rSK4 channel mediates K+ secretion induced by muscarinic agonists in the rat proximal colon and that transcription of the rSK4 channel is downregulated to prevent K+ loss during dietary K+ depletion.

Electrogenic ion transport in mammalian colon involves an ammonia-sensitive apical membrane K+ conductance

It is remarkable that high ammonia concentrations can be present within the colonic lumen without compromising normal epithelial function. We investigated the impact of luminal ammonia on Cl- secretion in native tissue. Stripped human colonic mucosa and unstripped rat distal colon were used. Paired samples were mounted in modified Ussing chambers for electrophysiological studies. In rat distal colon, apical ammonia dose-dependently blocked forskolin-activated short-circuit current with an IC50 to approximately 5 mM. Basolateral NH4Cl was less effective. Luminal methylamine (50 mM), chromanol 293B (10-50 microM), and Ba2+ (5 mM) blocked cAMP-activated short-circuit current but apical clotrimazole (100 microM) was without effect. In stripped human colonic mucosa, luminal but not basolateral NH4Cl (10 mM) and luminal Ba2+ (5 mM) suppressed forskolin-activated short-circuit current. Ammonia may be an endogenous regulator of colonic water and salt secretion. Apical K+ channels may be involved in the regulation of cAMP-stimulated Cl- secretion in mammalian colon.

Beta-adrenergic stimulation of Na(+)-K(+)-2Cl(-) cotransport activity in the rabbit lens

Experimental maneuvers known to increase cellular cAMP levels evoked a stimulation in the K(+) influx across the anterior surfaces of isolated rabbit lenses, as measured by 86Rb(+) uptake. For this, the lenses were mounted in a modified Ussing-type chamber and exposed to the radiolabel under short-circuit conditions. The enhanced, cAMP-elicited flux was attributed to the basolateral Na(+)-K(+)-2Cl(-) cotransporter given its preclusion by bumetanide, a highly selective inhibitor of this symport, and the ineffectiveness of ouabain in mitigating the stimulation. The ouabain- plus bumetanide-insensitive K(+) uptake, which is about 10% of the total influx and represents passive entry of the radiolabel, was not affected by cAMP-elevating conditions. Forskolin, an activator of adenylyl cyclase; epinephrine, a non-selective adrenergic agonist; and the beta-selective agents, isoproterenol and terbutaline, were among the drugs used to elicit the increase in bumetanide-sensitive K(+) inflow. In experiments with isoproterenol, the stimulated influx evoked by the agonist was inhibited in lenses simultaneously exposed to propranolol. Other observations included that the stimulation of bumetanide-sensitive K(+) influx with forskolin was eliminated in lenses pretreated with the protein kinase inhibitors, staurosporine or H-89. However, these drugs were ineffective in preventing the increased influx produced by calyculin A, a phosphatase inhibitor, suggesting modulation of the cotransporter by at least two independent pathways. The cAMP-generating stimuli also produced an inhibition of the short-circuit current across the lens and an increase in translens resistance. These latter effects suggest that cAMP elevation also evokes an inhibition in an epithelial conductance(s) simultaneously to the stimulation of the cotransporter. As such, this study provides the first indication for the regulation of lens transport by adrenoceptors, presumably of the beta-2 subtype.

K(+) channels as therapeutic drug targets

K(+) channels play critical roles in a wide variety of physiological processes, including the regulation of heart rate, muscle contraction, neurotransmitter release, neuronal excitability, insulin secretion, epithelial electrolyte transport, cell volume regulation, and cell proliferation. As such, K(+) channels have been recognized as potential therapeutic drug targets for many years. Unfortunately, progress toward identifying selective K(+) channel modulators has been severely hampered by the need to use native currents and primary cells in the drug-screening process. Today, however, more than 80 K(+) channel and K(+) channel-related genes have been identified, and an understanding of the molecular composition of many important native K(+) currents has begun to emerge. The identification of these molecular K(+) channel drug targets should lead to the discovery of novel drug candidates. A summary of progress is presented.

The flavonol quercetin activates basolateral K(+) channels in rat distal colon epithelium

1. The flavonol quercetin has been shown to activate a Cl(-) secretion in rat colon. Unlike the secretory activity of the related isoflavone genistein, quercetin's secretory activity does not depend on cyclic AMP; instead, it depends on Ca(2+). We investigated the possible involvement of Ca(2+) dependent basolateral K(+) channels using apically permeabilized rat distal colon epithelium mounted in Ussing chambers. 2. In intact epithelium, quercetin induced an increase in short-circuit current (I(sc)), which was diminished by the Cl(-) channel blockers NPPB and DPC, but not by glibenclamide, DIDS or anthracene-9-carboxylic acid. The effect of the flavonol was also inhibited by several serosally applied K(+) channel blockers (Ba(2+), quinine, clotrimazole, tetrapentylammonium, 293B), whereas other K(+) channel blockers failed to influence the quercetin-induced increase in I(sc) (tetraethylammonium, charybdotoxin). 3. The apical membrane was permeabilized by mucosal addition of nystatin and a serosally directed K(+) gradient was applied. The successful permeabilization was confirmed by experiments demonstrating the failure of bumetanide to inhibit the carbachol-induced current. 4. In apically permeabilized epithelium, quercetin induced a K(+) current (I(K)), which was neither influenced by ouabain nor by bumetanide. Whereas DPC, NPPB, charybdotoxin and 293B failed to inhibit this I(K), quinine, Ba(2+), clotrimazole and tetrapentylammonium were effective blockers of this current. 5. We conclude from these results that at least part of the quercetin-induced Cl(-) secretion can be explained by an activation of basolateral K(+) channels.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

The effect of cAMP on electrogenic Na(+) absorption in the rat distal colon

Cyclic AMP is a ubiquitous second messenger produced in cells in response to extracellular stimulants. The aim of this study was to examine its role in the regulation of amiloride-sensitive electrogenic Na(+) absorption in the rat distal colon by measuring the short-circuit current (I(sc)) and (22)Na(+) fluxes in a chamber-mounted mucosal sheet. Forskolin stimulated the amiloride-inhibitable I(sc) and amiloride-inhibitable (22)Na(+) absorption. 8Br-cAMP also stimulated the amiloride-inhibitable I(sc). Furthermore, isoproterenol, acting via beta-adrenergic activation, stimulated the amiloride-inhibitable I(sc). The isoproterenol-induced increase in the amiloride-sensitive I(sc) was largely suppressed by H89, a protein kinase A inhibitor. In conclusion, cAMP can upregulate amiloride-sensitive electrogenic Na(+) absorption in the rat distal colon.

Cell volume-induced changes in K+ transport across the rat colon

The effect of cell swelling and cell shrinkage on K+ transport across the rat colonic epithelium was studied by measuring unidirectional fluxes, uptake and efflux of 86Rb+, a marker for K+. Exposure to a hypotonic medium stimulated the secretory, serosa-to-mucosa flux of K+, whereas exposure to a hypertonic medium inhibited the absorptive, mucosa-to-serosa flux of K+ in the distal, but not in the proximal colon. Neither manoeuvre had any effect on the uptake of K+ across the apical or the basolateral membrane. Cell swelling induced a sustained increase in the apical and basolateral K+ efflux from both colonic segments, whereas cell shrinkage reduced the efflux. Ba2+ (10(-2) mol l(-1)) inhibited the swelling-induced stimulation of the apical, quinine (10(-3) mol l(-1)) that of the basolateral K+ efflux in the distal colon. Incubation of the tissue in Ca2+-free buffer or La3+, which blocks Ca2+-influx into the epithelium, strongly reduced the basal K+ efflux across the basolateral membrane. The same was observed with brefeldin A, a blocker of the transport of newly synthesized proteins out of the endoplasmatic reticulum. Swelling-induced K+ efflux, however, was not reduced. In the presence of colchicine, an inhibitor of the polymerization of microtubules, swelling evoked only a transient increase in mucosal efflux, which, especially in the proximal colon, fell after 6 min to the level of the isotonic control period. These results demonstrate that the cell volume is involved in the regulation of transepithelial K+ transport across the rat colonic epithelium and suggest a role of the cytoskeleton in the control of a part of the volume-sensitive K+ channels.

Expression and function of colonic epithelial KvLQT1 K+ channels

1. KvLQT1 (KCNQ1) is a voltage-gated K+ channel essential for repolarization of the heart action potential. Defects in ion channels have been demonstrated in cardiac arrhythmia. This channel is inhibited potently by the chromanol 293B. The same compound has been shown to block cAMP-dependent electrolyte secretion in rat and human colon. Therefore, it was suggested that a K+ channel similar to KvLQT1 is expressed in the colonic epithelium. 2. In the present paper, expression of KvLQT1 and its function in colonic epithelial cells is described. Reverse transcription-polymerase chain reaction analysis of rat colonic mucosa demonstrated expression of KvLQT1 in both crypt cells and surface epithelium. When expressed in Xenopus oocytes, KvLQT1 induced a typical delayed activated K+ current. 3. As demonstrated, the channel activity could be further activated by increases in intracellular cAMP. These and other data support the concept that KvLQT1 is forming a component of the basolateral cAMP-activated K+ conductance in the colonic epithelium.

Adrenoceptor-mediated secretion across the rat colonic epithelium

Norepinephrine evoked a biphasic change in short-circuit current (Isc) across the proximal and distal colon of the rat. The (1) phase of the current response consisted of a transient increase, which was followed by a long-lasting decrease during the (2) phase. The (1) phase, which is assumed to represent Cl(-) secretion, was resistant against classical adrenoceptor antagonists, but was inhibited by the beta(3)-adrenoceptor antagonist 3-(2-ethylpenoxy)-1-[(1S-1,2,3, 4-tetrahydronaphth-1-ylaminol-(2S)-propranol oxalate (SR 59230A) in the proximal colon and by the non-selective beta-adrenoceptor antagonist bupranolol in both colonic segments. Vice versa, the increase in Isc was mimicked by the beta(3)-adrenoceptor agonist, (R*, R*)-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]pheno xyacetic acid (BRL 37344). The (2) phase of the norepinephrine-induced Isc, which is assumed to represent K(+) secretion, was inhibited by yohimbine in the proximal colon, suggesting the mediation by alpha(2)-adrenoceptors, whereas in the distal colon, both alpha- and beta-adrenoceptors are involved, as shown by the sensitivity against, e.g. phentolamine and propranolol. These adrenoceptors seem to be located - at least in part - at extraepithelial sites because the (1) phase of the norepinephrine response was sensitive to indomethacin, and the (2) phase, both to indomethacin and tetrodotoxin.

Multiple action sites of flufenamate on ion transport across the rat distal colon

The antisecretory effects of flufenamate in the rat distal colon were investigated with the Ussing-chamber and the patch-clamp method as well as by measurements of the intracellular Ca(2+) concentration using fura-2-loaded isolated crypts. Flufenamate (5.10(-4) mol l(-1)) suppressed the short-circuit current (Isc) induced by carbachol (5.10(-5) mol l(-1)), forskolin (5.10(-6) mol l(-1)) and the Isc induced by the membrane-permeable analogue of cyclic AMP, CPT - cyclic AMP (10(-4) mol l(-1)). Indomethacin (10(-6) - 10(-4) mol l(-1)) did not mimic the effect of flufenamate, indicating that the antisecretory effect of flufenamate is not related to the inhibition of the cyclo-oxygenase. When the basolateral membrane was depolarized by a high K(+) concentration and a Cl(-) current was induced by a mucosally directed Cl(-) gradient, the forskolin-stimulated Cl(-) current was blocked by flufenamate, indicating an inhibition of the cyclic AMP-stimulated apical Cl(-) conductance. When the apical membrane was permeabilized by the ionophore, nystatin, flufenamate decreased the basolateral K(+) conductance and inhibited the Na(+) - K(+)-ATPase. Patch-clamp experiments revealed a variable effect of flufenamate on membrane currents. In seven out of 11 crypt cells the drug induced an increase of the K(+) current, whereas in the remaining four cells an inhibition was observed. Experiments with fura-2-loaded isolated crypts indicated that flufenamate increased the basal as well as the carbachol-stimulated intracellular Ca(2+) concentration. These results demonstrate that flufenamate possesses multiple action sites in the rat colon: The apical Cl(-) conductance, basolateral K(+) conductances and the Na(+) - K(+)-ATPase.

A constitutively open potassium channel formed by KCNQ1 and KCNE3

Mutations in all four known KCNQ potassium channel alpha-subunit genes lead to human diseases. KCNQ1 (KvLQT1) interacts with the beta-subunit KCNE1 (IsK, minK) to form the slow, depolarization-activated potassium current I(Ks) that is affected in some forms of cardiac arrhythmia. Here we show that the novel beta-subunit KCNE3 markedly changes KCNQ1 properties to yield currents that are nearly instantaneous and depend linearly on voltage. It also suppresses the currents of KCNQ4 and HERG potassium channels. In the intestine, KCNQ1 and KCNE3 messenger RNAs colocalized in crypt cells. This localization and the pharmacology, voltage-dependence and stimulation by cyclic AMP of KCNQ1/KCNE3 currents indicate that these proteins may assemble to form the potassium channel that is important for cyclic AMP-stimulated intestinal chloride secretion and that is involved in secretory diarrhoea and cystic fibrosis.

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.

Activation of Ca(2+)- and cAMP-sensitive K(+) channels in murine colonic epithelia by 1-ethyl-2-benzimidazolone

1-Ethyl-2-benzimidazolone (EBIO) caused a sustained increase in electrogenic Cl(-) secretion in isolated mouse colon mucosae, an effect reduced by blocking basolateral K(+) channels. The Ca(2+)-sensitive K(+) channel blocker charybdotoxin (ChTX) and the cAMP-sensitive K(+) channel blocker 293B were more effective when the other had been added first, suggesting that both types of K(+) channel were activated. EBIO did not cause Cl(-) secretion in cystic fibrosis (CF) colonic epithelia. In apically permeabilized colonic mucosae, EBIO increased the K(+) current when a concentration gradient was imposed, an effect that was completely sensitive to ChTX. No current sensitive to trans-6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2, 2-dimethylchromane (293B) was found in this condition. However, the presence of basolateral cAMP-sensitive K(+) channels was demonstrated by the development of a 293B-sensitive K(+) current after cAMP application in permeabilized mucosae. In isolated colonic crypts EBIO increased cAMP content but had no effect on intracellular Ca(2+). It is concluded that EBIO stimulates Cl(-) secretion by activating Ca(2+)-sensitive and cAMP-sensitive K(+) channels, thereby hyperpolarizing the apical membrane, which increases the electrical gradient for Cl(-) efflux through the CF transmembrane conductance regulator (CFTR). CFTR is also activated by the accumulation of cAMP as well as by direct activation.

The protein tyrosine kinase pathway is not involved in the regulation of K+ transport across the rat colon

The protein tyrosine kinase inhibitor, genistein, is known to activate the cystic fibrosis transmembrane regulator (CFTR) Cl- channel and to inhibit K+ currents across the rat colonic epithelium. The aim of the present study is to answer the question whether these effects are involved in the regulation of transepithelial K+ transport. Therefore, the action of genistein on K+ transport in rat proximal and distal colon was studied by measuring unidirectional fluxes, uptake and efflux of Rb+ in mucosa-submucosa preparations. All effects of genistein (5 x 10(-5) mol L(-1)) were tested in the presence of a low concentration of forskolin (2 x 10(-7) mol L(-1)), because prestimulation of the cAMP pathway has been shown to be a prerequisite for a secretory action of genistein. Forskolin caused an increase in the serosa-to-mucosa flux of Rb+ (J(Rb)sm) thereby stimulating net K+ secretion in the proximal and distal colon. None of these effects was further enhanced after administration of genistein. Neither mucosal uptake of Rb+, representing mainly the activity of the H+-K+-ATPase in the distal colon, nor serosal Rb+ uptake, representing, e.g. the activity of the Na+-K+-2Cl- cotransporter, were affected by genistein. Also the efflux of Rb+ across the apical or the basolateral membrane, an indicator for the apical and basolateral K+ conductance, was unchanged in the presence of genistein. These results demonstrate that the K+ channels inhibited by genistein are not involved in transepithelial K+ transport.

Nitric oxide inhibits potassium transport in the rat distal colon

The effect of the nitric oxide (NO) pathway on K+ (measured using 86Rb) transport in adult rat distal colon was investigated in muscle-stripped segments of colons mounted in Ussing chambers. When added to the mucosal solution, the endogenous precursor of NO, L-arginine (30 mM), inhibited both mucosal-to-serosal and serosal-to-mucosal 86Rb fluxes and caused a prolonged decrease of short-circuit current (Isc). This effect was significantly reduced by the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) but not by D-NAME. Mucosal application of S-nitroso-N-acetyl-penicillamine (SNAP) inhibited mucosal-to-serosal 86Rb flux without affecting serosal-to-mucosal transport. Serosal addition of two different exogenous NO donors, sodium nitroprusside (0.1 mM) and SNAP (0.2 mM), decreased serosal-to-mucosal 86Rb flux, whereas Isc increased. The SNAP-induced decrease in 86Rb flux was abolished by 1H-(1,2,4)oxodiazolo(4,3-a)quinoxalin-1-one (0.2 mM), a selective inhibitor of NO-stimulated soluble guanylyl cyclase, and by methylene blue (0.01 mM). Addition of 8-bromo-cGMP (2 x 10(-4) M) in the presence of an inhibitor of cGMP-specific phosphodiesterase mimicked the effects of NO-donating compounds. This study provides evidence that NO inhibits K+ transport in the rat distal colon via a cGMP-dependent pathway. The effect on net K+ transport may depend on the side of NO action.

Mechanisms of carbachol-induced alterations in K+ transport across the rat colon

The effect of carbachol, an agonist of the Ca2+ pathway, on K+ transport in rat proximal and distal colon was studied by measuring unidirectional fluxes, uptake, and efflux of Rb+, a marker for K+, in mucosa-submucosa preparations. Unidirectional ion flux measurements revealed that carbachol stimulated K+ secretion in the proximal colon by a marked increase in the serosa-to-mucosa flux (J(Rb)sm) and a more moderate rise in the mucosa-to-serosa flux (J(Rb)ms). In the distal colon carbachol had no effect on J(Rb)ms but J(Rb)sm was reduced after a transient increase finally resulting in an inhibition of K+ secretion. Carbachol caused a stimulation of mucosal Rb+ uptake in the distal colon, which was diminished in the presence of inhibitors of the apical H+-K+-ATPase, vanadate and ouabain. In contrast, in the proximal colon the serosal Rb+ uptake was enhanced by carbachol, an effect, which could be prevented by bumetanide, an inhibitor of the basolateral Na+-K+-2Cl(-)-cotransporter. Efflux experiments revealed that carbachol caused a transient increase of apical and basolateral Rb+ permeability in both colonic segments. In the distal colon, stimulated K+ efflux to the serosal side was reduced by quinine, efflux to the mucosal side was blocked by tetraethylammonium. In the proximal colon, carbachol-activated apical and basolateral K+ efflux were inhibited by Ba2+. In conclusion, these data suggest that in the distal colon carbachol stimulates the H+-K+-ATPase and the basolateral K+ efflux through quinine-sensitive K+ channels, whereas in the proximal colon carbachol induces K+ secretion due to a stimulation of the basolateral Na+-K+-2Cl(-)-cotransporter and an increased efflux to the luminal side via Ba2+-sensitive apical K+ channels.

Segment-specific effects of epinephrine on ion transport in the colon of the rat

The effect of epinephrine on transport of K+, Na+, Cl-, and HCO-3 across the rat colon was studied using the Ussing chamber technique. Epinephrine (5 x 10(-6) mol/l) induced a biphasic change in short-circuit current (Isc) in distal and proximal colon: a transient increase followed by a long-lasting decay. The first phase of the Isc response was abolished in Cl--poor solution or after bumetanide administration, indicating a transient induction of Cl- secretion. The second phase of the response to epinephrine was suppressed by apical administration of the K+ channel blocker, quinine, and was concomitant with an increase in serosal-to-mucosal Rb+ flux, indicating that epinephrine induced K+ secretion, although this response was much smaller than the change in Isc. In addition, the distal colon displayed a decrease in mucosal-to-serosal and serosal-to-mucosal Cl- fluxes when treated with epinephrine. In the distal colon, indomethacin abolished the first phase of the epinephrine effect, whereas the second phase was suppressed by TTX. In the proximal colon, indomethacin and TTX were ineffective. The neuronally mediated response to epinephrine in the distal colon was suppressed by the nonselective beta-receptor blocker, propranolol, and by the beta2-selective blocker, ICI-118551, whereas the epithelial response in the proximal colon was suppressed by the nonselective alpha-blocker, phentolamine, and by the selective alpha2-blocker, yohimbine. These results indicate a segment-specific action of epinephrine on ion transport: a direct stimulatory action on epithelial alpha2-receptors in the proximal colon and an indirect action on secretomotoneurons via beta2-receptors in the distal colon.

Beta-adrenergic stimulation of cellular K+ uptake in rat distal colon

We recently demonstrated that the ratio between colonic K+ absorptive and K+ secretive pathways was higher in infant than in adult rats. To test the hypothesis that hormones selectively affect these pathways during ontogeny we examined the effect of adrenergic agonists on cellular K+ uptake in distal colon from infant (10-day-old) and adult (50-day-old) rats. Here we describe that adrenaline (10(-5) M) increased total and ouabain-insensitive 86Rb uptake in both age groups, but it did not affect ouabain-sensitive 86Rb uptake. This stimulation was more pronounced in adult than in infant rats. The effect of adrenaline was mediated via beta-adrenergic receptors. Incubation in vitro with beta-agonist, isoproterenol, stimulated SCH-28080-sensitive, i.e. H+, K(+)-ATPase-dependent, 86Rb uptake in adult but not in infant rats. The threshold dose of beta-agonist was at 10(-7) M, and the maximal activation was observed at 10(-5) M. In vivo inhibition of beta-adrenergic system with propranolol caused a significant decrease in H+, K(+)-ATPase-dependent 86Rb uptake in infant but not in adult colon. In conclusion, this study suggests that the higher colonic K+ absorption in infant rats may be as a result of a selective beta-adrenergic up-regulation leading to stimulation of the apical H+, K(+)-ATPase.

K+ and Cl- conductances in the distal colon of the rat

1. K+ and Cl- conductances and their putative regulation have been characterized in the rat colonic epithelium by Ussing-chamber experiments, whole-cell and single-channel patch-clamp recordings. 2. The apical Cl- conductance is under the control of intracellular cAMP. An increase in the concentration of this second messenger induces transepithelial Cl- secretion due to the activation of an apical 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB)- and glibenclamide-sensitive Cl- conductance. 3. In addition to the apical Cl- conductance, the basolateral membrane is equipped with Cl- channels. They are stimulated by cell swelling and play a role in cell volume regulation and transepithelial Cl- absorption. 4. The basolateral K+ conductance is under the dominant control of intracellular Ca2+. An increase in the cytosolic Ca2+ concentration leads to the opening of basolateral K+ channels, which causes a hyperpolarization of the cell membrane, indirectly supporting Cl- secretion owing to an increase in the driving force for Cl- exit. The predominant effect of cAMP on the basolateral K+ conductance is an inhibitory one, probably due to a decrease in the intracellular Ca2+ concentration. 5. The apical K+ conductance, which is involved in transepithelial K+ secretion, is stimulated by an increase in the intracellular Ca2+ concentration. 6. The differential regulation of apical and basolateral ion conductances in the epithelium of the rat distal colon provides an interesting example for the mechanisms underlying vectorial transport of ions across polarized cells.

Importance of basolateral K+ conductance in maintaining Cl- secretion in murine nasal and colonic epithelia

1. Epithelia lining the nasal passages and descending colon of wild-type and cystic fibrosis (CF) mice were examined by the short-circuit current technique. Additionally, intracellular Ca2+ ion determinations were made in nasal epithelial cells. Forskolin produced anion secretory currents in wild-type and CF nasal epithelia. It produced similar effects in wild-type colonic epithelia, but not in colonic epithelia from CF mice. 2. After electrogenic Na+ transport was blocked with amiloride and electrogenic Cl- secretion was stimulated with forskolin, the ability of K+ channel blockers to inhibit the forskolin-induced Cl- current was determined. The order of efficiency for nasal epithelium was: Ba2+ > clofilium >>> TEA = azimilide >>> trans-6-cyano-4-(N-ethylsulphonyl-N-methylamino)-3-hydroxy-2, 2-dimethyl-chromane (293B) = charybdotoxin, whereas for the colonic epithelium the order was: Ba2+ = 293B >>> azimilide = TEA >>> clofilium = charybdotoxin. 3. 1-Ethyl-2-benzimdazolinone (1-EBIO) was able to generate large Cl--secretory currents in colonic epithelia which were partially sensitive to charybdotoxin, with the remaining current being inhibited by 293B. In nasal epithelia 1-EBIO produced only a small transient effect on current. 4. Forskolin released intracellular Ca2+ in nasal epithelial cells; this activity was attenuated when more powerful Ca2+-releasing agents were applied first. 5. It is concluded that an action on basolateral cAMP-sensitive K+ channels is an important determinant of the maintained responses to forskolin in nasal and colonic epithelia, in addition to the effects on the cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane. In CF nasal epithelia the activation of calcium-activated chloride channels (CACs) substitutes for the effect on CFTR. On the basis of the different orders of potency of the blocking agents and the differential response to 1-EBIO it is concluded that the cAMP-sensitive K+ channels are different in the airways and the gut.

Inhibition of a K+ conductance by the phosphatase inhibitor calyculin A in rat distal colon

Basal membrane permeability of epithelial cells from the lower third and the middle of rat colonic crypts is dominated by a K+ conductance as shown by ion replacement experiments. Calyculin A, an inhibitor of protein phosphatases, induced a depolarization of these cells. The depolarization was concomitant with an inhibition of membrane current. The current inhibited by calyculin A had a reversal potential identical with the theoretical K+ equilibrium potential indicating that the drug inhibits a basal K+ conductance. The efficiency of calyculin A was comparable with that of other well-known K+ channel blockers such as Ba2+, tetraethylammonium or quinine. In the intact tissue, calyculin A exerted an inhibitory action on forskolin-induced anion secretion, an effect which may be explained by the decrease in the driving force for Cl- exit after inhibition of cellular K+ conductance. Together with previous results, these data suggest an inhibition of epithelial K+ conductance by phosphorylation.

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.

Nonionic diffusion of short-chain fatty acids across rat colon

Short-chain fatty acid (SCFA) transport across the colon may occur by nonionic diffusion and/or via apical membrane SCFA-/HCO3- exchange. To examine the relative importance of these processes, stripped segments of rat (Ratus ratus) proximal and distal colon were studied in Ussing chambers, and the unidirectional fluxes of radiolabeled SCFA butyrate, propionate, or weakly metabolized isobutyrate were measured. In N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) or 1 or 5 mM HCO3- Ringer, decreases in mucosal pH stimulated mucosal-to-serosal flux (Jm-->s) of all SCFA, decreases in serosal pH stimulated serosal-to-mucosal flux (Js-->m), and bilateral pH decreases stimulated both fluxes equally. These effects were observed whether the SCFA was present on one or both sides of the tissue, in both proximal and distal colon, in the absence of luminal Na+, and in the presence of either luminal or serosal ouabain. Changes in intracellular pH or intracellular [HCO3-] did not account for the effects of extracellular pH. Luminal Cl- removal, to evaluate the role of apical membrane Cl-/SCFA- exchange, had no effect on Jm-->s but decreased Js-->m 32% at pH 6.5 and 22% at 7.2. Increasing SCFA concentration from 1 to 100 mM, at pH 6.4 or 7.4, caused a linear increase in Jm-->s. We conclude that SCFA are mainly transported across the rat colon by nonionic diffusion.

Regulation of apical and basolateral K+ conductances in rat colon

1. Apical administration of an ionophore, nystatin, and basolateral depolarization by K+ were used to investigate the regulation of apical and basolateral electrogenic transport pathways for K+ in the rat proximal and distal colon. 2. Administration of nystatin (100 micrograms ml-1 at the mucosal side), in the presence of Na+ and in the presence of a serosally directed K+ gradient, stimulate a large increase in short-circuit current (ISC) and tissue conductance in both colonic segments. This response was composed of a pump current generated by the Na(+)-K(+)-ATPase and of a current cross a quinine-sensitive basolateral K+ conductance. 3. The pump current, measured as Na(+)-dependent or scilliroside-sensitive current in the absence of a K+ gradient, was significantly greater in the distal than in the proximal colon. The pump current was unaltered by pretreatment of the tissue with forskolin (5 x 10(-6) mol 1(-1)). 4. The current across the basolateral K+ conductance, measured as current in the presence of a serosally directed K+ gradient either in the absence of Na+ or in the presence of scilliroside, was increased by the cholinoreceptor agonist, carbachol (5 x 10(-5) mol 1(-1)), but inhibited by forskolin (5 x 10(-6) mol 1(-1)). 5. Basolateral K+ depolarization induced a negative ISC in both colonic segments, which was inhibited by the K+ channel blocker quinine (10(-3) mol 1(-1)) at the mucosal side), but was resistant to tetraethylammonium (5 x 10(-3) mol 1(-1) at the mucosal side). This K+ current across an apical K+ conductance was stimulated in both colonic segments by carbachol, whereas forskolin had no effect, although control experiments revealed that forskolin was still able to open an apical Cl- conductance under these conditions. 6. These results demonstrate that an increase in intracellular Ca2+ concentration induced by carbachol causes an increase in the basolateral and the apical K+ conductance, thereby inducing K+ secretion in parallel with an indirect support of Cl- secretion due to the hyperpolarization of the cell membrane. In contrast, the dominating effect of an increase in the intracellular cyclic AMP concentration is inhibition of a basolateral K+ conductance; a mechanism which might contribute to the inhibition of K+ absorption.