External Ni2 + and ENaC in A6 cells: Na+ current stimulation by competition at a binding site for amiloride and Na+

In cultured A6 monolayers from distal Xenopus kidney, external Ni2+ stimulated active Na+ uptake via the epithelial Na+ channel, ENaC. Transepithelial capacitance measurements ruled out exocytosis of ENaC-containing vesicles underlying the Ni2+ effect. Na+ current noise analysis was performed using the neutral Na(+) -channel blocker 6-chloro-3,5-diamino-pyrazine-2-carboxamide (CDPC) and amiloride. The analysis of CDPC-induced noise in terms of a three-state channel model revealed that Ni2+ elicits an increase in the number of open channels as well as in the spontaneous open probability. While Ni2+ had no influence on CDPC-blocker kinetics, the macroscopic and microscopic blocking kinetics of amiloride were affected. Ni2+ turned out to compete with amiloride for a putative binding site but not with CDPC. Moreover, external Na(+)--known to compete with amiloride and so producing the "self-inhibition" phenomenon--and Ni2+ exerted mutually exclusive analogous effects on amiloride kinetics. Na+ current kinetics revealed that Ni2+ prevents ENaC to be downregulated by self-inhibition. Co2+ behaved similarly to Ni2+, whereas Zn2+ did not. Attempts to disclose the chemical nature of the site reacting with Ni2+ suggested cysteine but not histidine as reaction partner.

Effects of extracellular Mg2+ on transepithelial capacitance and Na+ transport in A6 cells under different osmotic conditions

The electrophysiological characteristics of monolayers of cultured renal epithelial A6 cells were studied under short-circuit conditions. Replacing basolateral isosmotic (260 mOsm/kg H2O) media by hyposmotic (140 mOsm/kg H2O) solutions transiently increased the transepithelial capacitance (C(T)) by 57.3+/-2.3% after 16 min. The transepithelial Na+ current (I(Na)) increased concomitantly from 4.2+/-0.7 to 26.1+/-2.6 microA/cm2 with a time course that was noticeably slower, reaching its maximum after 60 min of hypotonicity. The transepithelial conductance (G(T)) increased synchronously with I(Na). Analysis of blocker-induced noise in I(Na), using the amiloride analogue 6-chloro-3,5-diaminopyrazine-2-carboxamide (CDPC), showed that the hypotonic shock increased Na+ channel density (N(T)) at the apical border. The presence of 10 mM Mg2+ on both sides of the epithelium suppressed the hypotonicity-induced C(T) increase to 14.3+/-0.5%, whereas the I(Na) increase was even larger than without Mg2+. Both effects of Mg2+ were located at an extracellular, basolateral site, because apical administration was without effect, whereas the acute basolateral addition of Mg2+ at the moment of the hypotonic shock was sufficient. Interaction between Mg2+ and Ca2+ influenced the behaviour of C(T). At constant osmolality (200 mOsm/kg H2O) 10 mM Mg2+ increased I(Na), leaving C(T) unaffected, whereas 10 mM Ca2+ stimulated both I(Na) and CT. In the presence of 1 mM Mg2+, however, the Ca(2+)-induced CT increase was abolished. The failure of CT to increase during stimulation of I(Na) by Mg2+ suggests that the divalent cation activates pre-existing channels in the apical membrane. Noise analysis showed that the natriferic effects of Mg2+ were also mediated by an increase in NT. The moderate initial increase in CT in the presence of Mg2+ under hypotonic conditions, occurring in parallel with increases in GT and I(Na), reflects most likely Na+ channel insertion induced by the hypotonic treatment. However, the large, transient, Mg(2+)-sensitive increase in CT, not correlated with increases in GT and I(Na), seems to be unrelated to Na+ channel recruitment.

Structure and regulation of insect plasma membrane H(+)V-ATPase

H(+) V-ATPases (V-ATPases) are found in two principal locations, in endomembranes and in plasma membranes. The plasma membrane V-ATPase from the midgut of larval Manduca sexta is the sole energizer of all transepithelial secondary transport processes. At least two properties make the lepidopteran midgut a model tissue for studies of general aspects of V-ATPases. First, it is a rich source for purification of the enzyme and therefore for structural studies: 20 larvae provide up to 0.5 mg of holoenzyme, and soluble, cytosolic V(1) complexes can be obtained in even greater amounts of up to 2 mg. Second, midgut ion-tranport processes are strictly controlled by the regulation of the V-ATPase, which is the sole energizer of all ion transport in this epithelium. Recent advances in our understanding the structure of the V(1) and V(o) complexes and of the regulation of the enzyme's biosynthesis and ion-transport activity will be discussed.

Intracellular pH shifts in cultured kidney (A6) cells: effects on apical Na+ transport

We report, for the epithelial Na+ channel (ENaC) in A6 cells, the modulation by cell pH (pHc) of the transepithelial Na+ current (INa), the current through the individual Na+ channel (i), the open Na+ channel density (No), and the kinetic parameters of the relationship between I(Na) and the apical Na+ concentration. The i and N) were evaluated from the Lorentzian INa noise induced by the apical Na+ channel blocker 6-chloro-3, 5-diaminopyrazine-2-carboxamide. pHc shifts were induced, under strict and volume-controlled experimental conditions, by apical/basolateral NH4Cl pulses or basolateral arrest of the Na+/H+ exchanger (Na+ removal; block by ethylisopropylamiloride) and were measured with the pH-sensitive probe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. The changes in pHc were positively correlated to changes in INa and the apically dominated transepithelial conductance. The sole pHc-sensitive parameter underlying INa was No. Only the saturation value of the INa kinetics was subject to changes in pHc. pHc-dependent changes in No may be caused by influencing Po, the ENaC open probability, or/and the total channel number, NT = No/Po.

Secretory apical Cl- channels in A6 cells: possible control by cell Ca2+ and cAMP

Distal kidney cells (A6) from Xenopus laevis were cultured to confluency on porous supports. Tissues were mounted in Ussing-type chambers to measure short-circuit current (Isc), transepithelial conductance and capacitance, and to analyse the fluctuation in Isc. In the absence of apical NaCl, but with normal basolateral NaCl Ringer's solution, extracellular addition of ATP, oxytocin, a membrane-permeant cAMP derivative, and forskolin produced a transient increase of the electrical parameters. Noise analysis revealed a spontaneous Lorentzian component. All responses depend strictly on the presence of basolateral Cl- and are caused by the activation of an apical (CFTR type) Cl- permeability. Repetitive treatment with ATP (or oxytocin) resulted in refractoriness. ATP and oxytocin acted antagonistically, whereas cAMP and ATP had additive effects. Incubation with the vesicular Ca2+ pump inhibitor thapsigargin or application of the Ca2+ channel blocker nifedipine elicited finite but variable Cl- channel activity. After treatment with nifedipine or thapsigargin, the response to oxytocin was severely impaired. We speculate that not only cAMP but also cell Ca2+ plays a crucial role in the activation of CFTR in A6. Ca2+ may be multifunctional but the rise in capacitance (apical area) observed with all stimulants strongly suggests its involvement in, and contribution to, exocytosis in the process of the CFTR-mediated transcellular Cl- movements.

Apical Cl- channels in A6 cells

Short-circuit current (Isc), transepithelial conductance (Gt), electrical capacitance (CT) and the fluctuation in Isc were analyzed in polarized epithelial cells from the distal nephron of Xenopus laevis (A6 cell line). Tissues were incubated with Na+- and Cl--free solutions on the apical surface. Basolateral perfusate was NaCl-Ringer. Agents that increase cellular cAMP evoked increases in Gt, CT, Isc and generated a Lorentzian Isc-noise. The responses could be related to active, electrogenic secretion of Cl-. Arginine-vasotocin and oxytocin caused a typical peak-plateau response pattern. Stimulation with a membrane-permeant nonhydrolyzable cAMP analogue or forskolin showed stable increases in Gt with only moderate peaking of Isc. Phosphodiesterase inhibitors also stimulated Cl- secretion with peaking responses in Gt and Isc. All stimulants elicited a spontaneous Lorentzian noise, originating from the activated apical Cl- channel, with almost identical corner frequency (40-50 Hz). Repetitive challenge with the hormones led to a refractory behavior of all parameters. Activation of the cAMP route could overcome this refractoriness. All agents caused CT, a measure of apical membrane area, to increase in a manner roughly synchronous with Gt. These results suggest that activation of the cAMP-messenger route may, at least partly, involve exocytosis of a vesicular Cl- channel pool. Apical flufenamate depressed Cl- current and conductance and apparently generated blocker-noise. However, blocking kinetics extracted from noise experiments could not be reconciled with those obtained from current inhibition, suggesting the drug does not act as simple open-channel inhibitor.

Na+ dependence of single-channel current and channel density generate saturation of Na+ uptake in A6 cells

In high-resistance, salt-absorbing epithelia the apical amiloride-sensitive Na+ channel is the key site for regulation of salt and water balance. The saturation of macroscopic Na+ transport through these channels was investigated using A6 epithelial monolayers. The relation between transepithelial Na+ transport (INa) and apical Na+ concentration ([Na+]ap) under short-circuit conditions was studied. Michaelis-Menten analysis of the saturable short-circuit current (Isc) yielded an apparent Michaelis-Menten constant (KmI) of 5 mmol/l and a maximal current (Imax) of 8 microA/cm2. The microscopic parameters underlying INa, namely the single-channel current (i) and the open channel density (No), were investigated by the analysis of current fluctuations induced by the electroneutral amiloride analogue CDPC (6-chloro-3, 5-diaminopyrazine-2-carboxamide). A two-state model analysis yielded the absolute values of i (0.18 +/- 0.01 pA) and No (65.38 +/- 9.57 million channels/cm2 of epithelium) at [Na+]ap = 110 mmol/l containing 50 mumol/l CDPC. Our data indicate that in A6 cells both i and No depend on [Na+]ap. Between 3 and approximately 20 mmol/l the density of conducting pores, No, decreases sharply and behaves again as an almost [Na+]ap-independent parameter at higher [Na+]ap. The single-channel current clearly saturates with an apparent Michaelis-Menten constant, Kmi, of approximately 17 mmol/l. Thus, the [Na+]ap dependence of No as well as the limited transport capacity of the amiloride-sensitive Na+ channel are both responsible for the saturation of INa.

The H+ pump in frog skin (Rana esculenta): identification and localization of a V-ATPase

We here report on studies on the frog skin epithelium to identify the nature of its excretory H+ pump by comparing transport studies, using inhibitors highly specific for V-ATPases, with results from immunocytochemistry using V-ATPase-directed antibodies. Bafilomycin A1 (10 microM) blocked H+ excretion (69 +/- 8% inhibition) and therefore Na+ absorption (61 +/- 17% inhibition after 60 min application, n = 6) in open-circuited skins bathed on their apical side with a 1 mm Na2SO4 solution, "low-Na+ conditions" under which H+ and Na+ fluxes are coupled 1:1. The electrogenic outward H+ current measured in absence of Na+ transport (in the presence of 50 microM amiloride) was also blocked by 10 microM bafilomycin A1 or 5 microM concanamycin A. In contrast, no effects were found on the large and dominant Na+ transport (short-circuit current), which develops with apical solutions containing 115 mm Na+ ("high-Na+ conditions"), demonstrating a specific action on H+ transport. In immunocytochemistry, V-ATPase-like immunoreactivity to the monoclonal antibody E11 directed to the 31-kDa subunit E of the bovine renal V-ATPase was localized only in mitochondria-rich cells (i) in their apical region which corresponds to apical plasma membrane infoldings, and (ii) intracellularly in their neck region and apically around the nucleus. In membrane extracts of the isolated frog skin epithelium, the selectivity of the antibody binding was tested with immunoblots. The antibody labeled exclusively a band of about 31 kDa, very likely the corresponding subunit E of the frog V-ATPase. Our investigations now deliver conclusive evidence that H+ excretion is mediated by a V-ATPase being the electrogenic H+ pump in frog skin.

K+ CHANNEL PERMEATION AND BLOCK IN THE MIDGUT EPITHELIUM OF THE TOBACCO HORNWORM MANDUCA SEXTA

The K+-secreting larval midgut of Manduca sexta in vitro was voltage- or current-clamped. In contrast to Tl+, NH4+ and Na+, both Rb+ and K+ generated a short-circuit current, although with different saturation kinetics. The dependence of the short-circuit current on Rb+/K+ mole fraction gave no evidence for multi-ion occupation of the basolateral K+ channels. After 'functionally' eliminating the apical membranes using the ionophore amphotericin B and the 'apical K+ pump' blockers trimethyltin chloride or Tl+, the K+ channels could be more closely investigated. By measuring zero-current potentials, permeability ratios PX/PK were estimated using an adapted version of the Goldman­Hodgkin­Katz voltage equation. Their sequence was K+ (1) = Tl+ > Rb+ (0.38) > NH4+ (~0.3) > Cs+ (0.03) > Na+ (~0). The K+ channels could not be blocked by basally applied Cs+, Na+ or tetraethylammonium. Blockade of K+ current by Ba2+ was typically voltage-dependent, but only at moderate transbasal voltages. The relative electrical distance delta of the Ba2+ binding site from the basal channel opening was determined to be 0.2. At zero transbasal voltage, the apparent inhibition constant for barium KBa* was 1.7 mmol l-1.

CYCLIC AMP STIMULATION OF ELECTROGENIC UPTAKE OF Na+ AND Cl- ACROSS THE GILL EPITHELIUM OF THE CHINESE CRAB ERIOCHEIR SINENSIS

Split gill lamellae (epithelium plus cuticle) of hyperregulating Chinese crabs acclimated to fresh water were mounted in a modified Ussing chamber. Active and electrogenic absorption of sodium and chloride were measured as positive amiloride-sensitive and negative Cl--dependent short-circuit currents (INa, ICl), respectively. Both currents were characterized before and after treatment of the tissue with theophylline or dibutyryl cyclic AMP. Both drugs increased INa and ICl. A simple circuit analysis showed that INa stimulation reflected a marked increase in the transcellular Na+ conductance, whereas the respective electromotive force was unchanged. The Michaelis constant (KNa) for Na+ current saturation was decreased after INa stimulation, indicating an increased affinity of the transport mechanism for its substrate. Consequently, the affinity for the Na+ channel blocker amiloride decreased as expected for a competitive interaction between substrate and inhibitor. Analysis of the amiloride-induced current-noise revealed a marked increase in the number of apical Na+ channels after INa stimulation with theophylline, whereas there was little change in the single-channel current. Stimulation of Cl- absorption was accompanied by a substantial increase in both transcellular conductance and electromotive force, indicating an activation of the apical H+ pump that provides the driving force for active Cl- uptake via apical Cl-/HCO3- exchange and basolateral Cl- channels.

AN INVESTIGATION OF THE MIDGUT K+ PUMP OF THE TOBACCO HORNWORM (MANDUCA SEXTA) USING SPECIFIC INHIBITORS AND AMPHOTERICIN B

Active K+ secretion in isolated posterior midguts of Manduca sexta was studied by measuring the short-circuit current. One aim of this study was to verify the postulate from biochemical reports that the cooperative apical arrangement of a vacuolar-type H+-ATPase (V-ATPase) and a K+/H+ antiporter drive the short-circuit current. Hence, we tested several specific inhibitors of the V-ATPase on the in vitro midgut preparation. Nitrate, bafilomycin A1, 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-Cl) and amiloride all reduced the short-circuit current. This suggests that the H+-ATPase is involved in transepithelial K+ secretion. However, even at relatively high doses of these inhibitors, the block of the short-circuit current was not complete. Two other agents, thallium ions (Tl+, at millimolar concentrations) and trimethyltin chloride (TMT, 50 µmol l-1), did abolish the short-circuit current. Apical, but not basal, use of the ionophore amphotericin B largely eliminated the short-circuit current. This supports the view that the current-generating source resides in the apical membranes. An apical (and probably intracellular) site of action for NO3-, Tl+ and TMT is suggested by the observation that basal amphotericin B is needed for blockage by NO3- but does not, however, influence the effect of Tl+ and TMT. Likely sites of action are the V-ATPase (for nitrate and TMT) and the K+/H+ antiporter (for Tl+).

INSECT ION HOMEOSTASIS

The constant composition of body fluids in insects is maintained by the cooperative interaction of gastrointestinal and urinary tissues. Water follows ionic movements, which are driven by the basolateral Na+/K+-ATPase and/or the apical 'K+(or Na+) pump'. The latter now is thought to be the functional expression of a parallel arrangement of a proton-motive V-ATPase and a K+(or Na+)/nH+ antiport. This review focuses on the pathways for the movement of monovalent inorganic ions through epithelia involved in ion homeostasis. A graphical summary compares the principal findings with respect to cation secretion in lepidopteran caterpillar midgut goblet cells (K+) and in brush-border cells of Malpighian tubules (K+, Na+).

K+ current stimulation by Cl- in the midgut epithelium of tobacco hornworm (Manduca sexta). II. Analysis of Ba(2+)-induced K+ channel conduction noise

Chloride-stimulated K+ secretion by Manduca sexta midgut (5th-instar larvae) was measured as K(+)-carried short-circuit current of the tissue mounted in an Ussing chamber. "Microscopic" parameters, such as single-channel current and channel density for the rate-determining passive transport step across the basolateral goblet cell membrane (i.e. K+ channels), were estimated by means of current-fluctuation analysis of the K+ channel blockade by haemolymph-side Ba2+ ions. Ba2+ was equally effective with Cl- or gluconate (Glu-) as the principal ambient anion. The Ba(2+)-induced K+ channel conduction noise is reflected by a Lorentzian, or relaxation, noise component in the power spectrum of the K+ current fluctuations. A reduced Lorentzian plateau value, but an unchanged corner frequency, were observed when Cl- was replaced by Glu-. The results from the analysis of a "two-state" model of K+ channel block by Ba2+, with respect to the anion-replacement effects, suggest that the observed changes in K+ current and Lorentzian plateau value mirror a complex change of the underlying parameters: Cl- omission reduces single channel current but increases channel density so that the product of single channel current and channel density is smaller in Glu- than in Cl-. It seems likely that basolateral K+ channels (1) are subject to anionic gating ligands, and (2) depend on anions with respect to the rate of K+ transfer through an open K+ channel.

K+ current stimulation by Cl- in the midgut epithelium of tobacco hornworm (Manduca sexta). I. Kinetics and effect of Cl(-)-site-specific agents

Goblet cells in the midgut epithelium of the tobacco hornworm (Manduca sexta larva, 5th instar) actively secrete K+. This can be measured as short-circuit current (Isc) when the tissue is mounted in an Ussing chamber and bathed in K(+)-rich standard saline containing 32 mmol K+.l-1. Isc depends strictly on basolateral (i.e. haemolymph side) K+ and is therefore termed K+ current, IK. Basolateral, but not apical, chloride, bromide and iodide stimulate IK when compared to the baseline current recorded with gluconate-, nitrate- or thiocyanate-containing salines. So-called "Cl(-)-specific" transport inhibitors (frusemide, 9-anthracene carboxylic acid, diphenylamine carboxylic acid and 4,4'-diisothiocyana-to-stilbene-2,2'-disulphonic acid) reduce IK when added to the basolateral bath, whether Cl- or gluconate is the principal ambient anion. Cl- stimulates IK according to saturation kinetics. The Michaelis-Menten-type, K+ concentration-dependent, saturation of IK is altered in a highly specific manner when gluconate is replaced by Cl-: maximal K+ current, as well as the apparent Michaelis constant, are increased by a factor of 4. Since IK develops in these conditions exclusively via basolateral, Ba(2+)-blockable K+ channels, these results can be understood if it is assumed that haemolymph Cl- interferes with the K+ channel by simultaneously lowering the binding affinity for K+ ions and increasing their subsequent transfer rate across the basolateral goblet cell membrane.

Invertebrate epithelial Na+ channels: amiloride-induced current-noise in crab gill

Epithelial sheets (including cuticle) from posterior gills of the freshwater-adapted euryhaline crab Eriocheir sinensis were obtained according to the method of Schwarz and Graszynski ((1989) Comp. Biochem. Physiol. 92A, 601-604; (1989) Verh. Dtsch. Zool. Ges. 82, 211 and (1989) Arch. Int. Physiol. Biochim. 97, C45). With external NaCl-saline, the outward-directed short-circuit current (Isc) could hardly be influenced by external amiloride up to 100 mumol/l but was, on the contrary, strictly dependent on apical Cl- (Onken, Graszynski and Zeiske (1991) J. Comp. Physiol. B 161, 293-301). In absence of external chloride an inward-directed, amiloride-inhibitable Isc was observed which depended on external Na+ (thus, Isc approximately INa) in a two-step, saturating mode. The Isc-block by amiloride obeyed saturation kinetics (half-maximal at less than or equal to 1 mumol/l, suggesting apical Na(+)-channels). Only for Na+ concentrations below 100 mmol/l we found an indication for a competitive interaction between Na+ and amiloride at the channel. Current fluctuation analysis revealed the presence of an amiloride-induced relaxation (Lorentzian) component in the Isc-noise (so-called 'blocker-noise'). The Lorentzian parameter-shifts with increasing amiloride concentration indicate first-order kinetics of the blocker with its apical receptor. Using a 'two-state' blocking model we calculated, for amiloride concentrations between 2 and 5 mumol/l, a mean single-channel current of 0.46 pA and a mean channel density of 250.10(6) cm-2.

A vacuolar-type proton pump energizes K+/H+ antiport in an animal plasma membrane

In this paper we demonstrate that a vacuolar-type H(+)-ATPase energizes secondary active transport in an insect plasma membrane and thus we provide an alternative to the classical concept of plasma membrane energization in animal cells by the Na+/K(+)-ATPase. We investigated ATP-dependent and -independent vesicle acidification, monitored with fluorescent acridine orange, in a highly purified K(+)-transporting goblet cell apical membrane preparation of tobacco hornworm (Manduca sexta) midgut. ATP-dependent proton transport was shown to be catalyzed by a vacuolar-type ATPase as deduced from its sensitivity to submicromolar concentrations of bafilomycin A1. ATP-independent amiloride-sensitive proton transport into the vesicle interior was dependent on an outward-directed K+ gradient across the vesicle membrane. This K(+)-dependent proton transport may be interpreted as K+/H+ antiport because it exhibited the same sensitivity to amiloride and the same cation specificity as the K(+)-dependent dissipation of a pH gradient generated by the vacuolar-type proton pump. The vacuolar-type ATPase is exclusively a proton pump because it could acidify vesicles independent of the extravesicular K+ concentration, provided that the antiport was inhibited by amiloride. Polyclonal antibodies against the purified vacuolar-type ATPase inhibited ATPase activity and ATP-dependent proton transport, but not K+/H+ antiport, suggesting that the antiporter and the ATPase are two different molecular entities. Experiments in which fluorescent oxonol V was used as an indicator of a vesicle-interior positive membrane potential provided evidence for the electrogenicity of K+/H+ antiport and suggested that more than one H+ is exchanged for one K+ during a reaction cycle. Both the generation of the K+ gradient-dependent membrane potential and the vesicle acidification were sensitive to harmaline, a typical inhibitor of Na(+)-dependent transport processes including Na+/H+ antiport. Our results led to the hypothesis that active and electrogenic K+ secretion in the tobacco hornworm midgut results from electrogenic K+/nH+ antiport which is energized by the electrical component of the proton-motive force generated by the electrogenic vacuolar-type proton pump.

Effect of mucosal H+ and chemical modification on transcellular K+ current in frog skin

A transcellular K+ current (IK) was established across the skin of the frog Rana temporaria, whose apical K+ permeability had been previously stimulated by exposure to K(+)-rich media. Short-term (less than or equal to 15 s) mucosal pH-titration of IK indicated two titrated groups (A and B), with apparent pKA of 6 and pKB of 3. The height of the titration steps, A and B, varied from skin to skin. Intracellular (i) H(+)-sensitive microelectrode studies on Rana esculenta skin (which lacks apical PK) were conducted in order to assess possible changes in pHi and basolateral K+ conductance as a consequence of the rise in mucosal [H+]. Cell pH decreased only at mucosal pH lower than 5.4 which caused a drop in basolateral K+ conductance as estimated from I-V records of the serosal membranes. These effects were much too slow to account for the fast mucosal pH effects on IK (Rana temporaria). Thus, we conclude that the two-step titration curves reflect solely the interaction of external H+ with the mucosal side of apical membrane K+ channels. Exposure to the SH-reagent PCMB, and to the carboxy-modifying EEDQ markedly reduced total IK at neutral pH; however, PCMB seemed to preferentially affect titration step B while EEDQ virtually eliminated step A. When the saturating IK kinetics were studied at different mucosal pH, protons showed a 'mixed' type inhibition of K+ current in the range of titration step A; at pH values less than 5, protons blocked IK by competition with K+ ions. These results are compatible with the presence of two K+ channel populations in the apical membrane which are discernible by their different interactions with external protons and chemical modifiers.

The chloride-stimulated K(+)-secretion by insect midgut and its modification in the presence of osmotic gradients: a short-circuit current and noise-analysis study

K(+)-secretion in the midgut of the larval moth, Manduca sexta, was studied by measuring the kinetics of the lumen-directed short-circuit current (Isc) and the conduction noise from basolateral K+ channel block by Ba2+. Hemolymph chloride as well as hypotonicity both stimulate this K+ current (IK). The kinetic nature of the stimulation is, however, different in each case. Analysis of blocker noise supports, to a large degree, the interpretation obtained from kinetics, namely: chloride ions do not act via changes in cell volume but influence ion turnover and channel number.

A novel synergistic stimulation of Na+-transport across frog skin (Xenopus laevis) by external Cd2+- and Ca2+-ions

Isolated skin of the clawed frog Xenopus laevis was mounted in an Ussing-chamber. The transcellular sodium-current (INa) was identified either as amiloride-blockable (10(-3) mol/l) short-circuit current (ISC), or by correcting ISC for the shunt-current obtained with mucosal Tris. A dose of 10 mmol/l Cd2+ applied to the mucosal side increased the current by about 70%. The half-maximal effect was reached at a Cd2+-concentration of 2.6 mmol/l (in NaCl-Ringer). The quick and fully reversible effect of Cd2+ could not be seen when 10(-3) mol/l amiloride was placed in the outer, Na+-containing solution, nor when Na+ was replaced by Tris. This suggests that Cd2+ stimulates INa. Cd2+ interfered with the Na+-current self-inhibition, and therefore with the saturation of INa by increasing the apparent Michaelis constant (KNa) of this process. The "INa recline" after stepping up mucosal [Na+] was much reduced in presence of Cd2+. Ca2+-ions on the mucosal side had an identical effect to Cd2+, and 10 mmol/l Ca2+ increase INa by about 100%. The half-maximal effect was obtained with 4.4 mmol/l Ca2+. The mechanism of INa-stimulation by Ca2+ did not seem to differ from that of Cd2+. Thus, although of low Na+-transport capacity, Xenopus skin appears to be as good a model for Na+-transporting epithelia as Ranidae skin, with the exception of the calcium effect which, so far, has not been reported for Ranidae.

Current-noise analysis of the basolateral route for K+ ions across a K+-secreting insect midgut epithelium (Manduca sexta)

The isolated midgut of a lepidopteran larva (Manduca sexta), 5th instar was investigated with voltage-clamp and fluctuation analysis techniques. With high K+ insect saline on both sides the outward-directed short-circuit current (Isc) was carried by K+ (IK) from serosal to mucosal compartment. IK could be blocked, in a dose-dependent manner by serosal Ba2+ ions. There was no current with serosal Na+. Noise analysis of IK revealed a Lorentzian component in the power spectrum when Ba2+ was present in the serosal solution. The Ba2+/receptor kinetics show pseudo-first order characteristics only at low [Ba2+]s. For [Ba2+]s greater than KBa, the apparent Ba2+ association rate decreases with a hyperbolic course as a function of serosal [Ba2+] which could indicate some "substrate-inhibition"-like interaction of Ba2+ at its receptor site. It is concluded that the serosal membranes of the K+-secreting intestinal cells contain the common type of Ba2+-blockable K+ channel which provides the serosal pathway for K+ during secretion which is ultimately driven by the mucosally-located electrogenic K+-ATPase.

Impairment of Na+ transport across frog skin by Tl+: effects on turnover, area density and saturation kinetics of apical Na+ channels

Na+ transport across abdominal skin of the frogs, Rana temporaria and Rana esculenta was followed by measuring Na+-dependent short-circuit, current (INa) kinetics and INa fluctuations induced by triamterene, a diuretic. Exposure of the skin to serosal Tl+ led to a pronounced and irreversible drop in INa and INa-blocker noise. At low serosal Tl+ concentrations, we observed mainly a decrease in the apparent Michaelis constant for INa saturation while, at larger [Tl+], the maximal INa dropped irreversibly. Tl+ acts even when serosal Tl+ "transporters" like the Na+-K+ pump, or the K+ channel are nonfunctional. The rate constants for the triamterene/Na+ channel reaction were unchanged after Tl+ whereas the relaxation noise from channel blockage decreased in amplitude. Noise analysis in terms of a two-state blocking model suggested that Tl+ poisoning results in a small decrease in single-channel current through apical Na+ pathways, as well as in a drastic and irreversible drop in channel density. The impairment of Na+ transport by Tl+ can be attributed to the above cited concerted events at the level of the apical membrane.

Ionic channels in epithelial cell membranes

This review focused on results obtained with methods that allow studies of ionic channels in situ, namely, patch clamping and current-noise analysis. We reported findings for ionic channels in apical and basolateral plasma membranes of various tight and leaky epithelia from a wide range of animal species and tissues. As for ionic channel "species," we restricted ourselves to the discussion of cation-specific (Na+ or K+), hybrid (Na+ and K+), and Cl- channels. For the K+-specific channels it can be said that their properties in conduction (multisite, single file), selectivity (only "K+-like" cations), and blocking behavior (Ba2+, Cs+, TEA) much resemble those observed for K+ channels in excitable membranes. This seems to include also the Ca2+-activated "maxi" K+ channel. Thus, K+ channels in excitable membranes and K+ channels in epithelia appear to be very closely related in their basic structural principles. This is, however, not at all unexpected, because K+ channels provide the dominant permeability characteristics of nearly all plasma membranes from symmetrical and epithelial cells. An exception is, of course, apical membranes of tight epithelia whose duty is Na+ absorption against large electrochemical gradients in a usually anisosmotic environment. Here, Na+ channels dominate, although a minor fraction of membrane permeability comes from K+ channels, as in frog skin, colon, or distal nephron. Epithelial Na+ channels are different from excitable Na+ channels in that they 1) are far more selective and 2) seem to be chemically rather than electrically gated. Furthermore, their specific blockers belong to very different chemical families, although a guanidinium/amidinium moiety is a common feature (TTX vs. amiloride). [For a more detailed summary of Na+ channel properties see sect. IV H.] Most interesting is the occurrence of relatively nonselective cationic (hybrid) channels in apical membranes of tight epithelia, like larval or adult frog skin. Here, not only the weak selectivity is astonishing but also the fact that these channels react with so-called K+-channel-specific (Ba2+, TEA) as well as with Na+-channel-specific (amiloride, BIG) compounds. Moreover, this cross-reactivity does not seem to be inhibitory but, on the contrary, stimulating. Clearly these channels may become a fascinating object with which to assess whether Na+ and K+ channels are not only structurally but also genetically related and whether they can somehow be converted into each other.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+-sensitive, spontaneously fluctuating, cation channels in the apical membrane of the adult frog skin epithelium

The fluctuations in transepithelial current through the abdominal skin of bullfrogs (Rana catesbeiana) were analysed while the transepithelial voltage was clamped to zero. A Lorentzian component in the power spectrum was recorded when the skin was bathed with Ca2+ free NaCl Ringer's on both sides. After replacement of all mucosal Na+ by choline the Lorentzian component disappeared. The application of mucosa positive potentials enhanced the plateau of the relaxation noise component while it was depressed by mucosa negative potentials. These observations showed that the current associated with the relaxation noise, was carried by Na+ moving in the inward direction. Divalent cations added to the mucosal solution in micromolar concentrations depressed the relaxation noise immediately, which is indicative for an apical localization of the fluctuating channels. The relaxation noise depended strongly on the pH of the mucosal medium: alkalinization enhanced the relaxation noise while acidification depressed the fluctuations. Micromolar concentrations of the diuretic amiloride, which is known to block the Na+ entry into the cellular compartment, enhanced the Na+-dependent relaxation noise while at higher concentrations an inhibitory effect was observed. From these observations it was concluded that the relaxation noise is caused by inward Na+ movement through fluctuating channels which are localized in the apical membrane. These channels seem to constitute a pathway in parallel with the amiloride-blockable channels. Ionic substitution of Na+ by other monovalent cations showed that these channels are also permeable for K+, Rb+, NH4+, Cs+ and Tl+, but not for Li+. Divalent cations in micromolar concentrations completely occlude these fluctuating channels. Therefore, this pathway will be blocked for monovalent cations when normal Ca2+ containing Ringer's are used as mucosal bathing medium.

The sensitivity of apical Na+ permeability in frog skin to hypertonic stress

Na+ transport across abdominal skins of the frog species Rana esculenta and Rana pipiens was analyzed by recording short-circuit current (Isc), transepithelial conductance (Gt), and the current noise generated by the random blockage of apical Na+ channels by the diuretic, amiloride. Specific Na+ current (INa) and conductance (GNa), as reflected by the amiloride-sensitive part of Isc and Gt, respectively, were markedly depressed after addition of some osmotically active substances, like sugars or alcohols to the mucosal Na+-Ringer solution. These hypertonicity-induced reactions were fast and fully reversible, even at mucosal osmolarities of 1 Osmol. With mucosal solutions of moderate hyperosmolarity a recovery of INa and GNa was observed in presence of the osmotic gradient. This "regulatory" current showed to be carried by Na+ through the Na+-specific apical channels. Contrary to the fast current drop during the initial phase of hyperosmotic shocks, the "osmoregulation" was considerably slower. The recovery of INa was only complete at smaller osmotic gradients but became more and more suppressed at higher osmolarities. Steady-state analysis of the kinetics of the Na+-specific current revealed that the current depression by osmotic shocks obeys Michaelis-Menten kinetics. This current depression at high osmolarities, as well as during the initial phase before "osmoregulation" with small osmotic gradients, can be described in terms of a non-competitive inhibition. This was also suggested by Na+-concentration jump experiments indicating a reduction of the maximal, apical Na+ permeability as mechanism of the hypertonicity-induced drop in INa. The INa kinetics after complete "osmoregulation" were, however, indistinguishable from the isotonic control condition.(ABSTRACT TRUNCATED AT 250 WORDS)

Cl- - and K+-related fluctuations of ionic current through oxyntic cells in frog gastric mucosa

Transport and conductance pathways for Cl- and K+ were studied in frog gastric mucosa using noise analysis techniques. The current-noise power spectra exhibited both Cl- - and K+-dependent characteristics. In Cl- -containing solutions, reductions in Cl- transport were associated with reductions of the overall noise power. Such changes appeared to reflect the movement of Cl- through the apical (mucosal) membranes of oxyntic cells. In Cl- -free solutions a K+-dependent Lorentzian component was detected in the power spectrum when applying a mucosally or serosally directed transepithelial K+ concentration gradient. This component was enhanced by 1) stimulating the oxyntic cells with histamine and 2) appropriate voltage clamping. It was reduced by mucosal Ba2+ in resting tissues and enhanced by mucosal Ba2+ in stimulated tissues. The K+ noise measured in gastric mucosae in Cl- -free solutions appeared also to be generated at the apical membranes of oxyntic cells. This is in analogy to previous findings in other gastrointestinal epithelia with fluctuating apical K+ channels. In the gastric mucosa these channels may play a key role in the mechanism of electrogenic H+ secretion.

The interaction of "K+-like" cations with the apical K+ channel in frog skin

The apparent permeability of the apical K+ channel in the abdominal skin of the frog (Rana temporaria) for different monovalent cations was tested by comparing the short-circuit current (SCC) obtained after imposition of serosally directed ionic concentration gradients. Furthermore, the SCC was subjected to noise analysis. Of various cations tested, only the "K+-like" ions NH+4, Rb+ and Tl+, besides K+, were found to permeate the apical K+ channel, as reflected by SCC- and fluctuation analysis: (i) The SCC could be depressed by addition of the K+-channel blocker Ba2+ to the mucosal solution. (ii) With the K+-like ions (Ringer's concentration), a spontaneous Lorentzian noise was observed. Plateau values were similar for K+ and Tl+, and smaller for NH+4 and Rb+. The corner frequencies clearly increased in the order K+ less than NH+4 less than Tl+ much less than Rb+. The SCC dose-response relationships revealed a Michaelis-Menten-type current saturation only for pure K+- or Tl+-Ringer's solutions as mucosal medium, whereas a more complicated SCC behavior was seen with Rb+ and especially, NH+4. For K+-Tl+ mixtures an anomalous mole-fraction relationship was observed: At low [Tl+]/[K+] ratios, Tl+ ions appeared to inhibit competitively the K+ current while, at high [Tl+]/[K+] ratios, Tl+ seemed to be a permeant cation. This feature was also detected in the noise analysis of K+-Tl+ mixtures. Long-term exposure to mucosal Tl+ resulted in an irreversible deterioration of the tissue. The SCC depression by Ba2+ was of a simple saturation-type characteristic with, however, different half-maximal doses (NH+4 less than K+ less than Rb+). Ba2+ induced a "blocker noise" in presence of all permeant cations with corner frequencies that depended on the Ba2+ concentration. A linear increase of the corner frequencies of the Ba2+-induced noise with increasing Ba2+ concentration was seen for NH+4, Rb+ and K+. With the assumption of a pseudo two-state model for the Ba2+ blockade the on- and off-rate constants for the Ba2+ interaction with the NH+4/Rb+/K+ channel were calculated and showed marked differences, dependent on the nature of the permeant ion. The specific problems with Tl+ prevented such an analysis but SCC- and noise data indicated a comparably poor efficiency of Ba2+ as Tl+-current inhibitor. We attempted a qualitative analysis of our results in terms of a "two-sites, three-barriers" model of the apical K+ channel in frog skin.

A fluctuation analysis study of the development of amiloride-sensitive Na+ transport in the skin of larval bullfrogs (Rana catesbeiana)

In the presence of the Na+ -channel blocker amiloride, the short-circuit current across the skins of bullfrog tadpoles in metamorphic stages XIX-XXIV was subjected to fluctuation analysis. The resulting power spectra contained a Lorentzian component of which the plateau value (S0) decreased while the corner frequency (fc) increased as the mucosal amiloride concentration was increased from 0.5 to 24 microM. From the linear relationship between the fc values and the amiloride concentrations it was possible to determine the binding (k'01) and unbinding (k10) constants for amiloride to its receptor on the Na+ channel. With these parameters as well as short-circuit current and S0 values, the current through the individual Na+ channels (i) was calculated (average 0.58 pA). It did not increase significantly during late metamorphosis. The density of Na+ channels (M) in the apical membrane, on the other hand, increased significantly. It would appear that the increase in short-circuit current which occurs at this time is due primarily to an increase in amiloride-blockable Na+ channels. Unexpectedly, a Lorentzian component could be fitted to power spectra in amiloride-treated skins (stages XIX-XXI) which showed no amiloride-sensitive short-circuit current. Moreover, the typical increase in fc with the amiloride concentration did not occur in these animals.

Noise analysis reveals K+ channel conductance fluctuations in the apical membrane of rabbit colon

In this paper we describe current fluctuations in the mammalian epithelium, rabbit descending colon. Pieces of isolated colon epithelium bathed in Na+ or K+ Ringer's solutions were studied under short-circuit conditions with the current noise spectra recorded over the range of 1-200 Hz. When the epithelium was bathed on both sides with Na+ Ringer's solution (the mucosal solution contained 50 microM amiloride), no Lorentzian components were found in the power spectrum. After imposition of a potassium gradient across the epithelium by replacement of the mucosal solution by K+ Ringer's (containing 50 microM amiloride), a Lorentzian component appeared with an average corner frequency, fc = 15.6 +/- 0.91 Hz and a mean plateau value So = (7.04 +/- 2.94) x 10(-20) A2 sec/cm2. The Lorentzian component was enhanced by voltage clamping the colon in a direction favorable for K+ entry across the apical membrane. Elimination of the K+ gradient by bathing the colon on both sides with K+ Ringer's solutions abolished the noise signal. The Lorentzian component was also depressed by mucosal addition of Cs+ or tetraethylammonium (TEA) and by serosal addition of Ba2+. The one-sided action of these K+ channel blockers suggests a cellular location for the fluctuating channels. Addition of nystatin to the mucosal solution abolished the Lorentzian component. Serosal nystatin did not affect the Lorentzian noise. This finding indicates an apical membrane location for the fluctuating channels. The data were similar in some respects to K+ channel fluctuations recorded from the apical membranes of amphibian epithelia such as the frog skin and toad gallbladder. The results are relevant to recent reports concerning transcellular potassium secretion in the colon and indicate that the colon possesses spontaneously fluctuating potassium channels in its apical membranes in parallel to the Na+ transport pathway.

Poorly selective cation channels in the skin of the larval frog (stage less than or equal to XIX)

The abdominal skin of bullfrog larvae (Rana catesbeiana) was placed in an Ussing-type chamber, and its transepithelial electrical parameters were recorded with mucosal solutions of different ionic composition. With "K+-like" cations (K+, NH+4, RB+, Cs+) the power spectra of the fluctuations in short-circuit current displayed a Lorentzian component (fc = 30 - 40 Hz). The relaxation noise could be suppressed by addition of the K+ -channel blockers Ba2+ and TEA to the mucosal solution. Also, in presence of the ionophore antibiotic nystatin the Lorentizian noise was abolished. The Na+ -channel probes amiloride and benzimidazolyl-2-guanidine (BIG) both enhanced the relaxation noise obtained with the K+-like cations but, with Na+ and Li+, also caused the rise of a relaxation component above the background noise. In presence of amiloride or BIG, the addition of Ba2+, TEA and nystatin still abolished the Lorentizian noise. It can be concluded that the relaxation-noise source is located in the apical cell membranes of the tadpole skin. These spontaneously fluctuating cation channels do not seem to strictly discriminate between K+-like ions (K+, NH+4, Rb+, Cs+) and Na+-like ions (Na+, Li+). On the other hand, well-known specific probes for K+ channels (Ba2+, TEA) and for Na+ channels (amiloride, BIG) interact with this apical cation channel. It is possible that the poorly selective channel plays a role in the ontogenesis of the specific Na+ transport in the maturing frog skin.

Na+ channels and amiloride-induced noise in the mammalian colon epithelium

(1) The effects of the Na+-channel blocker, amiloride, on the short-circuit current carried by Na+ was studied with fluctuation analysis, in rabbit descending colon epithelium. (2) In the presence of mucosal amiloride, the power spectrum of the Na+-current noise showed a Lorentzian component. When the Na+ current was reduced by increasing the blocker concentrations, the Lorentzian plateau decreased and corner frequency increased. Macroscopic short-circuit current and current-noise data are evidence for a two-state mechanism of the blocker interaction with the Na+ channel. (3) On- and off-rate constants for the blocker-receptor reaction, single-channel currents and Na+-channel density were calculated at room temperature and at 37 degrees C. Also, the activation energy for the amiloride-receptor reaction was estimated. The microscopic parameters obtained for the Na+ channel in the colon were similar to those found for Na+ channels in other tight epithelia.

Apical K+ channels in frog skin (Rana temporaria): cation adsorption and voltage influence gating kinetics

Open-close kinetics of fluctuating K+ channels in the apical frog skin membrane were studied with noise analysis of the K+ current (IK). The mucosa to serosa directed IK was obtained with serosal NaCl- and mucosal KCl-Ringer under voltage clamp conditions. Mucosal protons (pH greater than 4), several polyvalent metal ions, and choline shifted the plateaus (SO) of the Lorentzian component in the IK noise spectrum to higher, but the corner frequency (fc) to lower values. SO was lowered at pH less than 4, due to a K+-channel block by H+. Ca2+, Sr2+, H+ (pH greater than 4) and choline did not affect IK. A slight reduction of IK was seen with Mg2+, Mn2+, Co2+, Ni2+, Zn2+, Cu2+ and La3+. At pH greater than 4, the H+-induced shifts in SO an fc were almost abolished in solutions of high mucosal Ca2+ concentrations. Clamping the transepithelial potential difference to more positive values (with respect to the serosa) shifted the Lorentzian parameters SO and fc in the same way as the cations did. As with protons, mucosal Ca2+ interferred with the effect of voltage. The interference of cationic (probably fixed charge screening) and voltage effects suggests a common, more general mechanism of action, namely alterations in K+-channel fluctuation kinetics by changes in local electrical fields. On this basis, the rates for the open-close reaction of K+ channels and their mean lifetime were calculated. We found that e.g. increasing [Ca2+]O from 1-10 mM caused no change of the mean open time, but increased the mean time "closed" of the K+ channel by a factor of about 1.5. Other mucosal cations, as well as depolarizing clamp potentials are thought to have the same effect.

Ba2+-induced conductance fluctuations of spontaneously fluctuating K+ channels in the apical membrane of frog skin (Rana temporaria)

We studied the influence of mucosal Ba2+ ions on the recently described (Zeiske & Van Driessche, 1979a, J. Membrane Biol. 47:77) transepithelial, mucosa towards serosa directed K+ transport in the skin of Rana temporaria. The transport parameters G (conductance), PD (potential difference), Isc (short-circuit current, "K+ current"), as well as the noise of Isc were recorded. Addition of millimolar concentrations of Ba/+ to the mucosal K+-containing solution resulted in a sudden but quickly reversible drop in Isc. G and Isc decreased continuously with increasing Ba2+ concentration, (Ba2+)o. The apparent Michaelis constant of the inhibition by Ba2+ lies within the range 40-80 microM. The apical membrane seems to remain permselective for K+ up to 500 microM (Ba2+)o. Higher (Ba2+)o, however, appears to induce a shunt (PD falls, G increases). This finding made an accurate determination of the nature of the inhibition difficult but our results tend to suggest a K+-channel block by K+-Ba2+ competition. In the presence of Ba2+, the power spectrum of the K+ current shows a second Lorentzian component in the low-frequency range, in addition to the high-frequency Lorentzian caused by spontaneous K+-channel fluctuations (Van Driessche & Zeiske, 1980). Both Lorentzian components are only present with mucosal K+ and can be depressed by addition of Cs+ ions, thus indicating that Ba2+ ions induce K+-channel fluctuations. The dependence of the parameters of the induced Lorentzian on (Ba2+)o shows arise in the plateau values to a maximum around 60 microM (Ba2+)o, followed by a sharp and progressive decrease to very low values. The corner frequency which reflects the rate of the Ba2+-induced fluctuations, however, increases quasi-linearly up to 1 mM (Ba2+)o with a tendency to saturate at higher (Ba2+)o. Based on a three-state model for the K+ channel (having one open state, one closed by the spontaneous fluctuation and one blocked by Ba2+) computer calculations compared favorably with our results. The effect of Ba2+ could be explained by assuming reversible binding at the outer side of the apical K+ channel, thereby blocking the open channel in ;competition with K+. The association-dissociation of Ba2+ at its receptor site is thought to cause a chopping of the K+ current, resulting in modulated current fluctuations.

Spontaneous fluctuations of potassium channels in the apical membrane of frog skin

1. The previously demonstrated K+-dependent short-circuit current through the skin of frog species Rana temporaria (Zeiske & Van Driessche, 1979), bathed with mucosal K+- and serosal Na+-Ringer solution, was investigated with current-fluctuation analysis. 2. The current-noise spectra were recorded in the frequency range from 1 to 800 Hz and showed a Lorentzian component with a mean plateau value S0 = (1.50 +/- 0.05).10(-20) A2.s.cm-2 and a corner frequency of fc=(81.0 +/- 3.4)Hz(n=14). 3. S0 increased with mucosal K+ concentration, [K]o, while fc remained almost unchanged. A decrease in S0 was observed when serosal Na+ was replaced by K+. 4. Mucosal Cs+ (10 mM) depressed, reversibly, the K+-dependent current noise to the level of the background noise. Moreover, a linear decrease in fc with increasing Cs+ concentration was observed. 5. Among the other tested alkali cations, Rb+ was the only blocker though less potent than Cs+. Tetraethylammonium, 4-aminopyridine, 2.4.6-triaminopyrimidine and amiloride had no effect. 6. Alterations in the transcellular transport of Na+ contained in a mucosal solution with high [K]o resulted in significant changes in K+ current noise. 7. The current-fluctuation intensities decreased with increasing contact time to high [K]o; these changes were concomitant with the previously reported time dependence of the short-circuit current (Zeiske & Van Driessche, 1979). 8. The K+-dependent fluctuations are thought to originate from K+-selective pathways in the apical cell membranes. The description of the K+-current noise by a single Lorentzian suggests that the "K+ channels" switch randomly between an open and closed state. 9. Assuming a two state model for the channel-kinetics, the single channel current i and the channel density M were calculated as i=(0.37 +/- 0.05)pA and M=(0.53 +/- 0.08) mu-2 (n=13).

Saturable K+ pathway across the outer border of frog skin (rana temporaria): kinetics and inhibition by Cs+ and other cations

The reaction of abdominal skins of the frog species Rana temporaria on mucosal K+-containing solutions was studied in an Ussing-type chamber by recording transepithelial potential difference (PD), short-circuit current (SCC) and conductance (G). With Na-Ringer's as serosal medium, a linear correlation between PD and the logarithm of the mucosal K+-concentration ([K]o) was obtained. The K+-dependent SCC saturated with increasing [K]o, and could quickly and reversibly be depressed by addition of Rb+, Cs+, and H+. Li+, Na+, and NH4+ did not influence K+ current. A large scatter was obtained for kinetic parameters like the slope of the PD-log[K]o-line (18--36.5 mV/decade), the apparent Michaelis constant (13--200 mM), and the maximal current of the saturable SCC (6--50 microa . cm-2), as well as for the degree of inhibition by Cs+ ions. This seemed to be caused by a time-dependent change during long time exposure to high [K]o (more than 30 sec), thereby inducing a selectivity loss of K+-transporting structures, together with an increase in SCC and G and a decrease in PD. Short time exposure to K+-containing solutions showed a competitive inhibition of K+ current by Cs+ ions, and a Michaelis constant of 6.6 mM for the inhibitory action of Cs+. Proton titration resulted in a decrease of K+ current at pH less than 3. An acidic membrane component (apparent dissociation constant 2.5 x 10(-3) M) is virtually controlling K+ transfer. Reducing the transepithelial K+-concentration gradient by raising the serosal potassium concentration was accompanied by the disappearance of SCC and PD.