Influence of extracellular Cl concentration on Cl transport across isolated skin or Rana pipiens

Fast and slow voltage modulation of apical Cl- permeability in toad skin at high [K+]

The influence of voltage on the conductance of toad skin was studied to identify the time course of the activation/deactivation dynamics of voltage-dependent Cl- channels located in the apical membrane of mitochondrion-rich cells in this tissue. Positive apical voltage induced an important conductance inhibition which took a few seconds to fully develop and was instantaneously released by pulse inversion to negative voltage, indicating a short-duration memory of the inhibiting factors. Sinusoidal stimulation at 23.4 mM [Cl-] showed hysteresis in the current versus voltage curves, even at very low frequency, suggesting that the rate of voltage application was also relevant for the inhibition/releasing effect to develop. We conclude that the voltage modulation of apical Cl- permeability is essentially a fast process and the apparent slow components of activation/deactivation obtained in the whole skin are a consequence of a gradual voltage build-up across the apical membrane due to voltage sharing between apical and basolateral membranes.

Proton pump activity of mitochondria-rich cells. The interpretation of external proton-concentration gradients

We have hypothesized that a major role of the apical H(+)-pump in mitochondria-rich (MR) cells of amphibian skin is to energize active uptake of Cl- via an apical Cl-/HCO3(-)-exchanger. The activity of the H+ pump was studied by monitoring mucosal [H+]-profiles with a pH-sensitive microelectrode. With gluconate as mucosal anion, pH adjacent to the cornified cell layer was 0.98 +/- 0.07 (mean +/- SEM) pH-units below that of the lightly buffered bulk solution (pH = 7.40). The average distance at which the pH-gradient is dissipated was 382 +/- 18 microns, corresponding to an estimated "unstirred layer" thickness of 329 +/- 29 microns. Mucosal acidification was dependent on serosal pCO2, and abolished after depression of cellular energy metabolism, confirming that mucosal acidification results from active transport of H+. The [H+] was practically similar adjacent to all cells and independent of whether the microelectrode tip was positioned near an MR-cell or a principal cell. To evaluate [H+]-profiles created by a multitude of MR-cells, a mathematical model is proposed which assumes that the H+ distribution is governed by steady diffusion from a number of point sources defining a set of particular solutions to Laplace's equation. Model calculations predicted that with a physiological density of MR cells, the [H+] profile would be governed by so many sources that their individual contributions could not be experimentally resolved. The flux equation was integrated to provide a general mathematical expression for an external standing [H+]-gradient in the unstirred layer. This case was treated as free diffusion of protons and proton-loaded buffer molecules carrying away the protons extruded by the pump into the unstirred layer; the expression derived was used for estimating stationary proton-fluxes. The external [H+]-gradient depended on the mucosal anion such as to indicate that base (HCO3-) is excreted in exchange not only for Cl-, but also for Br- and I-, indicating that the active fluxes of these anions can be attributed to mitochondria-rich cells.

Conductive chloride flux across amphibian skin: inhibition by indacrinone and cobalt ion

When amphibian skin was incubated under conditions in which transepithelial sodium transport was abolished, a conductive transepithelial Cl- flux arose when Cl- was removed from one of the compartments. This flux was matched by short-circuit current and it accounted entirely for transepithelial conductance. Cl- influx was larger than efflux; it was linearly related to the magnitude of transepithelial Cl- concentration difference. When applied to the epithelial surface of the tissue, divalent metal cations such as Co2+, and the ethacrynic acid derivative, indacrinone, reduced rapidly and reversibly both transepithelial Cl- (in)flux and short-circuit current. Frog skin proved to be more sensitive to these inhibitors than toad skin. Further characterization of transepithelial Cl- pathway(s) should benefit from the fact that Cl- across amphibian skin can easily be monitored by the short-circuit current method, and from the availability of agents which inhibit this passive flux rapidly and reversibly.

Roles of external and cellular Cl- ions on the activation of an apical electrodiffusional Cl- pathway in toad skin

This study is concerned with the short-circuit current, Isc, responses of the Cl(-)-transporting cells of toad skin submitted to sudden changes of the external Cl- concentration, [Cl]o. Sudden changes of [Cl]o, carried out under apical membrane depolarization, allowed comparison of the roles of [Cl]o and [Cl]cell on the activation of the apical Cl- pathways. Equilibration of short-circuited skins symmetrically in K-Ringer's solutions of different Cl- concentrations permitted adjustment of [Cl]cell to different levels. For a given Cl- concentration (in the range of 11.7 to 117 mM) on both sides of a depolarized apical membrane, this structure exhibits a high Cl- permeability, P(Cl)apical. On the other hand, for the same range of [Cl]cell but with [Cl]o = 0, P(Cl)apical is reduced to negligible values. These observations indicate that when the apical membrane is depolarized P(Cl)apical is modulated by [Cl]o; in the absence of external Cl- ions, intracellular Cl- is not sufficient to activate P(Cl)apical. Computer simulation shows that the fast Cl- currents induced across the apical membrane by sudden shifts of [Cl]o from a control equilibrium value strictly follow the laws of electrodiffusion. For each experimental group, the computer-generated Isc versus [( Cl]cell - [Cl]o) curve which best fits the experimental data can only be obtained by a unique pair of P(Cl)apical and Rb (resistance of the basolateral membrane), thus allowing the calculation of these parameters. The electrodiffusional behavior of the net Cl- flux across the apical membrane supports the channel nature of the apical Cl- pathways in the Cl(-)-transporting cells. Cl- ions contribute significantly to the overall conductance of the basolateral membrane even in the presence of a high K concentration in the internal solution.

Forskolin activates gated Cl- channels in frog skin

We investigated the effect of forskolin on Cl- movements across the isolated epithelium of frog skin. With Cl- on both sides, forskolin (50 mumol/l) increased the transepithelial conductance considerably and elicited significant Cl- secretion. Establishing transepithelial Cl- gradients markedly increased the Cl- currents (ICl). During forskolin treatment, the power density spectra (PDS) of the fluctuation in transepithelial current contained a Lorentzian component that depended on the presence of Cl- in the bathing solutions. Mucosal as well as serosal diphenylamine-2-carboxylic acid (DPC; 1 mmol/l) partially depressed ICl as well as the Lorentzian noise component. Microelectrode recordings from cells involved in transepithelial Na+ absorption showed that forskolin activates gated Cl- channels in a cellular pathway in parallel with the Na+-transporting granulosum cells of the frog skin. The activation of the Cl- -dependent currents and Lorentzian noise was rather variable, and adaptation of the animals to solutions that contained 40 or 60 mmol/l NaCl increased the sensitivity to forskolin. In skins of salt-adapted animals, oxytocin (0.1 U/ml) also slightly activated the Cl- pathway. On the other hand, oxytocin and 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP; 1 mmol/l) were without effect in control skins.

Comparative roles of voltage and Cl ions upon activation of a Cl conductive pathway in toad skin

(1) Combined use of external Cl concentration pulses and apical membrane depolarization permitted to compare the roles of apical voltage and Cl ions upon the activation of a skin Cl conductance, GCl, which is assumed to reflect activation of the permeability of a Cl pathway. (2) Apical membrane depolarization induced by skin hyperpolarization, or by short-circuiting skins with high K Ringer's on the inner side, failed to activate GCl in the absence of external Cl, GCl remaining negligible. Under apical membrane depolarization, a step elevation of [Cl]0 slowly activated GCl as characterized by a sigmoidal current response of slow onset concomitant to a slow conductance increase. External Cl removal had the reverse effect, slowly inactivating GCl. (3) With the apical membrane in the normal polarized state, a step increase of [Cl]0 slowly activated GCl to submaximal values. This indicates that the interaction of Cl ions with the apical membrane partially activates GCl in the absence of apical membrane depolarization. (4) Activation of GCl was interpreted on the basis of a direct effect of Cl ions upon the apical membrane, having been attributed to the apical membrane voltage an indirect role. Voltage would affect the Cl distribution across the apical membrane, and, as a result, the Cl concentration at a proposed regulatory site which modulates the apical membrane permeability to Cl ions.

Potential dependence of unidirectional chloride fluxes across isolated frog skin

Isolated frog skins were voltage clamped at transepithelial potentials (Vt) ranging from -60 mV to 60 mV to measure transepithelial 36Cl- fluxes from the apical to the basolateral bathing solution (J13) and in the opposite direction (J31). The potential dependence of fluxes obtained in Na+-free choline Ringer's indicates the presence of conductive and nonconductive components that probably correspond to fluxes through paracellular and cellular pathways, respectively. Rectification of fluxes with reversal of the potential reflects a structural asymmetry, presumably in surface charge density. The data are consistent with a charge density of one negative charge per 280 A2 on the apical side. A new model for passive Cl- transport was developed that includes surface charge asymmetry and specifically accounts for the observed variation of conductance with potential. In normal frog Ringer's, J13 was larger than J31 at zero potential (active Cl- transport), J13 rose exponentially with increasing positive potential to reach a maximum at 40 mV (approximately open-circuit), and the predicted partial Cl- conductance exceeded the measured conductance leading to the conclusion that when J13 is largely driven by Na+ transport, much of the coupling occurs via nonconductive pathways. Theophylline stimulates Cl- transport that also occurs via nonconductive pathways as Vt becomes more positive.

Concentration dependence of halide fluxes and selectivity of the anion pathway in toad skin

The isolated toad (Bufo bufo) skin was mounted under voltage-clamp conditions in a chamber shown to cause no significant edge damage. The serosal side of the skin was bathed with NaCl-Ringer's, and the passive voltage-sensitive anion conductance studied in its fully voltage activated state, V = -80 mV (apical bath negative). The active sodium currents were eliminated by replacing external Na+ with K+. With [Cl-]o varying between 1.45 mM and 110 mM (gluconate substitution) and [I-]o = 3 mM, the total clamping current (y) and the sum of halide currents (x), estimated from flux measurements, were related by y = 1.0x-3.7 microA cm-2 (r2 = 0.98, n = 50 preparations). The increase in [Cl-]o produced a sigmoidal increase in Cl- influx and clamping current, with the rate coefficient for the influx increasing with [Cl-]o for 1.45 less than [Cl-]o less than 60 mM, but decreasing slightly again as [Cl-]o was further raised to 110 mM. A similar relationship was obtained between the rate coefficient for the Br- influx and [Br-]o, and the I- influx and [Cl-]o, indicating that these three ions are transported by a pathway that is activated by Cl-o and Br-o. The rate coefficients for the influxes ranked as follows, I-:Cl-:Br- = 0.7:1:1.3. The I-/Cl- selectivity was shown to be independent of the degree of Cl-o activation of the anion pathway, and identical with the I-/Cl- selectivity of a furosemide-sensitive, conductive pathway. With [Cl-]o, [Br-]o, or [I-]o = 110 mM, the currents ranked as follows, Cl-:Br-:I- = 1:0.68:0.06, indicating that Cl-, to a lesser extent Br-, and I-, poorly activate the conductive anion pathway. External I- was a potent inhibitor of the Cl-o activation of the Cl- conductance. The unidirectional I- fluxes ([I-]o = [I-]i = 3 mM, [Cl-]o = [Cl-]i = 110 mM) revealed passive transport for V less than -50 mV, active transport for V = o mV, and exchange diffusion for V = 50 mV, confirming our previous finding that depending on the transepithelial potential, the toad skin exhibits three modes of anion transport. A model that shares some properties with that of the anion transport system of the red cell membrane accounts for our findings, and for an inwardly directed active Cl- flux in terms of Cl-/HCO3- exchange.

Basolateral membrane chloride transport in isolated epithelia of frog skin

Isotopic methodology was used to characterize Cl- transport in isolated epithelia of frog skin (northern Rana pipiens) bathed in Cl--rich Ringer solution and short-circuited. Cl- content of epithelia measured when loaded to 36Cl specific activity equilibrium averaged 139.6 neq/mg dry wt. The kinetics of 36Cl efflux was biexponential and consistent with binding or compartmentalization of approximately 30% of tissue Cl- within the intracellular pool. Because efflux of 36Cl to the apical solution was immeasurable, it was concluded that apical membranes were virtually impermeable to Cl- and that basolateral membranes were highly permeable to Cl- with a mean unidirectional Cl- efflux of 21.7 microA/cm2. Both furosemide (1 mM) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (5 X 10(-4) M) inhibited markedly the basolateral membrane chloride fluxes within seconds, as measured in chamber experiments. As inhibition of Cl- flux occurred in the absence of a change of the electrical parameters of apical and basolateral membranes, the mechanisms of Cl- transport appeared to be electroneutral and, for the most part at least, not coupled to the fluxes of Na+ and K+. Transepithelial Cl- fluxes averaged near 1 microA/cm2, proceeding via transport routes in parallel to the cells of the stratified epithelium. No correlation existed between the "shunt" resistance measured in the presence of 100 microM amiloride (greater than 1,000 omega X cm2) and the partial conductance to Cl-.

Opposite effects of indacrinone (MK-196) on sodium and chloride conductance of amphibian skin

Indacrinone, a drug chemically related to ethacrynic acid, usually stimulated reversibly short circuit current and sodium influx when applied to the epithelial surface of amphibian skin. Concomitantly, transepithelial conductance, gt, decreased, provided chloride was the main anion in the incubation fluid. Electrophysiological analysis including microelectrode impalement indicated that the drug increased the sodium-conductance at the apical border of the impaled (most likely granular) cells. The decrease in gt thus points at shunt conductance being reduced with indacrinone, sometimes drastically. Decrease of transepithelial chloride flux with the drug as well as lack of effect of the drug on gt in the absence of chloride on the epithelial side demonstrate the influence of indacrinone on a chloride specific pathway. Whether this is along a paracellular route or through a cellular compartment not coupled to granular cells (mitochondria-rich cells?) cannot be decided on the basis of the present data.

Exchange diffusion, electrodiffusion and rectification in the chloride transport pathway of frog skin

Measurements of chloride flux ratios across frog skin at different clamping voltages showed that chloride transport at clamping voltages from 0 mV to and beyond the spontaneous potential is probably electrodiffusion. At reversed potentials a significant fraction of chloride transport could be described formally as exchange diffusion. Chloride conductance was found to be highly voltage dependent, being largest at hyperpolarizing clamping voltages. The transition from the less conducting state to the more conducting one was studied by recording the time course of the current after a step change in clamping voltage from 0 mV to hyperpolarizing voltages. The shape of the curve is sigmoidal, and the relative rate of change of current increases with increasing hyperpolarization. It is proposed that the change in conductance is governed by the same mechanism as in the toad skin, namely a change in chloride permeability due to voltage gating of chloride channels. The time course of transepithelial conductance after addition of amiloride to the outside solution indicates that a fraction of the decrease in conductance is due to closure of chloride channels caused by the change in intracellular potential due to the inhibition of the sodium channels.

Relationship of transepithelial electrical potential to membrane potentials and conductance ratios in frog skin

Previous studies in anuran epithelia have shown that, after clamping the transepithelial voltage in symmetrical sequences for 4-6 min there is near-constancy of the rate of active Na transport and the associated oxidative metabolism, with a near-linear potential dependence of both. Here we have investigated in frog skin the cellular electrophysiolgical events associated with voltage clamping (Vt = inside-outside potential). Increase and decrease of Vt produced converse effects, related directly to the magnitude of Vt. Hyperpolarization resulted in prompt decrease in inward transepithelial current It and increase in fractional outer membrane resistance fRo (as evaluated from small transient voltage perturbations) and in outer membrane potential Vo. Overshoot of Vo was followed by relaxation to a quasi-steady state in minutes. Changes in fRo were progressive, with half times of some 1-5 sec. Changes in transepithelial slope conductance gt were more variable, usually preventing precise evaluation of the outer and inner cell membrane conductances go and gi. Nevertheless, it was shown that go is related inversely to Vt and Vo. Presuming insensitivity of gi to Vt, the dependence of Go on Vo in the steady state much exceeds that predicted by the constant field equation. Apparent inconsistencies with earlier results of others may be attributable to differences in protocol and the complex dependence of go on Vo and/or cellular-current. In contrast to previous findings in tight epithelia at open circuit, differences in Vt were associated with substantial differences in fRo and inner membrane potential Vi. Hyperpolarization of Vt over ranges commonly employed in studies of active transport ad metabolism appears to increase significantly the electrochemical work per Na ion transported.

Influence of cellular and paracellular conductance patterns on epithelial transport and metabolism

Theoretical analysis of transepithelial active Na transport is often based on equivalent electrical circuits comprising discrete parallel active and passive pathways. Recent findings show, however, that Na+ pumps are distributed over the entire basal lateral surface of epithelial cells. This suggests that Na+ that has been actively transported into paracellular channels may to some extent return to the apical (mucosal) bathing solution, depending on the relative conductances of the pathways via the tight junctions and the lateral intercellular spaces. Such circulation, as well as the relative conductance of cellular and paracellular pathways, may have an important influence on the relationships between parameters of transcellular and transepithelial active transport and metabolism. These relationships were examined by equivalent circuit analysis of active Na transport, Na conductance, the electromotive force of Na transport, the "stoichiometry" of transport, and the degree of coupling of transport to metabolism. Although the model is too crude to permit precise quantification, important qualitative differences are predicted between "loose" and "tight" epithelia in the absence and presence of circulation. In contrast, there is no effect on the free energy of metabolic reaction estimated from a linear thermodynamic formalism. Also of interest are implications concerning the experimental evaluation of passive paracellular conductance following abolition of active transport, and the use of the cellular voltage-divider ratio to estimate the relative conductances of apical and basal lateral plasma membranes.