Interactive effects of ethanol and silver on sodium transport across toad skin

Both ethanol and silver ions have been shown to affect ion transport across various epithelia. This investigation was principally undertaken to further define mechanisms of silver ions and ethanol, and their possible interactions, on sodium transport across toad skin. Isolated toad skin, mounted between identical oxygenated amphibian bicarbonate Ringer solutions, maintained stable transepithelial potential differences (serosa positive) and short-circuit currents for several hours at 25 degrees C. It was observed that (1) ethanol inhibited the active transcellular component of sodium absorption and this effect was reversible; (2) inhibition of sodium transport by ethanol was directly proportional to the applied concentration; (3) pretreatment with silver ions prevented any ethanol effects; and (4) pretreatment with ethanol prevented any silver ion effects. It was concluded from these results that ethanol induced its inhibitory effects on membrane phospholipids thereby perturbing the function of a sulfhydryl ligand, while silver ion or silver chloride complex binding to this ligand would maintain its function in sodium transport despite the presence of ethanol.     

Effect of silver chloride on the short-circuit current across the isolated toad skin.

Since addition of 10^?4M AgNO₃ to either an inside or outside bathing medium containing sulfate had no effect on short-circuit current (SCC), a measure of net Na⁺ transport, or transmural potential difference (PD) in the isolated surviving toadskin, the effect of adding Ag⁺ to chloridebased Ringer solution was studied. Exposure of the outside bathing medium to 10^?4M AgNO₃ resulted in, after a 20 minute time lag, a 250 ± 51% (N=6) increase in SCC within 100 minutes as opposed to an immediate response which had a 350 ± 26% (N=8) increase in SCC by addition of 10^?4M AgNO₃ to the inside bathing solution. The dose response curve relating change in SCC to the Ag⁺ concentration added to the inside bathing medium was saturable at 10^?5M Ag⁺. The uptake of Ag⁺ by the tissue, as measured by atomic absorption spectrophotometry, showed no correlation to the relative change in SCC. Na⁺ flux experiments under short-circuited conditions showed that Ag⁺Cl^? stimulated only the unidirectional outside to inside Na⁺ flux. These results indicate that Ag⁺Cl^? enhances active sodium transport and that Ag⁺Cl^? binding to specific membrane groups is required for this effect.     

Membrane potential plays a dual role for chloride transport across toad skin

The Cl- -current through toad skin epithelium depends on the potential in a way consistent with a potential-controlled Cl- permeability. Computer analysis of the Koefoed-Johnsen Ussing two-membrane model provided with constant membrane permeabilities indicates that the voltage- and time-dependent currents are not caused by a trivial Goldmand-type rectification and ion redistributions following transepithelial potential pertubations. Extended with a dynamic Cl- permeability in the apical membrane according to a Hodgkin-Huxley kinetic scheme, the model predicts voltage clamp data which closely resemble experimental observations. This extension of the classic frog skin model implies that the Cl- permeability is activated by a voltage change caused by the inward Na+ current through the apical membrane.     

Cellular lithium and transepithelial transport across toad urinary bladder

Summary Toad urinary bladders were exposed on either their mucosal or serosal surfaces, or on both surfaces, to medium in which sodium was replaced completely by lithium. With mucosal lithium Ringer's, serosal sodium Ringer's, short-circuit current (SCC) declined by about 50 percent over the first 60 min and was then maintained over a further 180 min. Cellular lithium content was comparable to the sodium transport pool. With lithium Ringer's serosa, SCC was abolished over 60 to 120 min whether the mucosal cation was sodium or lithium. Measurements of cellular ionic composition revealed that the epithelial cells gained lithium from both the mucosal and serosal media. With lithium Ringer's mucosa and serosa, cells lost potassium and gained lithium and a little chloride and water, but these changes in cellular ions could not account for the current flow across the tissue under these conditions, which must, therefore, have been carried by a transepithelial movement of lithium itself. The inhibition by serosal lithium of SCC was overcome by exposure of the mucosal surface of the bladders to amphotericin B. Thus it reflected, predominantly, an inhibition of lithium entry to the cells across the apical membrane. It is suggested that this inhibition is a consequence of cellular lithium accumulation.     

Comparative effects of catecholamines, angiotensin II and antidiuretic hormone on chloride transport in toad skin

In this work we present data which show stimulation of Cl- transport in the isolated toad skin by four agonists: L-isoproterenol, L-adrenalin, angiotensin II and ADH. This response was demonstrated by raising mucosal amiloride concentration to block the sodium transport in the skin. With transepithelial sodium influx almost completely inhibited, it was likely that the response reflected transport events in the glands. Inhibition of the bioelectric parameters by removing chloride from the serosal bathing medium in the amiloride-inhibited preparation eliminated the response to all four agents, indicating that these responses are chloride dependent. The similarity of the bioelectric responses of the amiloride-treated preparation to db cAMP and to the four agents tested in this work add further evidence that this second messenger may account largely for the Cl- transport mechanism in the toad skin glands by increasing the apical membrane permeability to Cl-.     

Resolution of parameters in the equivalent electrical circuit of the sodium transport mechanism across toad skin

Summary In amphibian epithelia, amiloride reduces net sodium transport by hindering the entry of sodium to the active transport mechanism, that is, by increasing the series resistance (R_ser). Theoretically, therefore, analysis of amiloride-induced changes in potential differences and short-circuit current should yield numerical estimates of all the parameters in the equivalent electrical circuit of the sodium transport mechanism.The concept has been explored by analysis of such changes in toad skins (Xenopus laevis) bathed in hypotonic sulphate Ringer's, after exposure to varying doses of amiloride, or to amphotericin, dinitrophenol or Pitressin.The estimated values ofR_ser, of the electromotive force of the sodium pump (E_Na), and of the shunt resistance (R_sh) were independent of the dose of amiloride employed. Skins bathed in hypotonic sulphate Ringer's exhibited a progressive rise inE_Na. Amphotericin produced a fall inR_ser, while dinitrophenol caused a fall inE_Na; washout of the drugs reversed these effects. Pitressin produced a fall in bothR_ser andR_sh, with a rise inE_Na. These results are in accord with earlier suggestions regarding the site(s) of action of these agents.     

Dobutamine inhibits vasopressin-mediated water transport across toad bladder epithelium

This study aimed to investigate the effect of dobutamine on water transport across toad bladder epithelium. Water flow through the membrane was measured gravimetrically in bladder sac preparations. Dobutamine had no effect on basal water transport, but partially inhibited transport stimulated by vasopressin. Similarly, dobutamine exerted no influence on the hydrosmotic response to 8-chlorophenylthio-cAMP, but interfered with the response to phosphodiesterase inhibitor 1-methyl-3-isobutyl-xanthine. These results demonstrate that this catecholamine may inhibit vasopressin-stimulated water transport at a site prior to cAMP formation. The use of propranolol was ineffective in blocking the effect of dobutamine on transport stimulated by vasopressin, indicating that beta-adrenoceptors play no role in this effect. On the other hand, phentolamine significantly reduced the effect of dobutamine, indicating the involvement of alpha-adrenoceptors in such event. Rauwolscine also inhibited the effect of dobutamine, pointing to the specific contribution of the alpha(2)-adrenoceptors to this effect. Taken together, the results of this study demonstrate that dobutamine inhibits vasopressin-stimulated water transport in toad bladders through a mechanism mediated by the stimulation of alpha(2)-adrenoceptors, thus suggesting that such a drug may exert a direct cellular effect on membrane permeability to water in transporting epithelia. The current study may provide a better understanding of the effects of dobutamine on renal function by contributing towards the elucidation of its action mechanism.     

Ion transport by mitochondria-rich cells in toad skin

Summary The optical sectioning video imaging technique was used for measurements of the volume of mitochondria-rich (m.r.) cells of the isolated epithelium of toad skin. Under short-circuit conditions, cell volume decreased by about 14% in response to bilateral exposure to Cl-free (gluconate substitution) solutions, apical exposure to ouabain resulted in a large increase in volume, which could be prevented either by the simultaneous application of amiloride in the apical solution or by the exposure of the epithelium to bilateral Cl-free solutions. Unilateral exposure to a Cl-free solution did not prevent ouabain-induced cell swelling. It is concluded that m.r. cells have an amiloride-blockable Na conductance in the apical membrane, a ouabain-sensitive Na pump in the basolateral membrane, and a passive Cl permeability in both membranes. From the initial rate of ouabain-induced cell volume increase the active Na current carried by a single m.r. cell was estimated to be 9.9±1.3 pA. Voltage clamping of the preparation in the physiological range of potentials (0 to –100 mV, serosa grounded) resulted in a cell volume increase with a time course similar to that of the stimulation of the voltage-dependent activation were prevented by exposure of the tissue to a Cl-free apical solution. The steady-state volume of the m.r. cells increased with the clamping voltage, and at –100 mV the volume was about 1.15 times that under short-circuit conditions. The rate of volume increase during current passage was significantly decreased by lowering the serosal K concentration (K_i) to 0.5mm, but was independent of whether K_i was 2.4, 5, or 10mm. This indicates that the K conductance of the serosal membrane becomes rate limiting for the uptake of KCl when K_i is significantly lower than its physiological value. It is concluded that the voltage-activated Cl currents flow through the m.r. cells and that swelling is caused by an uptake of Cl ions from the apical bath and K ions from the serosal bath. Bilateral exposure of the tissue to hypo- or hypertonic bathing solutions changed cell volume without detectable changes in the Cl conductance. The volume response to external osmotic perturbations followed that of an osmometer with an osmotically inactive volume of 21%. Using this value and the change in cell volume in response to bilateral Cl-free solutions, we calculated an intracellular steady-state Cl concentration of 19.8±1.7mm (n=6) of the short-circuited cell.     

The effect of catecholamines on ion transport in the toad bladder

Noradrenalin (8 · 10⁻⁶ M) and adrenalin (6 · 10⁻⁶ and 6 · 10⁻⁷ M) were found to cause marked stimulation of short-circuit current (S.C.C.) in isolated toad bladder, but isoprenalin (8 · 10⁻⁷ M) was found to be without effect. The percentage rise in S.C.C. due to noradrenalin was found to be inversely proportional to the initial S.C.C. or total conductance of the bladder. Again in the case of noradrenalin the rise in S.C.C. was almost completely abolished by α-adrenergic blockade but not by β-blockade. This rise in S.C.C. was found not to be significantly different from the rise in net Na⁺ flux. Bidirectional Cl⁻ fluxes were estimated using ⁸²Br as a companion radionuclide to ³⁶Cl. No significant net Cl⁻ flux was apparent, either before or after addition of any of the three catecholamines tested. However, in some cases the unidirectional Cl⁻ fluxes rose markedly following addition of noradrenalin or of adrenalin and this change was not reflected in a change in total conductance. This anomaly was noted to occur in bladders whose initial conductance was of the order of 0.5 kΩ⁻¹ · cm⁻² or greater. The evidence presented suggests that two actions of catecholamines on ion transport in toad bladder are (a) to increase Na⁺ transport via stimulation of α-adrenergic sites and (b) at the concentrations tested to cause an increase in passive Cl permeability in bladders whose initial conductance is high.     

Barium inhibition of sodium ion transport in toad bladder

The effect of Ba²⁺ on Na⁺ transport and electrical characteristics of toad bladder was determined from change produced in short circuit current (I_sc, epithelial, apical and basal-lateral potentials (ψ_t, ψ_a, ψ_b), epithelial and membrane resistances (R_t, R_a, R_b) and shunt resistance (R_s). Mucosal Ba²⁺ had no effect. Serosal Ba²⁺ reduced I_sc, ψ_t, ψ_a, and ψ_b, but had no effect on R_t, R_a, R_b and R_s. Minimal effective Ba²⁺ concentration was 5 · 10^?5 M. The phenomenon was reversed by Ba²⁺ removal, but not by 86 mM serosal K⁺. Ba²⁺ inhibition of I_sc did not impair the response to vasopressin which was quantitatively the same as controls. ψ_a with Ba²⁺ equalled ψ_b. After Ba²⁺ inhibition, ouabain produced no further decrease in ψ_t and I_sc. Ba²⁺ exposure after ouabain did not decrease ψ_t and I_sc. The results suggest that Ba²⁺ inhibits the basal-lateral electrogenic Na⁺ pump.