Control of sodium permeability of the outer barrier in toad skin

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Abstract
The24Na efflux (J eff Na ) (i.e., the rate of appearance of24Na in the outer compartment) in the isolated short-circuited toad skin bathed by NaCl-Ringer's solution on both sides is composed of para- and transcellular components of almost equal magnitudes. This relies on the assumption that amiloride acts on the transcellular component only and could block it completely. Ouabain induces a large transient increase of the transcellular component. This increase, which starts within a few minutes after the addition of ouabain, is due to electrical depolarization of the outer barrier, rather than a consequence of blocking Na recirculation across the inner barrier. The subsequent decline ofJ eff Na , which takes place after the ouabain-inducedJ eff Na peak, is due to a progressive block of outer barrier Na channels with time, which can eventually be complete, depending on the duration of action of ouabain. As the external Na concentration was always kept high and constant in these experiments, the results indicate that a rise in cell Na concentration, and not in the outer bathing solution, is the signal that triggers the reduction of outer barrier Na permeability (P 0 Na ). Ouabain has no effect uponJ eff Na with Na-free solution bathing the outer and NaCl-Ringer's solution the inner skin surface, showing the importance of Na penetration across the outer barrier, and not across the inner barrier due to its low Na permeability, in the process of closing the Na channels of this structure. Step changes from Na 115mm to Na-free external solution, or vice-versa, may affect both the outer barrier electrical potential difference (PD0) and cell Na concentration (Na) c . Therefore, the behavior ofJ eff Na depends on which variable (if PD0 or (Na) c regulated outer barrier Na permeability) is most affected by step changes in outer bathing solution Na concentration. Amiloride in the control condition blocks the transcellular component ofJ eff Na . However, in the condition of approximate short-circuiting of the outer barrier and high cellular Na concentration induced by long term effects of ouabain, when the Na channels of the outer barrier are already blocked by elevated cell Na concentration, amiloride may induce the opposite effect, increasing Na permeability of the outer barrier. With outer barrier Na channels completely blocked by high cell Na concentration, PCMB in the outer bathing medium induces a large increase ofJ eff Na , rendering these channels again amiloride sensitive. The results are consistent with the notion that Na efflux from cell compartment to the outer bathing solution goes through the amiloride-sensitive Na channels of the apical border of the superficial cell layer of toad skin, with an apparent Na permeability modulated by cell ionic environment, most probably the cell Na concentration. The ensemble of the present results are consistent with Na permeability regulation taking place at the outer barrier level. However, this precise location could only be made unambiguously by measurements across the individual outer cell membranes.