Apical sodium entry in split frog skin: Current-voltage relationship

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Abstract
Apical Na+ entry into frog skin epithelium is widely presumed to be electrodiffusive in nature, as for other tight epithelia. However, in contrast to rabbit descending colon andNecturus urinary bladder, the constant field equation has been reported to fit the apical sodium current (N Na)-membrane potential (ψmc) relationship over only a narrow range of apical membrane potentials or to be inapplicable altogether. We have re-examined this issue by impaling split frog skins across the basolateral membrane and examining the current-voltage relationships at extremely early endpoints in time after initiating pulses of constant transepithelial voltage. In this study, the rapid transient responses in ψmc were completed within 0.5 to 3.5 msec. Using endpoints to 1 to 25 msec, the Goldman equation provided excellent fits of the data over large ranges in apical potential of 300 to 420 mV, from approximately −200 to about +145 mV (cell relative to mucosa). Split skins were also studied when superfused with high serosal K+ in order to determine whether theI Na-ψmc relationship could be generated purely by transepithelial measurements. Under these conditions, the basolateral membrane potential was found to be −10±3 mV (cell relative to serosa, mean±se), the basolateral fractional resistance was greater than zero, and the transepithelial current was markedly and reversibly reduced. For these reasons, use of high serosal K+ is considered inadvisable for determining theI Na-ψmc relationship, at least in those tissues (such as frog skin) where more direct measurements are technically feasible. Analysis of theI Na-ψmc relationships under baseline conditions provided estimates of intracellular Na+ concentration and of apical Na+ permeability of 9 to 14mm and of ∼3 × 10−7 cm · sec−1, respectively, in reasonable agreement with estimates obtained by different techniques.