Amiloride and calcium effect on the outer barrier of the frog skin

Amiloride (0.1 mM) as well as Ca++ (10 mM) inhibit Na+ transport across frog skin by blocking Na+ entrance across the outer barrier of the epithelium. The inhibition produced by amiloride consists of an "early" and a "late" phase which together account for almost a total inhibition of the short-circuit current (SCC). The analysis of the time course indicates that the two phases are due to the inhibition of superficially and deeply located Na sites, respectively, Ca++, instead, only blocks a fraction of the SCC, and this fraction seems to correspond to the inhibition of the same population of Na sites blocked by the "late" phase of amiloride effect. The location of the two populations of Na sites as well as the possible relationship between them are discussed in terms of maturation of the outermost cell layer.