Transient current changes and Na compartimentalization in frog skin epithelium

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Abstract
Experimental conditions are described in which transient and positive current responses across isolated frog skin epithelia can be elicited by sudden addition of Na and Li ions (2–40 mM) in the outer bathing solution. Subsequent return to outer Na (or Li) free conditions produce similar transient current changes but in the opposite direction. Analysis of the curve responses shows that the transient component of each curve is best described by a single, fast exponential term equation in case of Na addition to preparation unpoisonned with ouabain. In contrast, an equation including two exponential terms (a fast and a slow one) is required to fit the curve responses observed across ouabain treated epithelia of if Li is added outside. The transient responses were not significantly altered by substituying Cl for SO4 2− anions. They were completely prevented by Amiloride (5 · 10−5 M), increased by oxytocin (20 mU/ml) and markedly dependent upon the outer Na concentration. Interpreted in term of compartmental analysis, these observations suggest that a) the frog skin epithelium contains 2 separated but communicating compartments having different degrees of accessibility from outside; b) only that compartment filling at a fast rate (0.5 min) is involved in the transepithelial Na transport; c) the other one, filling at a rate of 4 to 7 min, is resplenished only under conditions where the basal pump system has a reduced activity. Tentative localization of these compartment is proposed.