Influx and efflux of sodium at the outer surface of frog skin.

The unidirectional Na influx j12, and Na efflux, j12, at the epithelial surface of the frog skin were determined under various experimental conditions. The j21 was taken as the difference between j12 and the simultaneously measured shortcircuit current (SCC). Errors in j12 determination originating from various transport rates within the skin were kept to a minimum using a normalization procedure. Under control conditions, j12 (1.20 muEquiv/cm-2. hr) was found to be only slightly larger than the SCC (1.10 MUEquiv/cm-2. hr). After inhibition of the transepithelial Na transport by amiloride, ouabain, low temperature and low Na concentration, the reduction of j12 and SCC was almost identical, indicating that the entrance of Na into the epithelium is rate limiting for the transepithelial transport. Compared to the control, j21 remained unchanged after amiloride and ouabain, but was insignificantly reduced at low temperature and significantly reduced at low Na concentration. These data are consistent with the assumption that the Na efflux follows mainly an extracellular pathway.