Abstract
1. The effects which alterations in the surface potential of the apical membrane of isolated Rana catesbeiana skin have on Na entry were examined. 2. Changes in the external ionic strength have little effect upon the rate of Na transport across the frog skin epithelium. 3. Uranyl ion (UO2(2+), 2.5 mM) induces a +145 mV change in the surface potential of phosphatidylserine monolayers, and a +60mV change in the surface potential of monolayers made from phosphatidylcholine. 4. UO2(2+) inhibits the short-circuit current (Isc) by a maximum of 20% in R. catesbeiana skin, while stimulating Isc by 40% in R. temporaria skin. Neither Isc stimulation nor inhibition by UO2(2+) can be seen in the presence of 10(-4) M-amiloride. 5. From points 1 and 2 above, we conclude that the surface charge density in the neighbourhood of the Na-selective entry site located in the apical membrane is small (greater than 1e-/600 A2). The results obtained using UO2(2+) suggest that Na entry is not affected by changes in the membrane surface potential.