Determination of the driving force for the sodium pump (ENa) and of active and passive conductances (GNa and Gsh) in isolated toad skin: influence of antidiuretic hormone.

1. Values of the sodium potential (ENa), active conductance (GNa) and passive conductance (Gsh) were measured in the isolated skin of the toad Pleurodema thaul placed in an Ussing chamber, and Isaacson's test was performed with 2,4,6-trieminopyrimidine (TAP) and with amiloride. 2. The numerical estimates obtained in the presence of TAP were ENa 122.85 +/- 15.17 mV, GNa 0.493 +/- 0.09 mS/cm2 and Gsh 1.145 +/- 0.23 mS/cm2. 3. After exposure to ADH these values were as follows: ENa 85.76 +/- 12.17 mV, GNa 1.191 +/- 0.20 mS/cm2 and Gsh 0.935 +/- 0.14 mS/cm2. 4. Addition of 0.5 x 10(2-) TAP produced a 53.90 +/- 5.10% decrease in transepithelial potential and a 37.90 +/- 4.90% fall in short-circuit current. 5. Exposure to ADH increased the transepithelial potential difference 34.20 +/- 13.20% and the short-circuit current to 78.00 +/- 20.50% above the control values. 6. Comparison of the efficiency and mechanism of action of TAP and amiloride in the determination of electrical parameters shows that both agents induce a similar decrease in Gsh, a finding which could indicate that TAP blocks toad skin apical membrane Na+ channels without affecting tight junction conductance.