Uptake of [3H]benzamil at different sodium concentrations. Inferences regarding the regulation of sodium permeability.

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Abstract
The effect of benzamil [BZ] on short-circuit current in frog skin was measured at different external Na concentrations. A linear relationship exists between the concentration of BZ reducing short-circuit current by 50% and the external Na concentration, indicative of some form of competitive antagonism between Na and BZ. Uptake of [3H]benzamil [3H-BZ] into isolated frog skin epithelium and whole skin (0.95 cm2 pieces) was measured at different external Na concentrations. With an Na concentration of 111 mM in the external medium the uptake of 3H-BZ is linear with concentration. Uptake amounted to 8.8 f[femto]mol nM-1, a value similar to the linear component of the uptake measured at low (1.1 mM) Na concentration. Using a variety of other conditions, the maximal number of specific binding sites for 3H-BZ was calculated from displaceable binding and the fractional occupancy, the latter being derived from the inhibition of short-circuit current. This approach gave similar binding site densities to those reported previously at low Na concentrations. The reduction in specific 3H-BZ uptake at high Na may result from 2 mechanisms, competition of Na with the ligand for an external binding site, and a reduction in the site density as the intracellular Na concentration increases. The saturation of Na transport which occurs at high Na concentration is likely a consequence of the reduced availability of entry sites, rather than saturation of the uptake process.