Amiloride and the sodium channel. The diuretic drugs amiloride and triamterene have been used as probes for sodium channels in the mucosal surface of isolated frog skin. Both substances interact competitively with sodium for the channel. Using 14C-amiloride measurements were made of the number of channels present in the mucosal surface under a variety of conditions. The data suggests that a reliable estimate of sodium channel density can be made. Antidiuretic hormone applied to the serosal surface was found to have no effect on the numbers of channels, while the current passing through each channel was increased. The implications of these findings for the mechanism by which hormone alters the mucosal sodium permeability are discussed.