Abstract

The spirolactones SC 9420 and SC 14266 both are effective inhibitors of the aldosterone-induced increase in in vitro sodium transport as measured in the isolated toad bladder. These compounds appear to be specific antagonists of aldosterone as the shortcircuit current response to exogenous vasopressin and glucose repletion were not altered by concentrations of spirolactone sufficient to inhibit the electrophysiologic effects of aldosterone. The results of kinetic analysis of aldosterone-spirolactone interaction fulfilled the criteria for competitive inhibition. Assuming that the tissue receptors for aldosterone are homogeneous, the relative affinity for these receptors was estimated by the ratio of the dissociation constants of aldosterone (K_D) and spirolactone (K_i). Using two separate derivations, each from independent data, the resulting ratios for aldosterone: SC 14266 were in reasonable agreement, i.e., 1:235 and 1:336. The average maximal effective aldosterone concentration (2 x K_D) was calculated to be 1 x 10^-8 M, while the inhibitor constant (K_i) for the spirolactone SC 14266 was 1 x 10^-6 M by direct plot and 2.6 x 10^-6 M when derived from a Lineweaver-Burk plot. The K_i for SC 9420 by direct plot was 5.25 x 10^-6 M and the K_D:K_i ratio was 1:681.