Studies on the binding of aldosterone in the toad bladder.

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Abstract
The technique employed distinguished that hormone which was absorbed or bound in the tissue from that in a free or soluble state. Physiological activity was associated with only the former, which comprised only 33 to 2% of the hormone accumulated by the tissue when exposed to physiologically active concentrations of aldosterone, 3 x 10-10 to 1 x 10-7 mole/liter. The bound or adsorbed hormone was resolvable into 2 sets: one with high affinity, Kassoc. = 1. 4 x 1014 and a maximal binding capacity of some 9 x 10-14 mole of aldosterone/g of tissue, and another of lower affinity, Kassoc. = 4 x 1012, binding 3 x 10-12 mole of hormone/g of tissue. The bound hormone was displaced readily from both sets of sites by d-aldosterone, deoxycorticosterone, cortisol, progesterone, and spirolactone (SC 14266) but not by cholesterol. Because the binding sites of the large set were saturated at concentrations of hormone corresponding to maximal physiological activity, whereas those of the small set were essentially saturated at concentrations too low to elicit a physiological response, it is concluded that the large set contains the receptors responsible for the hormonal effect. The characteristic latent period before onset of the hormonal effect is not attributable to a delay in the binding of the hormone with the tissue.