The Action of Aldosterone and Related Corticosteroids on Sodium Transport across the Toad Bladder*

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Abstract
Prolonged preincubation in steroid-free media enhanced the action of exogenous d-aldosterone on Na+ transport across the urinary bladder of the toad, Bufo marinus. The ability of aldosterone to augment Na+ transport was dependent on an adequate supply of substrate under these conditions. Aldosterone was as effective in stimulating Na+ transport against a fivefold gradient as it was in isomolar solutions. The effect of aldosterone was exerted solely on the unidirectional flux of Na+ from the mucosal to the serosal phases as judged by tracer studies with Na22. At a concentration of 7 x 10-7 moles per l, aldosterone was as active on addition to the musosal medium as on addition to the serosal medium. Structure-activity relationships were studied at equimolar concentrations (i.e., 7 x 10-7 [image]), and the order of effectiveness was found to be d-aldosterone= 9[alpha]-fluorocortisol - [image] desoxycorticosterone acetate 2-CH3 - 9[alpha]-fluorocortisol>2-CH3 - cortisol>progesterone no steroid.