Evidence for a two-site model of forskolin action in frog urinary bladder

In the present study several aspects of the osmotic water-flow-regulation mechanism in frog urinary bladder were examined, utilizing forskolin either as a direct hydrosmotic agent or in association with vasopressin. It was found that forskolin induces a hydrosmotic effect similar to the one induced by vasopressin. This effect is rapid, reversible and dose-dependent. The half-maximally effective concentration (Ec50FSK) is 1.37 microM forskolin. No additional effect on the osmotic water flow was observed when maximal concentrations of forskolin and vasopressin were given simultaneously. Moreover, forskolin can also markedly potentiate vasopressin-induced hydrosmotic response. This potentiation was maximal with submaximal doses of vasopressin, whereas it disappeared when the hormonal concentration was increased to very high levels. Therefore, forskolin increases vasopressin potency without affecting vasopressin efficacy. The Ec50FSK for the forskolin-induced increase in vasopressin potency was 11 nmol, about two orders of magnitude lower than the Ec50FSK for the direct effect of forskolin on the osmotic water transport. On the whole, our results are compatible with a two-site model of forskolin action in frog urinary bladder: a low affinity site (Ec50FSK = 1.37 microM) that mediates the direct action of forskolin on the osmotic water flow and a high affinity site (Ec50FSK = 11 nmol), which mediates the synergic effect of forskolin with the antidiuretic hormone.