Cardiac adenylate cyclase activity, positive chronotropic and inotropic effects of forskolin analogs with either low, medium or high binding site affinity

A series of in vitro studies were conducted examining the adenylate cyclase stimulation, positive chronotropic and inotropic effects of forskolin and nine analogs which exhibited a range of 3H]forskolin binding site affinities (K1) from 0.020 to 3.174 microM. A significant (P less than .001) linear correlation (r = 0.94) was found between binding site affinity and adenylate cyclase stimulation (EC50) for forskolin and the nine structural analogs. Adenylate cyclase activity was also significantly correlated with the positive chronotropic and inotropic effects of these substances on isolated guinea pig atria. Compounds with K1 values between 0.020 and 1.136 microM produced concentration-dependent increases in heart rate and contractile force in isolated spontaneous and electrically paced guinea pig atria, respectively. In contrast, an analog with a K1 of 3.174 microM caused significant (P less than .05) negative chronotropic and inotropic effects at concentrations above 10 microM. The optimal separation between positive inotropic and chronotropic activity was found with compounds displaying potent 3H]forskolin binding site affinity but moderate adenylate cyclase stimulation, i.e., K1 and EC50 values of approximately 0.05 to 0.10 and 3 microM, respectively. The results of this study show that the forskolin analog, P87-7692 7-desacetyl-7-(O-propionyl)-hydroxyl amino-carbonyl-forskolin], has marked activity with a wide separation between positive inotropic (248 +/- 41%) and chronotropic effects (43 +/- 13%) at 6.2 microM and may serve as a prototype for a forskolin-based cardiotonic.