Comparison of the effects of forskolin and isoproterenol on cyclic AMP levels and tension in bovine coronary artery.

The effects of isoproterenol and forskolin on cyclic AMP levels and tension were compared in helical strips of bovine coronary artery. Both compounds produced time- and dose-dependent increases in cyclic AMP levels and both compounds relaxed potassium-contracted arteries. However, in contrast to the results with isoproterenol, cyclic AMP elevation and relaxation were not well correlated at lower concentrations of forskolin. For example, 0.1 microM forskolin increased cyclic AMP levels in the arteries by approximately 5.5-fold but did not relax the muscles. A smaller elevation of cyclic AMP produced by 1.0 microM isoproterenol, on the other hand, was accompanied by an almost complete relaxation of the arteries. These results suggest that either cyclic AMP is not responsible for the relaxation caused by these agents, or some form of functional compartmentalization of cyclic AMP exists in this tissue.