An explanation of bell-shaped, dose-positive inotropic effect curves for denopamine in canine right ventricular muscle.

We investigated mechanisms by which denopamine produces bell-shaped, cumulative concentration-response curves for positive inotropic effect in canine right ventricular muscle. As in our previous study, the curves ascended at 10(-8) to 10(-6) M, reached a maximum at 3 X 10(-6) to 10(-5) M, and descended at higher concentrations. The ascending limb gave 6.58 +/- 0.21 for the pD2 value. Denopamine, when administered cumulatively up to 3 X 10(-5) to 3 X 10(-4) M, caused rightward shifts of concentration-response curves for the positive inotropic effect of isoproterenol. Application of a two-component fitting model to the concentration-response curves for isoproterenol in the presence of denopamine yielded two pA2 values of denopamine; 6.59 +/- 0.19 for the high- and 5.05 +/- 0.21 for the low-affinity state of beta 1 receptors. The bell-shaped curves for denopamine were successfully computer-simulated with the following assumptions. (a) Denopamine produces a positive inotropic effect by binding to the high-affinity state of beta 1 receptors with an apparent dissociation constant of 10(-6.59) M. (b) The fraction of the high-affinity state decreases as denopamine binds to the low-affinity state with an apparent dissociation constant of 10(-5.05) M. Cyclic AMP levels were changed by denopamine, isoproterenol, and a combination of both, roughly in a parallel way to changes in positive inotropic effect.