Interaction between isoproterenol and dihydropyridines in heart cells

The negative inotropic potencies (IC50) of the calcium channel blockers nicardipine (12 +/- 2 nM) and nifedipine (9 +/- 2 nM) measured in cultured chick embryo ventricular cells were not significantly different. Exposure of the cells to isoproterenol potentiated the negative inotropic effect of low concentrations of nicardipine (10(-9)-10(-8) M) and decreased the effect of higher concentrations (10(-7) M). In contrast, isoproterenol shifted the entire dose-response curve for nifedipine to the right. These results provide evidence that activation of beta-adrenoceptors modulates the negative inotropic effect of nifedipine and nicardipine differently.