Effects of verapamil on myocardial contractility, cardiac adenosine 3,'5'-monophosphate and heart phosphorylase.

The effects of verapamil on myocardial isometric force on contraction, cardiac adenosine 3,'5'-monophosphate (cyclic AMP) and heart phosphorylase alpha activity were studied in the isolated perfused rat heart. When hearts were perfused with verapamil (5.98 times 10- minus 8 M), force of contraction was reduced approximately 50% within 4 to 5 minutes; at this point, the concentration of cyclic AMP was significantly lower than control but phosphorylase alpha activity was unchanged. In hearts perfused continuously for 60 minutes with verapamil, force of contraction and cyclic AMP levels returned to normal within 20 minutes after administration of verapamil was begun. Isoproterenol (0.355 nmol/min) reversed the depressant effect of verapamil on cardiac contractility and restored heart cyclic AMP levels to normal. Methoxamine (35.5 nmol/min) given to verapamil-depressed hearts, caused contractile force to return to normal, but cardiac cyclic AMP levels remained low. Mephentermine (23.0 nmol/min) had no effect on cardiac contraction, cyclic AMP or phosphorylase alpha activity in hearts depressed by verapamil. It was concluded that with the concentration of verapamil used in these experiments, the drug caused a transient decrease in force of contraction and myocardial cyclic AMP. Both the depression in myocardial contractility and in cardiac cyclic AMP caused by verapamil were reversed promptly by isoproterenol, whereas methoxamine overcame acutely only the negative inotropic effect of verapamil. Mephentermine had no effect on hearts depressed by verapamil.