Coronary vasodilator and cardiac effects of (-) and (+) verapamil were investigated in two kinds of canine heart preparations. When administered intravenously to anesthetized open-chest dogs, (-) verapamil was only 1.5-2 times as potent as (+) verapamil in increasing coronary sinus outflow and in decreasing mean arterial pressure and coronary resistance. However, (-) verapamil was about 5 times as potent in producing a negative chronotropic effect, about 10 times as potent in producing a negative dromotropic effect, and about 15 times as potent in decreasing myocardial oxygen consumption as (+) verapamil. When injected into the anterior septal artery in the isolated, blood-perfused papillary muscle preparation of the dog, (-) verapamil was about 15 times as potent as (+) verapamil in producing a negative inotropic effect. However, in increasing blood flow through the anterior septal artery (-) verapamil was only about 2.5 times as potent as (+) verapamil. These results indicate that in equieffective doses in producing coronary vasodilatation (+) verapamil is far less cardiodepressant than the (-) isomer.
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