Augmentation of cardiac reflexes in the cat by the calcium antagonist verapamil

The effect of intravenous and intrapericardial administration of verapamil on cardiac reflexes was studied in chloralose-anesthetized cats. Cardiac reflexes were induced by either intrapericardial application of serotonin (80 micrograms/kg) and dimethylphenylpiperazinium iodide (DMPP) (20 micrograms/kg) or by occlusion of the ascending aorta for 30 s. Intrapericardial application of verapamil (3 and 6 micrograms/kg) did not change the epicardial chemoreflexes, whereas intravenous administration of verapamil in the normal clinical dose enhanced the cardiac reflexes induced by chemical or mechanical stimulation. These results indicate that verapamil does not act directly on the vagal afferent nerve fibers in the epicardium of the heart. We suggest that the augmentation of cardiac reflexes by verapamil might be caused by an action of verapamil on the sinoaortic baroreflex or baroreceptors.