Similarities and differences in the effects of verapamil, diltiazem and nifedipine on arterial baroreflexes of anesthetized cats.

The effects of verapamil, diltiazem and nifedipine on baroreflex sensitivity (BS) were investigated in anesthetized cats at equihypotensive doses of 100, 100 and 10 micrograms/kg, respectively. The gain in BS (ms/mmHg) was calculated from the ratio delta heart period/delta systolic pressure following evoked rises and falls in arterial pressure. Verapamil and diltiazem significantly augmented BS while nifedipine depressed BS during evoked rises in arterial pressure. By contrast, BS was significantly depressed by diltiazem and nifedipine in response to depressor challenges with nitroprusside, while it was unaffected by verapamil. The augmentatory action of verapamil on BS after pressor challenges, was abolished by bilateral vagotomy while the inhibitory action of nifedipine was still evident after vagotomy, suggesting a vagal augmentatory action for verapamil and an involvement of the cardiac sympathetic nerves in the inhibitory action of nifedipine on BS. The differences between the actions of the 3 drugs on BS are not readily explainable by our data but they cannot be attributed to alterations of cardiac autonomic function since at the doses employed none of the drugs affected the cardiac responses to carotid occlusion, i.v. isoproterenol or electrical stimulation of the right vagus. It is concluded that BS is affected differently by the 3 chemically unrelated calcium channel blockers in a way which could either be contributory or deleterious to the antihypertensive effectiveness of these drugs.