The baroreflex-induced changes in heart rate in chloralose anaesthetized and artificially ventilated cats (2.5-4.0 kg) before and after pretreatment with calcium channel blockers (CCBs) were compared. Baroreflex mediated changes in heart rate (HR) were elicited by raising and lowering the systemic blood pressure with intravenous injections of phenylephrine and sodium nitroprusside, respectively. The effects of three CCBs, verapamil, diltiazem and nifedipine administered either intravenously (i.v.) or intracisternally (i.c.) were studied. Verapamil administration markedly inhibited the reflex bradycardia as well as the tachycardia following either i.v. or i.c. administration. Intracisternally, a relatively smaller dose of verapamil produced an effect comparable in magnitude and duration, to a higher i.v. dose. The reflex bradycardia was inhibited following i.v., but not i.c. administration of nifedipine while the reflex tachycardia was not affected significantly by either i.v. or i.c. nifedipine. Intravenous diltiazem did not appear to affect the reflex bradycardia or tachycardia significantly. It is suggested that verapamil administration interacts with central cardiovascular integrating mechanisms to reduce the gain of the baroreflex function. Nifedipine and diltiazem are relatively free from this effect.
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