Effect of calcium channel blockers on baroreceptor reflex in anaesthetizedcats

The baroreflex-induced changes in heart rate in chloralose anaesthetized an d artificially ventilated cats (2.5-4.0 kg) before and after pretreatment w ith calcium channel blockers (CCBs) were compared. Baroreflex mediated chan ges in heart rate (HR) were elicited by raising and lowering the systemic b lood pressure with intravenous injections of phenylephrine and sodium nitro prusside, 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 b radycardia as well as the tachycardia following either i.v. or i.c. adminis tration. 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. administratio n 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 suggeste d that verapamil administration interacts with central cardiovascular integ rating mechanisms to reduce the gain of the baroreflex function. Nifedipine and diltiazem are relatively free from this effect. (C) 2000 Academic Pres s.