Ethanol abolishes clonidine-induced impairment of baroreflex control of heart rate in conscious rats

Our previous studies showed that the ability of ethanol or cholidine to alt er the baroreflex control of heart rate (baroreflex sensitivity, BRS) depen ds on the functional activity of aortic baroreflexes. In this study, we inv estigated the interaction between the two drugs on BRS in conscious rats wi th intact baroreflexes (shamoperated, SO) and after aortic baroreceptor den ervation (ABD). The slope of the curve relating increments in mean arterial pressure induced by phenylephrine to corresponding reflex bradycardic resp onses was taken as an index of BRS. Ethanol (1 g/kg IV) significantly (p < 0.05) attenuated BRS in SO rats (-1.7 +/- 0.13 versus -1.04 +/- 0.15 beats/ min/mm Hg) but not in ABD rats. Clonidine (30 mu g/kg, IV) elicited signifi cantly (p < 0.05) greater hypotensive responses in conscious ABD compared w ith SO rats. The BRS was not affected by clonidine administration in SO rat s but showed significant (p < 0.05) reductions in ABD rats. Ethanol (1 g/kg , IV) had no effect on the hypotensive response to subsequently administere d clonidine in ABD and SO rats; however, the effect of the two drugs on BRS was variable. In ABD rats, the BRS values before and after administration of ethanol and clondine were similar, suggesting that pretreatment with eth anol counteracted clonidine-evoked attenuation of BRS in this rat preparati on. In SO rats, the ethanol-clonidine combination produced a significant (p < 0.05) decrease in BRS, similar to the effect of ethanol when administere d alone. These data confirm earlier findings that the aortic baroreflex are modulates the interaction of ethanol and clonidine with baroreflex functio n. Further, the ability of ethanol to abolish clonidine-induced attenuation of BRS in ABD rats may relate to the compound effects of the two drugs on neuronal pathways participating in the central processing of baroreflexes i n these rats. (C) 1999 Elsevier Science Inc. All rights reserved.