Hypotensive effect of clonidine is not mediated by enhanced baroreflex gain in rats.
We wished to examine whether enhanced depressor baroreflexes contribute to the hypotensive effect of clonidine in rats. Clonidine's hypotensive effect and clonidine's effect on baroreflex gain mean heart rate/mean blood pressure, (delta HR/delta BP) were compared under two conditions: in conscious rats with intact baroreflex control of HR, and in pentobarbital-anesthetized (PA) rats in which baroreflex control of HR is obtunded. Clonidine produced greater hypotension and bradycardia in PA rats than in conscious rats, but this greater hypotension was not associated with enhancement of baroreflex gain. Instead, clonidine-induced hypotension in PA rats was associated with a further decrease in baroreflex gain, whereas hypotension in conscious rats was not associated with any change in baroreflex gain. Selective pharmacologic blockade of sympathetic (i.e., with atenolol treatment) or parasympathetic (i.e., with methyl-atropine treatment) control of HR showed that clonidine decreased baroreflex gain in PA rats through inhibition of sympathetic control of HR. This study suggests that clonidine's hypotensive effect is not associated with enhancement of reflex bradycardic responses in rats.
Watkins
L
L
Department of Pharmacology, University of North Carolina, Chapel Hill 27599-7455.
Maixner
W
W
D000758:Anesthesia; D000818:Animals; D001340:Autonomic Nerve Block; D001794:Blood Pressure / Q000187:drug effects*; D003000:Clonidine / Q000494:pharmacology*; D003864:Depression, Chemical; D006339:Heart Rate / Q000187:drug effects; D066298:In Vitro Techniques; D008297:Male; D010275:Parasympathetic Nervous System / Q000502:physiology; D010424:Pentobarbital; D011311:Pressoreceptors / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012018:Reflex / Q000187:drug effects*; D013564:Sympathetic Nervous System / Q000502:physiology
D010424:Pentobarbital; D003000:Clonidine