Selective effects of clonidine on the canine sinus node.
The direct and indirect chronotropic effects of clonidine (0.001--10 micrograms/ml) were studied by selective perfusion of the sinus node artery of 20 pentobarbital anesthetized dogs. Concentrations of 0.1, 1.0 and 10 migrograms/ml produced dose-related reductions in sinus rate of 3.7 +/- 0.9 (P less than .005), 6.9 +/- 1.8 (P less than .005) and 15.8 +/- 2.3 (P less than .0001) beats/min, respectivel. Sinus acceleration after right stellate ganglion stimulation at 16 and 32 Hz was significantly (P less than .05) inhibited by intranodal clonidine (0.1 microgram/ml). The positive chronotropic effect of norepinephrine (0.1 microgram/ml, sinus node artery) was not significantly affected by prior administration of clonidine. Atropine (1 mg/kg i.v. or 10 micrograms/ml, selectively perfused into the sinus node artery) had no effect on the sinus slowing produced by clonidine. Clonidine had an inconsistent effect on the response to vagal stimulation. Clonidine had an inconsistent effect on the response to vagal stimulation. Clonidine inhibits the heart rate increase after sympathetic nerve stimulation without affecting the heart rate response to norepinephrine infusion. The findings are consistent with previous observations that clonidine can inhibit norephinephrine release and demonstrate that this mechanism of action may occur within the sinus node.
D000818:Animals; D001285:Atropine / Q000494:pharmacology; D001794:Blood Pressure / Q000187:drug effects*; D003000:Clonidine / Q000494:pharmacology*; D004285:Dogs; D004558:Electric Stimulation; D005260:Female; D006339:Heart Rate / Q000187:drug effects*; D007275:Injections, Intravenous; D008297:Male; D018377:Neurotransmitter Agents / Q000378:metabolism; D009638:Norepinephrine / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects; D012849:Sinoatrial Node / Q000187:drug effects*; D013564:Sympathetic Nervous System / Q000502:physiology; D014628:Vagotomy; D014630:Vagus Nerve / Q000502:physiology
D018377:Neurotransmitter Agents; D011942:Receptors, Adrenergic, alpha; D001285:Atropine; D003000:Clonidine; D009638:Norepinephrine