Opioid modulation of baroreceptor reflex sensitivity in dogs.

Opioid peptides can modulate cardiovascular function both directly and by modifying specific responses. The action of opioids on baroreceptors was evaluated in anesthetized dogs. Baroreceptor responsiveness was determined by observing the peak heart rate and blood pressure responses as well as by evaluating the slope of a line relating heart period to blood pressure while pharmacologically altering blood pressure with doses of phenylephrine hydrochloride and sodium nitroprusside. These tests were performed before and after administration of naloxone (40 micrograms/kg). Naloxone significantly blunted baroreflex function (-55%). Treatment with atropine and vagotomy revealed that the action of naloxone appears to be dually mediated by the parasympathetic and sympathetic nervous systems. These data suggest that opioids can influence baroreflex function in the dog.