Effect of Naloxone on the Hypotensive Action of Clonidine in the Conscious, Normotensive Goat

The possible involvement of opioid receptors in the effects of clonidine was studied in conscious, normotensive goats. Clonidine was infused intravenously followed by an intravenous injection of naloxone. Blood pressure was recorded through an intraarterial catheter. All doses of clonidine (0.2-7 micrograms/kg) significantly lowered blood pressure and decreased heart rate. Hypotension after 0.2-0.7 micrograms/kg of clonidine was significantly reduced by naloxone (1 mg/kg), while naloxone did not antagonize hypotension induced by the larger doses of clonidine. The bradycardiac effect of 0.7-2 micrograms/kg of clonidine was reduced by naloxone while that of 0.2 and 7 micrograms/kg was not. Clonidine also induced sedation and respiratory irregularity, but these changes were not modified by naloxone. Opioid mechanisms possibly participate in the hypotensive action of clonidine, but less in the normotensive animal than in spontaneously hypertensive animals.