Evidence for antagonistic activity of endothelin for clonidine induced hypotension and bradycardia.

Effect of endothelin (ET) on clonidine induced cardiovascular effects was studied in male Sprague-Dawley rats. Clonidine (75 micrograms/kg, iv) produced significant decrease in blood pressure and heart rate. ET-1 (50 ng/kg, iv) pretreatment completely antagonized the hypotension and bradycardia induced by clonidine. ET-2 (50 ng/kg, iv) and ET-3 (50 ng/kg, iv) had similar antagonistic effect on clonidine induced hypotension and bradycardia. The antagonistic effect of ET lasted for several hours, however, 4 hours after ET pretreatment only partial blockade of clonidine induced hypotension and bradycardia was observed. This indicated that the antagonistic effect of ET was reversible. Initial hypertensive response induced by high dose of clonidine (750 micrograms/kg, iv) could not be antagonized by ET-1, ET-2 or ET-3, while phenoxybenzamine, an alpha adrenoceptor antagonist, blocked the hypertensive response of clonidine. Thus, ET has no antagonistic effect on the initial hypertensive response but antagonizes the hypotensive and bradycardic effect induced by clonidine. Clonidine induced hypotension and bradycardia are mediated through central alpha 2 adrenoceptors while hypertension is mediated through peripheral alpha 2 adrenoceptors. It is concluded that central alpha 2 adrenoceptors are different from peripheral alpha 2 adrenoceptors and ET antagonizes the effect of clonidine only on central alpha 2 adrenoceptors but has no antagonistic activity on peripheral alpha 2 adrenoceptors.