Chloroethylclonidine unmasks a non-alpha-adrenoceptor noradrenaline binding site in the rat aorta

The effect of chloroethylclonidine on noradrenaline-induced contractions of the rat aorta was studied. Chloroethylclonidine (1-5 X 10(-5) M) shifted noradrenaline dose-response curve to the right approximately 5000-fold without depressing the maximum. The response to noradrenaline after chloroethylclonidine was not antagonized by phenoxybenzamine (10(-7) M), prazosin (10(-7) M), WB 4101 (10(-7) M) nor yohimbine (10(-5) M) and is therefore not mediated via alpha 1-adrenoceptors. These results would suggest that there is a homogenous population of chloroethylclonidine-sensitive alpha 1-adrenoceptors in the rat aorta and that chloroethylclonidine treatment reveals a non-alpha-adrenoceptor noradrenaline binding site in this tissue.