Contributions of Alpha 1 and Alpha 2 -Adrenoceptors to Contractile Response in Canine Blood Vessels

Strips of canine saphenous vein, inferior vena cava, and femoral artery were studied isometrically in vitro to compare quantitatively the α1- and postsynaptic α2-adrenoceptor contributions to the contractile force generated by l-norepinephrine (NE). Effects mediated by each receptor type were measured independently by quantitative blockade of virtually all α1 -receptors with prazosin, or α2-receptors with rauwolscine. Appropriate concentrations of the antagonists were calculated from dissociation constants previously determined by binding or competition with [3H]prazosin or [3H]rauwolscine in tissue homogenates. The contribution of α1-adrenoceptors was larger than that of α2-receptors in all vessels. The α2-type was responsible for 38% of the maximum unblocked response to NE in saphenous vein, 32% in vena cava, and 28% in femoral artery. The occupation-response relationship for α1-receptors was almost linear, without the marked upward convexity reported in some other vessels. α2-Occupation-response curves were convex towards the occupation axis, with a relatively small response at low levels of occupation.