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Contributions of Alpha 1 and Alpha 2-Adrenoceptors to Contractile Response in Canine Blood Vessels
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Abstract
Strips of canine saphenous vein, inferior vena cava, and femoral artery were studied isometrically in vitro to compare quantitatively the α<sub>1</sub>- and postsynaptic α<sub>2</sub>-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 α<sub>1</sub> -receptors with prazosin, or α<sub>2</sub>-receptors with rauwolscine. Appropriate concentrations of the antagonists were calculated from dissociation constants previously determined by binding or competition with [<sup>3</sup>H]prazosin or [<sup>3</sup>H]rauwolscine in tissue homogenates. The contribution of α<sub>1</sub>-adrenoceptors was larger than that of α<sub>2</sub>-receptors in all vessels. The α<sub>2</sub>-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 α<sub>1</sub>-receptors was almost linear, without the marked upward convexity reported in some other vessels. α<sub>2</sub>-Occupation-response curves were convex towards the occupation axis, with a relatively small response at low levels of occupation.