Human vascular alpha-adrenoceptors. A study of peripheral arteries and veins in vitro and in vivo.

By means of a sensitive myograph and alpha-receptor subtype selective agonists and antagonists the postjunctional alpha-adrenoceptors were characterized in small omental and groin arteries and veins and in long saphenous veins obtained from 52 "healthy" adults during surgery. These vessels were found to have a mixed population of contraction mediating postjunctional alpha-receptors with a predominance of apha1-receptors in the arteries and a predominance of alpha2-receptors in the veins, although regional variations occurred. Noradrenaline was 5-10 more potent in the veins than in the arteries. Changes in diameter of the long saphenous vein in vivo were measured in 6 healthy subjects by means of infrared light and a phototransistor; the alpha1-receptor antagonist prazosin did not significantly antagonize the noradrenaline induced contraction of the saphenous vein in vitro, but was a potent antagonist in vivo whereasthe alpha2-receptor antagonist yohimbine was potent both in vitro and in vivo. Vessels obtained from the legs of 8 patients with severe ischemia due to atherosclerosis were studied in vitro. The contractile capacity of the arteries was generally poor whereas the veins behaved as vessels from health individuals. Eight patients with similar vascular disease were studied in vivo. Noradrenaline caused a more than 70% reduction of femoral artery blood flow in all patients without systemic effects. Yohimbine increased femoral blood flow in five patients, and decreased it in three, whereas prazosin increased it in all patients (by up to 150%). Papaverine increased the femoral blood flow by more than 350%. Prazosin, yohimbine and papaverine had all similar effects on the skin blood flow (measured by laser doppler technique), decreasing it in 6 patients and leaving it unchanged in 2 patients. Epidural anesthesia did not abolish the effects of noradrenaline. In spite of epidural anesthesia prazosin and papaverine increased femoral blood flow by up to 100% and 200% respectively