Endothelial α2-adrenoceptors in canine pulmonary and systemic blood vessels

Abstract

The present study was designed to determine whether α2-adrenoceptors mediate endothelium-dependent relaxations in veins and arteries of the pulmonary and systemic vasculature of the dog. Rings of either pulmonary or femoral artery and vein with and without endothelium were suspended for isometric force measurements in a buffered salt solution (37°C; gassed with 95% O2 and 5% CO2). Removal of the endothelium caused a shift to the left of the concentration-response curves to epinephrine, norepinephrine and the selective α2-adrenoceptor agonist UK 14,304, but not of that to the selective α1-adrenoceptor agonist phenylephrine. In the presence of α1- and β-adrenoceptor blockers, norepinephrine and UK 14,304 initiated relaxations of previously contracted rings. These relaxations were inhibited by the α2-adrenoceptor antagonist, rauwolscine, and were absent in rings without endothelium. In the pulmonary artery and femoral vein, the endothelium-dependent responses to UK 14,304 were masked in part by direct α2-adrenergic activation of the vascular smooth muscle.