Endothelial alpha 2-adrenoceptors in canine pulmonary and systemic blood vessels.

The present study was designed to determine whether alpha 2-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 degrees 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 alpha 2-adrenoceptor agonist UK 14,304, but not of that to the selective alpha 1-adrenoceptor agonist phenylephrine. In the presence of alpha 1- and beta-adrenoceptor blockers, norepinephrine and UK 14,304 initiated relaxations of previously contracted rings. These relaxations were inhibited by the alpha 2-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 alpha 2-adrenergic activation of the vascular smooth muscle.