Human postjunctional alpha-1 and alpha-2 adrenoceptors: differential distribution in arteries of the limbs

Experiments were performed in order to characterize the post-junctional alpha adrenoceptors that mediate contraction in arteries of human limbs. Blood vessels were obtained from patients undergoing amputation of an extremity for reasons other than vascular disease. Proximal (dorsalis pedis and arcuate arteries of the foot, superficial palmer arch of the hand) and distal (digital arteries of the foot and hand) blood vessels were studied from each limb. The blood vessels were removed within 60 min of amputation and were suspended for isometric tension recording in modified Krebs-Ringer bicarbonate solution. In proximal and distal arteries, alpha-1 adrenergic blockade with prazosin produced a nonparallel shift in the concentration-effect curve to high compared to low concentrations of the agonist. In contrast, alpha-2 adrenergic blockade with rauwolscine was more effective against responses evoked by low concentrations of norepinephrine. This suggests that the alpha-2 adrenergic component of the response to norepinephrine is a low-maximum effect compared to the alpha-1 adrenergic component. Prazosin was less potent and rauwolscine more potent in distal arteries, compared to proximal arteries which might indicate an increased alpha-2 adrenergic response in distal arteries. The selective alpha-1 adrenergic agonist, phenylephrine, produced similar responses in proximal and distal arteries. However, the selective alpha-2 adrenergic agonist, B-HT 920, caused greater contractile responses in distal arteries compared to proximal arteries. The results suggest that alpha-1 and alpha-2 adrenoceptors are present on the vascular smooth muscle of arteries of human limbs, and that alpha-2 adrenoceptors are more prominent on distal arteries. This may be related to an increased contribution of the distal arteries to thermoregulation.