Cooling enhances alpha2-adrenoceptor-mediated vasoconstriction in human hand veins

The contribution of different receptor subtypes in the contractile response during cooling in human hand vessels is of considerable interest in the understanding of cold-induced peripheral vasospasm as it appears in Raynaud's phenomenon. Subcutaneous vein segments from 50 patients undergoing hand operations not related to vascular disorders were examined in vitro. The temperature in the organ bath was initially 37 °C and was either continuously lowered to 10 °C or kept constant at 37 °C, 29 °C or 20 °C. The characteristics of the α-adrenoceptor-mediated motor response were elucidated with the use of the α₁-antagonist, prazosin, and the α₂-antagonist, yohimbine. A great variability between individuals in the proportions of α₁- and α₂-adrenoceptors was found. In the majority of the vessels continuous cooling to 25 °C augmented a noradrenaline-induced contraction. This augmentation was unaltered in the presence of prazosin but abolished by yohimbine, suggesting that it was mediated via the α₂-adrenoceptor. In the remaining vessels with a predominating α₁-adrenoceptor-mediated response a cold-induced relaxation was registered. This could be the result of a reduced α₁-adrenoceptor-mediated contraction at this low temperature. These varying reactions to cooling were unaffected by the β-antagonist, propranolol, and by endothelial denudation. The results obtained in corresponding experiments with the α₁-agonist methoxamine and α₂-agonist, oxymetazoline, were conflicting, probably due to the poor selectivity of these agonists in human tissues. The results suggest that in patients with peripheral cold-induced vasospasm administration of an α₂-adrenoceptor antagonist might be of therapeutic interest in order to relieve vasospasm in the hand vessels.