Precapillary vessels: effects of the sympathetic nervous system and of catecholamines

The study of receptor-operated control mechanisms involved in adrenergic transmission in the blood vessel wall has substantially advanced the understanding of the factors involved in the maintenance of arteriolar tone. As a result of this work, alpha-adrenoreceptors have been divided into two major subtypes termed alpha 1- and alpha 2-adrenoreceptors. Initially, the term alpha 1 was proposed as a synonym for the postjunctional alpha-adrenoreceptor. There is now a large body of evidence for the existence of postjunctional alpha-adrenoreceptors mediating the contraction of vascular smooth muscles that are resistant to blockade by the alpha 1-adrenoreceptor antagonist prazosin, but are blocked by alpha 2-antagonists and can be activated by selective alpha 2-agonists but not by selective alpha 1-agonists. The physiological significance of postjunctional alpha 2-adrenoreceptors is discussed in respect to factors such as their role in blood pressure control and of whether they are innervated or not, and the differences in contractile responses and in the postreceptor mechanisms involved in stimulation-contraction coupling following activation of each receptor subtype are reviewed. Although the situation is complex, on balance the data are compatible with the view that alpha 2-adrenoreceptor-mediated responses in vascular smooth muscle are more dependent on the physiological environment than are responses mediated by alpha 1-adrenoceptors. Thus, although postjunctional alpha 2-adrenoreceptors can be involved in sympathetic nerve transmission, they appear largely to fulfill a humoral role in vascular smooth muscle closely allied to the prevailing conditions, whereas alpha 1-adrenoreceptors are primarily involved in sympathetic neurotransmission, independent of local environmental conditions.