Precapillary Vessels

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Abstract
Summary: 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, α-adrenoreceptors have been divided into two major subtypes termed α1- and α2-adrenoreceptors. Initially, the term α1 was proposed as a synonym for the postjunctional α-adrenoreceptor. There is now a large body of evidence for the existence of postjunctional α-adrenoreceptors mediating the contraction of vascular smooth muscles that are resistant to blockade by the α1-adrenoreceptor antagonist prazosin, but are blocked by α2-antagonists and can be activated by selective α2-agonists but not by selective α1-agonists. The physiological significance of postjunctional α2-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 α2-adrenoreceptor-mediated responses in vascular smooth muscle are more dependent on the physiological environment than are responses mediated by α1-adrenoceptors. Thus, although postjunctional α2-adrenoreceptors can be involved in sympathetic nerve transmission, they appear largely to fulfil a humoral role in vascular smooth muscle closely allied to the prevailing conditions, whereas α1-adrenoreceptors are primarily involved in sympathetic neurotransmission, independent of local environmental conditions.

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