Receptors involved in the regulation of vascular tone

In vascular smooth muscle the occurrence of several rather different receptors can be demonstrated: alpha 1/ alpha 2- and beta 1/beta 2-adrenoceptors; muscarinic (mainly M2)-cholinoceptors; dopaminergic (DA1 and DA2), angiotensin II and serotonergic receptors (5HT1 and 5HT2). Apart from this variety in receptor subtypes a distinction should be made between pre- and postsynaptic (pre- and postjunctional) receptors. The characteristics and functionality of these numerous receptor subtypes are the subject of the present survey, with an emphasis on the selective agonists and antagonists which interact with the various receptor populations. Although dopaminergic, serotonergic and angiotensin II-receptors can be demonstrated to exist in vascular smooth muscle and also in the sympathetic system which innervates the circulatory tract, their relevance with respect to the regulation and maintenance of vascular tone remains obscure. It seems clear, in any way, that the levels of the circulating agonists of these receptors, that is dopamine, angiotensin II and serotonin, are too low to cause any significant degree of receptor activation. As far as can be judged at present, vascular tone is predominantly maintained by sympathetic stimuli, via the mediation of postsynaptic alpha 1- and alpha 2-adrenoceptors, vascular beta 2-receptors being of limited importance. Muscarinic receptors of the M2-subtype should be considered as well, although their quantitative role is probably rather variable and less important than that of alpha-adrenoceptors.