Spare alpha adrenoceptors in the peripheral circulation: excitation-contraction coupling.

Postsynaptic alpha adrenoceptors in arteries and veins represent a mixed population of alpha 1 and alpha 2 adrenoceptors, with both subtypes mediating vasoconstriction. In the peripheral arterial circulation, postsynaptic vascular alpha 1 adrenoceptors are found in the adrenergic neuroeffector junction, whereas postsynaptic vascular alpha 2 adrenoceptors are located extrajunctionally. In the venous circulation, it appears that alpha 2 adrenoceptors may be predominantly junctional, whereas alpha 1 adrenoceptors may be predominantly extrajunctional. In general, alpha 1 adrenoceptors play a more important functional role in arteries than in veins, with the converse being true for postsynaptic vascular alpha 2 adrenoceptors. The relationship between alpha-adrenoceptor occupancy and vasoconstrictor response is more favorable for postsynaptic vascular alpha 1 adrenoceptors than for alpha 2 adrenoceptors in both arteries and veins, and there is evidence for a receptor reserve in alpha 1 adrenoceptors in both the arterial and venous circulation. No reserve in postsynaptic vascular alpha 2 adrenoceptors is seen in the arterial circulation, but in isolated venous preparations, a reserve in alpha 2 adrenoceptors has been observed. It has been suggested that spare alpha 2 adrenoceptors found in veins, but not arteries, may be responsible, at least in part, for the exaggerated alpha 2-adrenoceptor-mediated response of veins relative to arteries.