Effects on Vascular Smooth Muscle

Postjunctional ^-adrenoceptors are always found in veins, arteries, and arterioles. Activation of these receptors results in the entry of extracellular calcium through receptor-operated channels and in the release of intra-cellularly stored calcium; this is brought about through the participation of the inositol triphosphate second-

messenger system. This system plays an important role in the regulation of blood pressure and vascular tone.

Vascular endothelium also plays an important role in maintaining vascular tone. The endothelium can modulate both vasodilation and vasoconstriction through its ability to locally synthesize and release vasodilators such as nitric oxide, endothelium-derived hyperpolarizing factor, and PGI2, and vasoconstrictors such as endothelin, which in turn directly affect vascular smooth muscle activity. Stimulation of ^-adrenoceptors located on the endothe-lial cells in certain vascular beds (such as the coronary artery) results in the release of nitric oxide and vasodilation.

In any blood vessel, the final integrated response to either neuronally released norepinephrine or to circulating epinephrine probably depends on the relative participation of at least four populations of «-adrenoceptors: postjunctional ar and ^-adrenoceptors mediate constriction of vascular smooth muscle, while pre-junctional and endothelial ^-adrenoceptors mediate vasodilation. An understanding of the vessel vascular response to adrenomimetic drugs also must include the effects of drugs on adventitial innervation, smooth muscle, and other vascular factors that may be present.

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