Alpha 2-adrenoceptors and endothelium-derived relaxing factor.
The endothelium can release potent vasodilator substances, in particular prostacyclin and endothelium-derived relaxing factor. The triggers for the release of endothelium-derived relaxing factor include increases in levels of shear stress, neurotransmitters, autacoids, platelet products, and hormones. The endothelium-dependent response to catecholamines involves stimulation of alpha 2-adrenoceptors on the endothelial cells. Indeed, in a number of blood vessels, selective alpha 2-adrenergic agonists cause endothelium-dependent relaxations. These are seen most typically in blood vessels with long-term exposure to high flows and high partial pressures of oxygen. In addition to the release of endothelium-derived relaxing factor, alpha 2-adrenergic agonists can stimulate postjunctional (postsynaptic) alpha 2-adrenoceptors on vascular smooth muscles. These receptors, which are more abundant in hypertensive blood vessels, activate the contractile process. However, the alpha 2-adrenergic vasoconstrictors act as partial agonists (with a limited receptor reserve) and hence their vasoconstrictor response is very sensitive to functional antagonists such as endothelium-derived relaxing factor. Thus, the presence of endothelial cells can blunt the vasoconstrictor response to these substances not only because of an augmented release of endothelium-derived relaxing factor but also because the vasoconstriction that they induce is particularly susceptible to the inhibitory effect of the factor.
Vanhoutte
P M
PM
Department of Physiology and Biophysics, Mayo Clinic, Rochester, Minnesota 55905.
Miller
V M
VM
D000818:Animals; D001685:Biological Factors / Q000378:metabolism / Q000502:physiology*; D002395:Catecholamines / Q000502:physiology; D004730:Endothelium, Vascular / Q000378:metabolism / Q000502:physiology*; D009569:Nitric Oxide; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D014664:Vasodilation
D001685:Biological Factors; D002395:Catecholamines; D011942:Receptors, Adrenergic, alpha; D009569:Nitric Oxide