The adrenergic receptors exert opposite physiologic effects in the vascular smooth muscle under activation:
alpha-1 receptors. Under NE binding alpha-1 receptors cause
vasoconstriction (contraction of the vascular smooth muscle cells decreasing the diameter of the vessels). These receptors are activated in response to shock or low blood pressure as a defensive reaction trying to restore the normal blood pressure. Antagonists of alpha-1 receptors (
doxazosin,
prazosin) cause
vasodilation (a decrease in vascular smooth muscle tone with increase of vessel diameter and decrease of the blood pressure). (See also
receptor antagonist)
alpha-2 receptors.
Agonists of alpha-2 receptors in the vascular smooth muscle lead to vasoconstriction. However, in clinical practice drugs applied intravenously that are agonists of alpha-2 receptors such as
clonidine lead to powerful vasodilation, which causes a decrease in blood pressure by presynaptic activation of the receptors in the sympathetic ganglia. This presynaptic effect is predominant and completely overrides the vasoconstrictive effect of the alpha-2 receptors in the vascular smooth muscle.[citation needed]
beta-2 receptors. Agonism of beta-2 receptors causes
vasodilation and
low blood pressure (i.e. the effect is opposite of the one resulting from activation of alpha-1 and alpha-2 receptors in the vascular smooth muscle cells). Usage of beta-2 receptor agonists as hypotensive agents is less widespread due to adverse effects such as unnecessary
bronchodilation in lungs and increase in
blood sugar levels.
Function
Vascular smooth muscle contracts or relaxes to change both the volume of blood vessels and the local
blood pressure, a mechanism that is responsible for the redistribution of the blood within the body to areas where it is needed (i.e. areas with temporarily enhanced oxygen consumption). Thus the main function of vascular smooth
muscle tone is to regulate the caliber of the blood vessels in the body. Excessive
vasoconstriction leads to
high blood pressure, while excessive
vasodilation as in shock leads to
low blood pressure.
Arteries have a great deal more
smooth muscle within their walls than
veins, thus their greater wall thickness. This is because they have to carry pumped blood away from the heart to all the organs and tissues that need the oxygenated blood. The
endothelial lining of each is similar.