Sympathetic amines exert effects on four different receptors:
Alpha-receptors provoke vasoconstriction of peripheral arteries when stimulated.
Beta-receptors increase atrial and ventricular contraction, increase heart rate by stimulating the sinus node, and enhance AV conduction.
Beta2-receptors promote bronchodilation and vasodilation.
Dopaminergic receptors, found in various tissues including blood vessels and nervous system tissues, are of two types: The activation of dopamine-1 receptors causes vasodilation in coronary, renal, mesenteric, and cerebral vascular beds, by stimulating adenylate cyclase and increasing cAMP. The activation of dopamine-2 receptors causes vasodilation, but by inhibiting transmission of sympathetic nerve endings.
What amines are alpha or beta selective?
Isoproterenol has practically no alpha-stimulating effect, with relatively pure beta-stimulation. It is clinically valuable in stimulating the sinus node and enhancing AV conduction in bradyarrhythmias. Its limitation in this setting is its hypotensive effect through its betaj properties.
Methoxamine is a practically a pure alpha-stimulant, helpful in hypotension in the cardiac catheterization laboratory.
Norepinephrine is predominantly alpha- and beta-stimulant and is valuable in cardiogenic shock following bypass surgery for stunned myocardium, and in hypotension.