a receptor, found on the surface of some cells of the sympathetic nervous system, that is stimulated by adrenergic substances resulting in constriction of blood vessels and contraction of most smooth muscle
See alpha receptor in American Heritage Dictionary 4
(ălˈfə-rĭ-sĕpˌtər)
noun
A site in the autonomic nervous system in which excitatory responses occur when adrenergic agents, such as norepinephrine and epinephrine, are released. Activation of alpha-receptors causes various physiological reactions, including the stimulation of associated muscles and the constriction of blood vessels. Also called alpha-adrenergic receptor.