An adrenergic alpha-agonist (or alpha-adrenergic agonists) are a class sympathomimetic agents that selectively stimulates alpha adrenergic receptors. The alpha-adrenergic receptor has two subclasses α1 and α2. Alpha 2 receptors are associated with sympatholytic properties. α Adrenergic agonists have the opposite function of alpha blockers. Alpha adrenoreceptor ligands mimic the action of epinephrine and norepinephrine signaling in the heart, smooth muscle and central nervous system, with norepinephrine being the highest affinity. The activation of α1 and α2 inhibits the enzyme Adenylate cyclase. This in turn leads to the inactivation of the secondary messenger Cyclic adenosine monophosphate and induces smooth muscle and blood vessel constriction.