Expressed by Alpha Adrenergic Receptor Genes in effector tissues innervated by postganglionic sympathetic adrenergic fibers, Alpha Adrenergic Receptors are one of two major classes of adrenergic receptors (alpha and beta) based on their reactions to norepinephrine and epinephrine, on cellular effects of receptor activation, and on relative affinities and reactions to synthetic blocking or stimulating agents. Alpha-adrenergic receptors respond to blocking agents such as phenoxybenzamine and phentolamine. When coupled to adenylate cyclase through Gi protein, Alpha Adrenergic Receptor stimulation results in decreased intracellular cyclic AMP. [ ]