THE catecholamines norepinephrine and epinephrine are key regulators of many physiologic events in human beings; norepinephrine acts primarily as a neurotransmitter released from sympathetic-nerve terminals, and epinephrine functions as a circulating hormone released from the adrenal medulla. These catecholamines initiate target-cell responses by binding to specific recognition sites, the adrenergic receptors. That receptors are the initial decoders of extracellular messages is a concept that has guided research on hormone and neurotransmitter action for many years. Studies of adrenergic receptors focused first on physiologic responses that are promoted by adrenergic stimulation and demonstrated the existence of two primary types of adrenergic . . .