The endogenous catecholamines epinephrine and norepinephrine have profound effects on smooth muscle activity, cardiac function, carbohydrate and fat metabolism, hormone secretion, neurotransmitter release, and central nervous system actions. These activities are mediated by GPCRs belonging to two subfamilies, the alpha- and beta-adrenergic receptors. The beta-adrenergic receptors, primarily the beta2 subtype, mediate relaxation of smooth muscle in many tissues, and beta2-selective agonists are the preferred drugs for stimulating bronchodilation in the treatment of asthma and chronic obstructive pulmonary disease. Activation of the beta-adrenergic receptors, primarily the beta1 subtype and to a lesser extent the beta2 subtype, acutely increases heart rate, cardiac output, and cardiac automaticity, and chronically increases cardiac myocyte apoptosis. As a result, beta-adrenergic receptor antagonists (beta blockers) are effective in the treatment of congestive heart failure and arrhythmia.