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-adrenoceptors. The three members of the alpha1 subclass of adrenoceptors, alpha1A, alpha1B and alpha1D, couple to Gq, and promote contraction of vascular and urinary tract smooth muscle, relaxation of intestinal smooth muscle, increased contractile force in the heart, and glycogenolysis and gluconeogenesis in the liver. The different subtypes have overlapping distributions and variably contribute to these effects depending on species and tissue; the alpha1A subtype plays a prominent role in urogenital smooth muscle contraction and renal artery contraction. Activation of alpha1 adrenoceptors also influences cell proliferation; alpha1A inhibits cell growth by arresting progression at the G1/S transition. The alpha1A subtype undergoes alternative splicing to generate four variants that differ at their C-termini, although these variants appear to be functionally identical.