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 β-adrenoceptors (Bylund et al., 1994). The three members of the α1 subclass of adrenoceptors, α1A, α1B and α1D, 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 α1A subtype plays a prominent role in urogenital smooth muscle contraction and renal artery contraction (Hrometz et al., 1999; Ruffolo and Hieble, 1999). Activation of α1 adrenoceptors also influences cell proliferation; α1A inhibits cell growth by arresting progression at the G1/S transition (Shibata et al., 2003). The α1A subtype undergoes alternative splicing to generate four variants that differ at their C-termini, although these variants appear to be functionally identical (Chang et al., 1998). Chemicon's cloned human α1A-expressing cell line is made in the Chem-1 host, which supports high levels of recombinant α1A expression on the cell surface and contains high levels of the promiscuous G protein Gα15 to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for antagonists of interactions between α1A and its ligands.