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 α- 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 a1 adrenoceptors also influences cell proliferation; α1A inhibits cell growth by arresting progression at the G1/S transition (Shibata et al., 2003). The α1A membrane preparations are crude membrane preparations made from our proprietary stable recombinant cell lines to ensure high-level of GPCR surface expression; thus, they are ideal HTS tools for screening of antagonists of α1A interactions with prazosin. The membrane preparations exhibit a Kd of 1.34 nM for [3H]-prazosin. With 5 mg/well α1A Membrane Prep and 1 nM [3H]-prazosin, a greater than 7-fold signal-to-background ratio was obtained.