Purity >98%. The Alpha1 -adrenergic receptors are Gq protein-coupled receptors that play a key role in the modulation of sympathetic nervous system activity and are the site of action for therapeutic agents, such as antihypertensive drugs. A-61603 is a selective Alpha1A -adrenergic receptor agonist that is at least 35-fold more potent at Alpha1A receptor sites than at Alpha1B or Alpha1D . Activation of the Alpha1A -adrenergic receptor by A-61603 has been reported to increase the frequency of spontaneous Ca2+ transients in rat ventricular myocytes in vitro (EC50 = 6.9 nM) much more potently than activation by phenylephrine (; EC50 = 2.3 μM).