Indication For the treatment of symptomatic BPH and mild to moderate hypertension. Mechanism of action In general, a1-adrenergic receptors mediate contraction and hypertrophic growth of smooth muscle cells. a1-Receptors are 7-transmembrane domain receptors coupled to G proteins, Gq/11. Three a1-receptor subtypes, which share approximately 75% homology in their transmembrane domains, have been identified: a1A (chromosome 8), a1B (chromosome 5), and a1D (chromosome 20). Terazosin is the first a1-receptor antagonist to demonstrate selectivity for the a1A-receptor. All three receptor subtypes appear to be involved in maintaining vascular tone. The a1A-receptor maintains basal vascular tone while the a1B-receptor mediates the vasocontrictory effects of exogenous a1-agonists. Activation of a1-receptors activates Gq-proteins, which results in intracellular stimulation of phospholipases C, A2, and