An antihypertensive agent that blocks postsynaptic α1-adrenergic receptors in the periphery (IC50 = 0.20 nM in dog aorta) and in the brain (IC50 = 1.7 nM in rat brain); also a potent antagonist at the melatonin MT3 receptor (Ki = 10.2 - 12.4 nM; IC50 = 7.8 nM in hamster)