Natural or synthetic Alpha-adrenergic Blocking Agents selectively bind to adrenergic receptors, or compete with natural adrenergic mediators, and inactivate alpha adrenergic receptors of the sympathetic nervous system. These agents block the action of natural sympathomimetic neurotransmitters (norepinephrine and related substances), and can initiate physiological responses such as vasodilation, bradycardia, and pupil contraction. (NCI04)