Alpha receptors mediate many important actions of endogenous catecholamines.
The antagonism which is observed in most cases is competitive. C. Cardiovascular Effects of Alpha 1 Antagonists 1. Alpha 1 antagonists block vasoconstriction induced by endogenous catecholamines. The resulting fall in peripheral resistance leads to a fall in mean blood pressure. The magnitude of this effect is dependent upon the degree of sympathetic tone at the time the antagonist is administered.