Determination of alpha-adrenergic blocking potency.

Determination of the alpha-adrenergic blocking potency of drugs in humans is usually done by measuring the shift in the blood pressure versus logarithm of intravenous phenylephrine dose-response relationship. Change in blood pressure activates homeostatic reflexes that may change this relationship. This study examines the effect of autonomic (beta 1- and beta 2-adrenergic, parasympathetic, and alpha-adrenergic) blockade on the dose versus blood pressure response relationship to sequential doses of phenylephrine in humans. Phenylephrine dose responses were conducted under controlled conditions, during propranolol and atropine infusion, during prazosin-induced alpha 1-adrenergic blockade, and during prazosin, propranolol, and atropine administration. Propranolol-atropine infusion decreased the threshold dose of phenylephrine required to increase mean blood pressure (p less than 0.00001), increased the slope of the phenylephrine dose versus increase in mean blood pressure relationship (p = 0.019), and and decreased the dose of phenylephrine required to increase mean blood pressure by 20 mm Hg (p less than 0.00001). Determination of the alpha-adrenergic blocking potency of prazosin was not affected by autonomic blockade with propranolol and atropine (dose ratio 5.2 before and 5.0 after autonomic blockade; p = 0.465). We conclude that beta 1- and beta 2-adrenergic and muscarinic blockade increase sensitivity to phenylephrine by increasing the slope and decreasing the threshold dose of the phenylephrine dose-response curve, and that alpha-adrenergic-blocking potency of prazosin may be determined with or without blocking homeostatic blood pressure regulatory mechanisms in humans.