The action of prazosin in human vascular preparations.

To identify the mode of action of prazosin, its effects on isolated human vascular preparations were studied. Isometric tension was recorded from spiral strips of dorsal metacarpal veins, common palmar digital arteries and uterine, splenic and ileocolic arteries. Prazosin was a potent antagonist of noradrenaline, but not 5-hydroxytryptamine or barium chloride in the metacarpal veins. Higher concentrations of prazosin were necessary to antagonize noradrenaline in the visceral arteries. The digital arteries were resistant to its sympatholytic effect. It is concluded that prazosin acts as an alpha-adrenoceptor antagonist of different potency in various vascular beds. It is postulated that the initial cardiovascular side effects of prazosin are due to venous pooling and that the long term antihypertensive effect is the result of reduced peripheral resistance because of interference with arterial sympathetic tone.