Effect of prazosin on veins: Inhibition of norepinephrine venoconstriction in dog forelimb

A dose–response curve was established based on increases in venous infusion pressure evoked by injection of a series of small doses of norepinephrine, before and after slow infusion of a prazosin solution into the accessory cephalic vein of anesthetized dogs. During the prazosin infusion, there was an increase in venous infusion pressure, which returned to control levels by the end of the infusion period. The reduction in arterial pressure that occurred during the infusion persisted throughout the experiment. Immediately following the prazosin infusion, venous constrictor responses to norepinephrine were greatly inhibited, an effect that was still present 1 hr later. Prazosin has a marked in vivo α‐adrenergic receptor inhibiting action in the accessory cephalic vein of the dog. This effect may be preceded by an agonistic action on these receptors at this site, as suggested by the increase in venous infusion pressure during the period of prazosin administration.