Inhibition of adrenaline-induced platelet aggregation by the orally administered α-adrenergic receptor blocker phentolamine (regitine®)

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Abstract
In eight healthy volunteers pretreatment with phentolamine 40 mg p o inhibited platelet aggregation (1st and 2nd phases) induced by low concentrations of adrenaline (2, 1 and 0.5 µM) in plasma from blood sampled 30 min after administration of the compound. The lower the concentration of adrenaline used, the greater was the degree of inhibition elicited. These results are indicative of competitive inhibition of the action of adrenaline on the platelet membrane by phentolamine. Four to six hours after administration of the compound, the aggregation characteristics had reverted to normal. It is concluded that the increased tendency toward platelet aggregation associated with elevated blood levels of catecholamines can be prevented by therapeutic doses of phentolamine.