Binding of phenoxybenzamine to rat platelets and platelet fractions

Irreversible binding of labelled phenoxybenzamine to platelets suspended in Tyrode solution was complete by 30 min and was proportional to the concentration incubated and non-saturable. Evidence for a reversible interaction was obtained at earlier time periods. Firmly bound phenoxybenzamine was found mainly in the supernatant (105,000 times g) of platelet homogenates. Small amounts were also detected in the 10,000 times g pellet. The labelled material in the supernatant fraction appeared to have a molecular weight of 500,000 as indicated by gel column chromatography. It was non-dialyzable, precipitated by 40-80% ammonium sulfate, and was absent in smooth muscle from the vas deferens. The material was found to bind norephinephrine but did not alter platelet aggregation. A platelet thrombosthenin fraction but not albumin or platelet fibrinogen bound phenoxybenzamine and had properties similar to the material in the supernatant fraction. The relationship of the phenoxybenzamine-binding material to thrombosthenin is discussed.