Responses of human arterial muscle to stable vasoactive substances released from human platelets by collagen and heat aggregated IgG.

The supernatant solutions obtained after aggregation or sonication of washed human platelets were superfused over preparations of human isolated digital arteries using a small volume bioassay method. The agents released from the platelets caused strong contractions of the artery strips. Platelet aggregation induced by 10 micrograms/ml collagen or by 100 micrograms/ml heat aggregated IgG, released 31.5% and 38.5% respectively, of the contractile activity produced by sonication of the platelets. The quantitative contractile effect of supernatants from platelets aggregated by 50 micrograms/ml IgG was significantly less than that for 100 micrograms/ml HA IgG. Similarly, the maximum contractile effect of supernatants from platelets aggregated by 300 ng/ml collagen was significantly less than that for 1 microgram/ml collagen. This suggests that the concentration of contractile agents released from platelets depends on the concentration of aggregating stimulus. Comparison with concentration-effect curves for exogenous serotonin suggests that if the contractility of the platelet supernatant occurring after sonication of platelets is solely due to serotonin, then it is present in a concentration of approximately 3.3 X 10(-6) mol/l (6.6 nmol per 10(9) platelets). It is suggested from this study that in certain clinical situations characterized by hypertension, and in which circulating immune complexes have been found, in vivo platelet activation by immune complexes may be releasing sufficient concentrations of serotonin to constrict peripheral blood vessels and contribute to the hypertension.