Role of platelet serotonin in the canine pulmonary response to endotoxin

There is evidence suggesting a role for platelet serotonin (5-HT) in the immediate pulmonary response to endotoxin in the dog (J. Appl. Physiol. 23: 47, 1967). To further define this role, autologous canine platelets were labeled with 5-[14C]HT in vitro and then reinfused. Subsequently Escherichia coli endotoxin (0.55:B-5, Difco), 2.5 mg/kg, was injected. Within 5 min dynamic compliance (CL) fell by more than 50%, and nonelastic resistance (RL) increased by more than 200%. Despite a 95% decrease in platelet count, less than 10% of the platelet 5-HT was released as determined by changes in the radioactivity of platelet-poor plasma (PPP) prepared from both aortic and pulmonary artery blood. As a positive control, injected of bovine collagen produced a similar decrease in platelet count that was associated with a significant increase in the radioactivity of aortic and pulmonary artery PPP. FInally, rapid injection of a dose of 5-HT equivalent to 25% of the 5-HT in circulating platelets did not cause a change in CL or RL equivalent to that produced by endotoxin. From these data we conclude that endotoxin injection does not cause immediate massive platelet activation and that platelet 5-HT does not play a major role in the immediate pulmonary response to endotoxin.