Endotoxin and dietary amines may increase plasma 5-hydroxytryptamine in the horse

Uptake of 5-hydroxytryptamine (5-HT) into platelets is an important mechani sm by which low plasma concentrations are maintained, and platelet activati on may therefore result in significant release of this vasoconstrictor. The present study examined the kinetics of active uptake of radiolabelled [H-3 ]5-HT by mashed equine platelets in vitro, and investigated the effects on this process of 4 other naturally occurring monoamines which may be release d from the caecum in conditions of carbohydrate overload. The release of [H -3]5-HT by platelets was also studied, since platelet accumulation and acti vation has been associated with acute laminitis. Release of [H-3]5-HT was m easured in response to platelet activating factor (PAF), unlabelled 5-HT an d the indirect activation of platelets by endotoxin in the presence of bloo d leucocytes. K-m value for the uptake of 5-HT by equine platelets was 2.4 +/- 0.6 mu mol/l and the V-max was 8.3 +/- 0.6 pmol [H-3]5-HT/10(7) platele ts/min. The rate of uptake of 5 mu mol/l [H-3]5-HT was significantly decrea sed by the uptake inhibitors fluvoxamine and clomipramine. The 4 other mono amines examined all inhibited the uptake of [H-3]5-HT in a noncompetitive m anner, decreasing V-max by between 17 and 82%. Incubation of platelets with LPS (0.1 mg/ml) in the absence of leucocytes did not result in significant release of [H-3]5-HT; however in the presence of leucocytes 3.8 +/- 1.7 pm ol [H-3]5-HT/10(7) platelets (mean +/- s.e.) were released. This release wa s significantly inhibited by parthenolide and WEB2086, but not by aspirin. This suggests that PAF from activated leucocytes was responsible for the 5- HT release, These data show that 5-HT uptake by equine platelets is a satur able process operating most efficiently at substrate concentrations in the low micromolar range. The noncompetitive inhibition of 5-HT uptake by other naturally occurring monoamines may result in increased plasma concentratio ns of 5-HT, as mould its release by endotoxin. Such a rise in plasma 5-HT c oncentrations may contribute to selective vasoconstriction in the equine di gital circulation.