Effect of physical stress on 5-hydroxytryptamine uptake kinetics (Km and Vmax) in human platelets

Platelet 5-HT uptake is possibly one mechanism by which plasma level of 5-HT is physiologically controlled. This mechanism may be of importance in the vascular 5-HT mediated tone and protect against thrombolysis and cardiovascular events. Physical stress is known to affect various platelet functions. The aim of this study was to investigate whether or not physical stress has any influence on platelet 5-HT uptake kinetics. In healthy adult persons (n = 13) the maximal rate (Vmax) and Michaelis-Menten constant (Km) of platelet 5-HT uptake (in vitro) were determined before (a), immediately after (b) and 1 hour after 50 min of squash playing (c). Immediately after squash Vmax and Km were elevates significantly as compared to the values before squash [Vmax(a): 77.0 +/- 31.0 (SD) pmol 5-HT/10(8) pl./min, Vmax(b): 109.9 +/- 48.4 (SD) pmol 5-HT/10(8) pl./min, p less than 0.01; Km(a): 5.16 +/- 1.73 (SD) x 10(-7) mol/1, Km(b): 7.59 +/- 2.37 (SD) x 10(-7) mol/1, p less than 0.05]. The percent elevations of Vmax (47%) and Km (42%) were of the same range. One hour after squash the values of Vmax(c) and Km(c) were found to be normalized. It is concluded that physical stress, like squash playing, does not affect net platelet 5-HT uptake in healthy persons at physiological plasma 5-HT concentrations. The significant increases of Vmax and Km of platelet 5-HT uptake are suggested to be adaptation reactions to keep the platelet net 5-HT uptake constant.