Platelet serotonin transporter (5HTt): physiological influences on kinetic characteristics in a large human population

The present study had two goals: first, to give a detailed description of a reliable method for full kinetic analysis of serotonin transporter (5HTt) on the membrane of human platelets, and second, as a main issue, to report on physiological influences on kinetic characteristics of this transmembrane transport on a large population of healthy individuals. Full kinetic analyses of platelet serotonin uptake were performed on 334 blood donors of both sexes by the use of ¹⁴C-radioisotopic method, which was first optimized according to assumptions of enzyme kinetic analyses, with regard to platelet concentration, duration of uptake, concentration of substrate as well as important technical parameters (underpressure of filtration, blanks, incubating temperature, etc). Kinetic parameters of platelet serotonin uptake in the whole population were for V_max: 142 ± 25.3 pmol 5HT/10⁸ platelets/minute and for K_m: 0.404 ± 0.089 µM 5HT. Besides the report on kinetic values of 5HT transporter protein, we have also described major physiological influences on the mentioned parameters, V_max, K_m and their derivative, V_max/K_m (transporter efficiency): range and frequency distribution of normal values, intraindividual stability over time, lack of age influence, gender dependence and seasonal variations. The report on kinetic values and main physiological influences on platelet serotonin transport kinetics, obtained by the use of thoroughly reassessed methodology, and on by far the largest human population studied until now, offers a reliable frame of reference for pathophysiological studies of this parameter in various clinical fields.