Serotonin level and serotonin uptake in human platelets: A variable interrelation under marked physiological influences

Although it is known that platelet serotonin level (PSL) depends directly on platelet serotonin uptake (PSU) through the plasma membrane, reports on their interrelation are inconsistent. The aim of this study was to systematically explore the relationship between these two platelet serotonin parameters in large human population. PSL and full-kinetics of PSU were determined on 318 blood donors (276 males, 42 females; 20–67 years). The overall correlation coefficient between PSL and maximal velocity of PSU was highly significant but unexpectedly low ( r = 0.269). Further analyses revealed lack of correlation among females, and variable association among males, depending on the subject age and season of measurements. Highly significant correlations were observed in spring–winter, while association was absent during summer–autumn. Lowering of PSL–PSU correlation with increased age was also demonstrated, showing modest interrelation among younger men and no interrelation in older population. By multiple regression analyses season was identified as the only independent predictor of PSL–PSU relationship. The results show prominent influence of biological (sex, age) and, especially, environmental (seasons) physiology on the intraindividual relationship between PSL and PSU. Although serotonin transporter activity plays an important role in determining PSL, the observed correlations indicate that other factors may predominate.