Compensatory effects of serotonin on the regulation of blood volume in healthy human subjects.

It has been demonstrated that water intake accelerates the release of serotonin from enterochromaffin cells in the rat jejunum, which is then transported through the portal vein into the blood. However, the physiological roles of serotonin in the regulation of blood volume remain unclear. We aimed to investigate the roles of serotonin in the physiological regulation of physical exercise-dependent sweating-mediated haemoconcentration in humans. All participants performed two times 30-min sessions of -up-and-down physical exercise with a 20-min rest. Changes in thermal perspiration rate, concentrations of serotonin and vasopressin in blood, and samples of blood and urine were measured just before and after repeated exercise. All participants presented increased vasopressin after the first trial but showed no change after the second exercise session. In contrast, blood serotonin levels increased dramatically after the second session. Urine osmolality increased significantly after the first exercise session. All the participants showed significantly decreased ratios of red blood cells, hemoglobin, and neutrophils before and after the second exercise. In contrast, no significant changes were observed in the ratios of total protein and lymphocytes. In conclusion, serotonin may play a crucial role in the compensatory regulation of sweating-mediated haemoconcentration in cooperation with vasopressin.