Age-related changes in serotonin content and its release reaction of rat platelets

The serotonin content of platelets, serum and plasma from rats of various ages was examined. In male rats, platelet serotonin content, which was about 0.65 nmol/10(8) platelets at young age (6-7 months), increased slightly at middle age (12-14 months) but decreased markedly at old age (25-26 months). Significant difference (P less than 0.05) was observed between young and old rats, and between middle-aged and old rats. In female rats, on the other hand, no age-related change in the platelet serotonin content was found. In both sexes, the serotonin content of rat sera changed with age in the same pattern as that of the platelets. No plasma serotonin was detected in rats of either sex and at any ages examined. Serotonin release from rat platelets was also studied using collagen and thrombin as stimulants. In males, the responsiveness of platelets to these two stimulants showed almost the same age-dependent changes. It was lower in middle-aged rats than in young rats but increased greatly in old rats. Significant difference (P less than 0.05) was observed between middle-aged and old rats. In females, collagen and thrombin had the opposite effect on the sensitivity of the platelets as age increased. The amount of serotonin released in response to collagen was low until middle age but increased markedly at old age, while the content of serotonin released by thrombin remained high until middle age and decreased greatly at old age. These results imply that age-related changes in the serotonin release reaction in rat platelets differed according to the stimulants used.