Involvement of thymic hormones in regulation of the pituitary-adrenal axis in mice.

The effects of thymic hormone thymosin (fraction 5) and tactivin on the adrenal glucocorticoid function were compared in BALB/c mice. An elevation in plasma corticosterone level was found 3 h after i.p. injection of thymosin (1 microgram/mouse) which was possibly caused by an activation of neuroendocrine structures. This appeared plausible because the pretreatment with dexamethasone (10 micrograms/mouse) abolished the effect of thymosin. In contrast, tactivin produced a decrease in plasma corticosterone if administered to mice with high basal level of the hormone. Tactivin added at doses from 0.00064-2 micrograms/ml together with ACTH (1.6 microIU/ml) to isolated adrenal cells hindered the stimulatory influence of ACTH on the production of corticosterone by the adrenal cells. Thus, the thymic hormone thymosin and tactivin showed opposite influences on the adrenal glucocorticoid function which appeared to be mediated through different mechanisms.