In vitro and in vivo effects of thymulin on rat testicular steroid synthesis.

Although the stimulatory effects of immunological hormone thymulin on ovarian function are documented, responses of the testis to thymulin are unknown. Effects of thymulin and thymulin augmentation of human chorionic gonadotropin (hCG) stimulation of testicular steroidogenesis were analyzed via incubation of testis from 3-, 6-, and 9-week-old rats with 0.1, 1, 10, and 100 ng/ml of thymulin or thymulin plus hCG (1 IU/ml). At three weeks of age, thymulin decreased testosterone and estrone synthesis. By 6 and 9 weeks of age, lower doses of thymulin (0.1 and 1 ng/ml) stimulated testosterone and estrone synthesis. Increased doses of thymulin (100 ng/ml) resulted in decreased testicular steroid synthesis. No thymulin augmentation of hCG stimulation of testicular steroidogenesis was noted in vitro. Responses in vitro may have been confounded with the endocrine status at the time of incubation. Thymulin levels are increased at 3 weeks of age and decline at 6 and 9 weeks of age whereas gonadotropin levels are just the opposite pattern; these in vivo endocrine changes may have influenced the in vitro responses. Analysis of in vivo response to thymulin was pursued in pituitary intact, hypophysectomized and hCG primed (12 h before thymulin injection) hypophysectomized rats (5 weeks of age) with 1 and 10 microg of thymulin injected intravenously. Thymulin had no effect on testicular steroidogenesis in hypophysectomized rats 30, 60, and 120 min post injection). In pituitary intact rats, thymulin reduced testosterone and estrone concentrations. In hCG primed hypophysectomized rats, thymulin injection was followed by significant increases in testosterone levels and declines in estrone concentrations. These studies indicated that any increases in testicular steroidogenesis from thymulin required gonadotropin stimulation and increased concentrations of thymulin had inhibitory effects on testicular steroidogenesis.