EFFECT OF NUTRITION ON TESTICULAR GROWTH AND PLASMA-CONCENTRATIONS OFGONADOTROPINS, TESTOSTERONE AND INSULIN-LIKE-GROWTH-FACTOR-I (IGF-I) IN PUBERTAL MALE SOAY SHEEP

Nutritional effects on puberty were studied in Soay rams. Testicular g rowth is initiated at birth in April and testes reach maximum size in October. Groups of eight lambs were fed for 18 weeks, starting in Augu st, a ration that restricted growth (Group R), the same diet ad libitu m (Group F), or a restricted diet for 8 weeks followed by ad libitum f eeding (Group R/F). Seasonal increases of plasma FSH, testis size, sex ual skin flush and plasma testosterone occurred with similar timing bu t reduced magnitude in Group R compared with Group F lambs. Testis siz e and sexual skin flush peaked in ail groups at 11 weeks (30 October); the testes of Group F animals were larger before the peak, but simila r in size thereafter, compared with testes from Group R/F, and larger throughout the experimental period than testes from Group R. Plasma te stosterone was higher in Group F than in Group R lambs from 7 to 17 we eks, but in Group R/F was similar to Group R before 10 weeks (23 Octob er) and similar to Group F thereafter. Testis size, plasma testosteron e, plasma insulin-like growth factor I (IGF-I) and liveweight were pos itively correlated. Ad libitum feeding in August-September (Group F) s timulated increased plasma FSH and LH above values for Group R, but ad libitum feeding initiated in October did not affect gonadotrophin con centrations (Group R/F). Therefore, the effects of improved nutrition on the hypothalamo-pituitary axis, which may have been mediated by cir culating IGF-I, were season-or age-dependent, and those on the testes included direct stimulation, independent of: changes in gonadotrophin concentrations. Nutrition modified the intensity, but not the timing, of peak pubertal reproductive activation.