EFFECTS OF NUTRITION ON TESTICULAR SIZE AND THE CONCENTRATIONS OF GONADOTROPINS, TESTOSTERONE AND INHIBIN IN PLASMA OF MATURE MALE SHEEP

The effects of nutrition on the hypothalamo-pituitary-gonadal axis wer e studied in three groups of six mature Merino rams that were fed for 56 days with a ration that maintained their initial live mass (interme diate diet: 675 g chaff plus 175 g lupins), the same ration with a lup in supplement (high diet: 675 g chaff plus 825 g lupins), or about hal f of the intermediate ration (low diet: 475 g chaff plus 125 g lupins) . Lupin seed provides a highly (95%) digestible source of energy and p rotein. Plasma concentrations of LH, FSH, testosterone and inhibin wer e measured in blood samples collected over 24 h on the day before diet ary treatments began (day - 1), then on days 0, 1, 5, 14, 28 and 56. C ompared with the intermediate diet, the high diet significantly increa sed live mass within 14 days and testicular size within 28 days, and t hese differences increased steadily throughout the experiment. Plasma FSH concentrations and LH pulse frequency increased within 5 days, but these effects were maintained for only 14 days. Decreasing the nutrit ional status reduced live mass and testicular size within 7 days, led to a low LH pulse frequency that persisted throughout the experiment, but did not affect FSH concentrations. Significantly less testosterone was secreted over 24 h in the low dietary group than in the intermedi ate or high group until day 28. The high group tended to secrete more than the intermediate group, but only at the beginning of the experime nt when LH pulse frequencies differed between these groups. The testos terone response to each endogenous LH pulse, or following an injection of ovine LH i.v. (200 ng kg(-1) live mass), was not related to testic ular size or dietary treatment at any stage of the experiment. Similar ly, plasma inhibin concentrations were not related to change of diet, despite large differences in testicular size. We concluded that the ef fects of nutritional status on testicular size in mature rams are at l east partly mediated through changes in gonadotrophin secretion. Both increases and decreases in food supply affected LH pulse frequency, su ggesting the involvement of hypothalamic mechanisms. However, the lack of an effect of a decrease in nutritional status on the secretion of FSH and inhibin and the inconsistent long-term relationship between LH pulse frequency and testicular size suggest that the effects of diet on testicular growth also involve mechanisms that are independent of c hanges in gonadotrophin secretion.