Effects of fasting on luteinizing hormone dynamics in the male rat.

It is well known that reproductive function is impaired in humans and animals when nutrient intake is inadequate. Fasting, one of the most severe nutritional insults, has been used experimentally to identify the major effects of nutritional deficiencies on reproductive processes. In the rat, circulating reproductive hormone concentrations are reduced during fasting. Although decreased luteinizing hormone-releasing hormone (LHRH) secretion from the hypothalamus may be responsible for the lower serum concentrations of reproductive hormones, pituitary and testicular function of fasted rats have not been considered in detail. We studied the luteinizing hormone (LH) dynamics (storage, secretion, circulation and excretion) in the male rat to determine if fasting alters the responsiveness of the testes or the pituitary to hormonal stimulation. Our results indicate that after a 4-d fast: 1) serum LH and testosterone (T) concentrations are reduced (P less than or equal to 0.001); 2) hypothalamic LHRH, pituitary LH and follicle-stimulating hormone (FSH) concentrations are unaffected, but testicular T content is reduced (P less than or equal to 0.001); 3) urine output of LH and FSH are reduced (P less than or equal to 0.001); 4) in vitro and in vivo LH responses of the pituitary to LHRH are not affected; and 5) hCG-stimulated in vitro T production by the testis is not affected. These results are consistent with the hypothesis that fasting inhibits LHRH secretion.