Effect of GH and IGF-I treatment on reproduction, growth, and plasma hormone concentrations in domestic nutria (Myocastor coypus).

The role of GH and IGF-I in the control of reproduction, growth, and hormone secretion in domestic nutria was examined. In the first series of experiments, we studied the effects of single and multiple (daily for 20 days) injections of recombinant hGH (15&mgr;g/animal) on plasma triiodothyronine (T3), thyroxine (T4), and progesterone (P) concentrations, as well as on the duration of pregnancy (time between start of mating and birth of pups), number of pups born, and body weight of adult females and their newborn pups. In the second series of experiments, the effects of single and multiple (daily for 28 days) injections of recombinant hIGF-I (1&mgr;g/animal) on plasma IGF-I, IGFBP-3, T3, T4 concentrations, the duration of pregnancy, and number of offspring delivered were assessed. It was found that either single or multiple GH treatment resulted in significant increase in plasma T3, T4, but not P concentration. Furthermore, it significantly increased the body weight of adults and newborn pups. No influence of GH on the duration of pregnancy and the number of offspring was observed. IGF-I treatment caused an increase in plasma IGF-I concentration, a reduction in plasma IGFBP-3, T3, and T4 concentrations, and a shorter duration of pregnancy but did not alter the number of pups delivered. Our observations suggest that GH and IGF-I may be involved in the control of hormone secretion, growth, and reproduction in domestic nutria. Reproductive processes are controlled by IGF-I rather than by GH, whilst GH may be involved in the stimulation of prenatal and postnatal growth. The differential effects of these substances on thyroid hormones and reproductive parameters suggest that the actions of GH on these processes are probably not mediated by IGF-I.