Factors influencing development of reproductive function in Brahman bulls

Abstract

The influences of age and dietary energy intake on the hypothalamic-hypophyseal-testicular axis of developing Brahman bulls are not well understood. Two experiments were conducted to examine these relationships. In the first experiment the effects of increased age and limited dietary energy intake on prepubertal development were examined. Brahman bulls were fed so they gained at either a moderate (.25 kg/d; MG) or optimal (.84 kg/d; HG) rate. Blood samples were obtained at four ages to evaluate changes in serum hormones. Following appearance of first motile spermatozoa in HG bulls, both the HG and MG bull of the pair group were slaughtered. The influence of limiting dietary energy was examined by evaluating in vitro median eminence (ME) GnRH release, testicular development and tissue weights. Serum concentrations of luteinizing hormone (LH), testosterone (T) and growth hormone (GH) increased with increasing age. Limiting dietary energy decreased T, increased GH but did not change LH. Basal ME GnRH release in vitro was lower in MG bulls. Potassium-induced ME GnRN release and ME GnRH concentrations were not changed by limiting dietary energy. Limiting dietary energy delayed testicular development, as evaluated by testicular histology, and decreased the weights of the reproductive, but not neuroendocrine tissues. In summary, limiting dietary energy delays the onset of puberty in bulls by depressing testicular function. Changes in both hypothalamic and hypophyseal function may also exist. The second study examined in vitro responsiveness of testicular cells to LH, a cyclic adenosine monophosphate (cAMP) analogue and a testosterone precursor (dehydroepiandrosterone; DHEA) at various ages. In vitro testosterone secretion increased with increasing age. Response to DHEA was also influenced by age. Testosterone production was increased by DHEA in all bulls and was lowest at 3 and 6 months, intermediate at 0 and 12 months and greatest at 9 and 15 months. Neither LH or cAMP stimulated testosterone production, suggesting the culture conditions did not optimize steroidogenic enzyme activity. Based on these results it appears that the proper culture conditions must be determined before studies can be undertaken to examine factors influencing testicular steroidogenesis.