To investigate the relationship between beta-endorphin and male reproductive function, blood and seminai plasma were collected from twenty Italian Friesian bulls, aged between l 0-18 months, with satisfactory semen quality and normal testosterone levels. Betaendorphin concentrations determined by radioimmunoassay were 11 0±50 pg/ml in blood and 960±540 pg/ml in seminai plasma (P<O,OOl). The same antibody was used for the immunohistochemical localization of the opioid peptide in specimens from the male genital tract of ten slaughtered Charolaise bulls. A strong to moderate immunostaining was present in the glandular epithelium of the prostate. A moderate to weak positive signal was also observed in the epithelium of the vesicular glands, epididymis, ductus deferens and ampulla. As in other species, beta-endorphin seems to be locally produced in the bovine male genital tract, resulting in a high level of the opioid peptide in seminai plasma compared to the concentration in blood (about 9:1). These findings indicate that beta-endorphin is involved in the local contro l of reproductive functions in the bull.
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