Luteinizing hormone secretion and gonadotropin releasing hormone binding in heifer and steer anterior pituitaries

Luteinizing hormone (LH) release and tissue content were studied in explanted bovine pituitaries from heifer and steer during superfusion in vitro. Compared to steer, heifer pituitaries contained significantly more LH and released more LH into the incubation medium. Quantitative response in LH release to gonadotropin releasing hormone (GnRH) stimulation was about equal in heifer and steer, but relative increase was more pronounced in steer. Specific 125I-GnRH binding to dispersed anterior pituitary (AP) cells and isolated plasma membranes was also analyzed. Concentration dependent binding curve of 125I-GnRH to dispersed cells exhibited sigmoid characteristics suggesting positive cooperativity. There was a sex dependent difference, the binding curve for heifer being shifted to the left and reaching saturation at lower GnRH concentrations than that for steer. "Multisigmoid" concentration dependent GnRH binding curves were obtained using isolated plasma membranes, revealing the presence of multiple binding sites. One of the binding sites, present in heifer, was absent in the steer derived materials, but could be induced by pretreatment of the steer plasma membranes with 17-beta estradiol as low as 50 pg/ml. It is concluded that the differences in GnRH binding may be responsible, at least in part, for the observed differences in LH secretion pattern between heifer and steer.