Plasma growth hormone (G/D, prolactin (PRL), and vitellogenin (VTG) concentrations were determined during the annual reproductive cycle of the frog Rana esculenta. Plasma GH and PRL were measured using a RIA that employed purified bullfrog (Rana catesbeiana) GH and PRL as standards and radioligand, and their respective antibodies. Using ELISA, plasma VTG titers were related to ovarian weight. GH, PRL, and VTG displayed different trends related to season and sex. In male frogs the GH and PRL trends have been found parallel, showing the highest concentrations (35 and 85 ng/ml, respectively) during the winter months. In the female frogs, the GH trend behaved differently from that in males; in fact, plasma GH changed during the annual reproductive cycle, showing two main peaks occurring during the reproductive period and the autumn ovarian recrudescence that are well correlated with the vitellogenesis as well as with the main changes in ovarian weight.
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