IN-VITRO EFFECTS OF MAMMALIAN AND AMPHIBIAN PROLACTINS ON HEPATIC VITELLOGENIN SYNTHESIS IN RANA-ESCULENTA

Male and female Rana esculenta liver was induced in an in-vitro system by homologous and Rana catesbeiana pituitary to synthesize and releas e vitellogenin, a lipoglycophosphoprotein precursor of yolk proteins, lipovitellins and phosvitins, in oviparous vertebrates. In the present experiments, the action of prolactin on hepatic vitellogenin synthesi s and release was investigated, using ovine prolactin and Rana catesbe iana prolactin. The effects of prolactin on hepatic vitellogenin synth esis displayed different trends related to sex; male liver was found t o be more responsive than female liver to both ovine and frog prolacti n; moreover, the response to prolactin was dose-related (r = 0.998; P< 0.05) in male but not in female liver. In both sexes, a high degree of seasonality in the responsiveness of the liver was found, since the v itellogenin levels induced by prolactin during the winter phase were s ignificantly (P<0.001) higher than those produced during the summer ph ase. Thus, there was no significant difference between the action of o vine and frog prolactin on vitellogenin synthesis; in fact, mammalian prolactins are structurally similar with regard to nucleotide and amin o acid sequences. The direct action of prolactin on hepatic vitellogen in synthesis in the frog Rana esculenta is discussed, on the basis of the role played by prolactin as an important growth modulatory hormone in fetal and adult tissues.