OVINE PLACENTAL-LACTOGEN IS A POTENT SOMATOGEN IN THE GROWTH-HORMONE (GH)-DEFICIENT RAT - COMPARISON OF SOMATOGENIC ACTIVITY WITH BOVINE GH

The somatogenic effects of recombinant ovine placental lactogen (oPL) were investigated in the GH-deficient dwarf rat and compared to those of identical doses of recombinant bovine GH (bGH) in three independent studies. Both oPL and bGH treatments resulted in an increase (P < 0.05) in body weight gain compared to that in saline controls, with oPL treatment being more potent than bGH (P < 0.05). In promoting linear growth, oPL was more potent (P < 0.05) than bGH in some instances. The nitrogen content of dry carcass matter was increased with oPL treatment compared to saline (P < 0.05), with a nonsignificant increase in bGH-treated animals. Carcass fat was similarly reduced by both oPL and bGH treatment (P < 0.05) compared to saline. Serum insulin-like growth factor-I (IGF-I) concentrations were increased significantly (P < 0.05) by both oPL and bGH treatments, with a significantly greater effect of oPL suggested in one study. No increase in hepatic IGF-I mRNA was evident with either treatment, suggesting that the increase in serum IGF-I is due to posttranscriptional mechanisms. The expression of IGF-binding protein-3 hepatic mRNA was increased (P < 0.05) with bGH treatment compared to that after saline treatment, but was unaffected by oPL treatment, indicating regulation by GH at the transcriptional level. The binding of [I-125]bGH to hepatic membrane preparations demonstrated no difference in specific binding compared to that in saline controls. However, [I-125]oPL specific binding was greater in oPL-treated animals (P < 0.05). Animals treated with bGH had reduced (P < 0.05) hepatic GH receptor mRNA compared to saline controls, but oPL treatment had no effect. Thus, oPL is a potent anabolic and lipolytic agent in the dwarf rat, exerting greater somatogenic effects on some parameters than bGH. Our data suggest differences in receptor binding and effects on GH receptor and IGF-binding protein-3 expression with these two treatments, raising the possibility of actions through different pathways or differential effects at the GH receptor level.