Ovine placental lactogen lacks direct somatogenic and anticatabolic actions in the postnatal lamb.

The metabolic effects of ovine placental lactogen (oPL) alone and in combination with bovine GH (bGH) were investigated in comparison with the identical dose of bGH alone in the well-fed postnatal lamb. The animals were treated by twice daily intramuscular injection for 5 days with oPL (n = 7), bGH (n = 7) or bGH+oPL (n = 7) at a dose of 0.3 mg/kg per day or saline (n = 9). bGH and bGH+oPL treatments, but not oPL treatment, resulted in significantly (P < 0.01) higher plasma IGF-I levels than saline treatment. The rate of net protein catabolism (NPC) was significantly (P < 0.05) reduced to the same extent by bGH and bGH+oPL treatment compared with saline treatment. In contrast, oPL did not affect the rate of NPC. Blood glucose and insulin:glucose ratio were significantly (P < 0.05) elevated in the bGH+oPL group, whereas they were not significantly altered by either bGH or oPL treatment. These results suggest that oPL alone is not somatogenic or anticatabolic in the postnatal lamb despite the earlier evidence that oPL can bind to the oGH receptor (oGHR). However, oPL appeared to augment some of the effects of bGH when administered together, particularly with respect to carbohydrate metabolism. Potentiation of the diabetogenic effect of bGH by oPL may lead to insulin resistance during pregnancy. The lack of any obvious actions by oPL treatment alone may support the hypothesis that oGHR homo-dimerization is required for the full activation of GHR-mediated effects, since oPL binding does not initiate homo-dimerization of the oGHR.