Growth-promoting effects of ovine placental lactogen (oPL) in young lambs: comparison with bovine growth hormone provides evidence for a distinct effect of oPL on food intake.

Abstract

The growth-promoting and metabolic effects of recombinant ovine placental lactogen (oPL) were compared with those of recombinant bovine growth hormone (bGH) in young lambs. Lambs were treated by twice daily subcutaneous injection with oPL (n = 16) or bGH (n = 16) at a dose of 0.1 mg/kg live weight/day or with saline (n = 16) for 21 days commencing on day 3 of life. Jugular blood samples were taken on days 0, 10 and 20 of treatment. Half the lambs in each group were slaughtered at 24 days, and the other half at 9 months of age. Both bGH and oPL treatments induced small but significant (P < 0.05) increases in circulating concentrations of insulin-like growth factor-I (IGF-I) on day 10 of treatment, but not on day 20. Neither treatment altered plasma concentrations of glucose, non-esterified fatty acids, urea or creatinine compared to those in saline-treated lambs. Relative to those of bGH-treated (0.24 +/- 0.01 kg/day) or saline-treated (0.25 +/- 0.01 kg/day) lambs, live weight gains of oPL-treated lambs (0.28 +/- 0.01 kg/day) were significantly (P < 0.05) increased during treatment and differences in live weight were still apparent at 9 months of age. Similarly, treatment with oPL, but not bGH, significantly (P < 0.01) increased daily energy intake. It is concluded that placental lactogen and growth hormone do not have identical biological actions. While oPL is growth-promoting in young lambs, this effect may be mediated by stimulating voluntary feed intake rather than by elevating circulating concentrations of IGF-I.