Differential regulation of plasma levels of insulin-like growth factors-I and -II by nutrition, age and growth hormone treatment in sheep.

Plasma levels of IGFs-I and -II were measured in 4-month-old ewe lambs (n = 20) and 2-year-old ewes (n = 16), which were well fed (n = 18) or fasted (n = 18) for 3 days. Half of each nutrition group was given daily (0900 h) injections of bovine GH (bGH, 0.1 mg/kg body weight per day) for 3 days. Blood samples were collected immediately before the GH injection every morning. Plasma IGFs were extracted by acid gel permeation chromatography using a Waters Protein Pak 125 column, fitted to a Pharmacia fast protein liquid chromatography system, then freeze-dried, reconstituted (at pH 7.4) and estimated by RIA. At the end of the experiment, IGF-I levels in plasma were increased (P < 0.01) by exogenous bGH in both fed ewes and lambs but not in the fasted animals; plasma IGF-I levels were depressed by fasting (P < 0.01) at all ages. IGF-I levels were also found to be significantly higher (P < 0.01) in ewes than lambs. In contrast, plasma IGF-II concentrations were depressed (P = 0.02) by administration of bGH in all groups and elevated in the ewes (P < 0.05) by fasting. However, the lambs showed no significant changes in IGF-II with fasting. The IGF-II levels were significantly higher (P < 0.001) in lambs than ewes. Results from the present study demonstrate that GH administration stimulated an increase in plasma IGF-I and induced a decrease in plasma IGF-II. On the other hand, fasting depressed plasma IGF-I and elevated plasma IGF-II in the sheep. A significant GH/nutrition interaction for IGF-I (P < 0.01), but not for IGF-II, and a significant nutrition/age interaction for IGF-II (P < 0.01), but not for IGF-I, in the present study suggest that GH has a greater stimulating effect on plasma levels of IGF-I in the fed rather than fasted sheep and that nutrition has a greater influence on plasma levels of IGF-II in the older rather than younger animals, indicating that plasma IGFs-I and -II are differentially regulated by nutrition, GH and developmental stage in postnatal sheep.