Abstract

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.

Journal of Endocrinology (1995) 147, 507–516

ABSTRACT

Tissue and plasma levels of insulin-like growth factor-I (IGF-I), and relative levels of liver IGF-I RNA, were measured in 6-month-old ewe lambs which were well fed (n = 10) or starved (n = 10) for 5 days. Half of each nutrition group was given daily (09.00 h) injections of human GH (hGH; 0·15 mg/kg body weight per day). Blood was sampled daily from 09.00 to 12.00 h at 15-min intervals through jugular vein catheters and the lambs were slaughtered 24 h after the fifth injection of hGH.

Tissue and plasma IGF-I was extracted using an acid-ethanol-cryo-precipitation technique and estimated by radioimmunoassay. Tissue IGF-I was corrected for retained plasma IGF-I using tissue and blood haemaglobin levels. Liver IGF-I RNA levels were monitored by in-situ hybridization.

Plasma IGF-I (nmol/l) was higher in both the fed group and the fed group given GH treatment. Tissue IGF-I from kidneys (nmol/kg) was also higher (P < 0·001) in the fed group. There was no significant difference in IGF-I concentrations in the muscle biceps femoris or liver between fed and starved lambs. Although GH treatment did not increase IGF-I levels in tissues significantly, IGF-I RNA levels in liver were increased (P = 0·02) in both fed and starved animals. The relative liver IGF-I RNA levels positively correlated with their corresponding tissue IGF-I levels in the fed group and the fed group given GH treatment. The lack of a significant IGF-I response to GH in tissues may be due to either the time at which tissues were sampled after the GH treatment or the dose of GH administered. However, the higher IGF-I concentrations in plasma and kidney from fed compared with starved animals and the positive correlations between liver IGF-I and IGF-I RNA levels suggest that tissue and plasma IGF-I is regulated by nutrition and GH, with nutrition playing a critical role in the regulation of tissue and plasma IGF-I in normal lambs.

Journal of Endocrinology (1993) 136, 217–224