Ontogeny and nutritional regulation of the serum growth hormone-binding protein in the rat

The ontogeny and the nutritional regulation of the serum growth hormone-binding protein in Wistar rats was studied in vitro using Ultrogel AcA34 filtration of serum incubated with 125I-bovine growth hormone. The level of the specific binding of GH to serum GH-binding protein was low in 1-week-old rats (female rats 2.3 +/- 0.9%; N = 6, and male rats 2.1 +/- 8%; N = 6) and increased with puberty, to reach 10-fold higher levels in 12-week-old adult female (26.1 +/- 2.3%; N = 6) and 5-fold higher levels in adult male rats (11.2 +/- 0.9%; N = 6). From 6 weeks of age and onwards, the level of serum GH-binding protein was significantly higher in female than in male rats, reflecting sexual dimorphism. The nutritional dependence of GH-binding protein was supported by the 46% decline of serum GH-binding protein levels in 6-week-old female rats fasted for 3 days (11.6 +/- 0.9 and 6.3 +/- 1.1% in control and fasted rats, respectively; N = 8/group; p less than 0.001). After 4 days of refeeding, no difference was found between control and experimental animals. During development and after nutritional manipulation, Scatchard analysis revealed that the changes in GH-binding protein were due to changes in binding capacity and not affinity. The levels of serum GH-binding protein were positively correlated with the levels of hepatic GH binding sites, suggesting that the regulation of both proteins is closely related during development and in states of nutritional sufficiency and deprivation.