Relationship of serum somatomedin-like activity and fibroblast proliferative activity with age and body weight gain in sheep

The relationship between serum growth factors and body weight gain was examined in five Dorset lambs. The lambs were weighed and bled by jugular puncture at 2-week intervals between 2 and 18 weeks of age. Somatomedin-like activity (Sm) declined from initially high concentrations at 2 weeks to fairly constant concentrations between 6 and 18 weeks. Relative weight gain--i.e., gain expressed as a percentage of body weight--declined in a manner similar to that of Sm. Mean relative weight gain and mean Sm for the eight 2-week intervals were significantly related (r = .84). Absolute body weight gain--i.e., gain expressed in kilograms--remained fairly constant throughout the study and was not significantly correlated to Sm (r = .15). Serum fibroblast proliferative activity (FPA) was measured as a possible indicator of collective activities of serum growth factors. FPA initially followed a pattern similar to that of Sm, decreasing between 2 and 6 weeks and plateauing until 12 weeks. After 12 weeks, FPA increased to concentrations similar to those observed at 2 weeks. The increase in FPA after 12 weeks was apparently due to an increase in a non-Sm growth factor and had no obvious relationship to body weight changes. Results of the in vitro cell assay system might have been more meaningful if cell type(s) other than WI-38 fibroblasts (e.g., myogenic cells) had been used for estimating collective activities of serum mitogenic factors. The data suggest that serum Sm-like activity may be important in the regulation of growth in sheep.