Insulin-like growth factor-I concentrations in plasma of intact and castrated male and female cattle at four ages.

The influence of age, sex and castration on plasma concentrations of insulin-like growth factor-I (IGF-I) and other metabolic hormones related to growth was studied in cattle. Plasma was sampled from bulls, steers, heifers, and ovariectomized heifers at 20-min intervals for 12 hr at 5, 8, 12, and 15 mo of age. Plasma samples from each animal taken during each 12-hr period were composited for analysis of IGF-I, testosterone, total estrogens, thyroxine, triiodothyronine, insulin, and glucose. The mean plasma IGF-I concentration in all cattle increased from 61.6 to 158.6 ng/ml as the animals aged (p less than .01). Over all ages, bulls had greater concentrations of IGF-I than steers, heifers, or ovariectomized heifers (P less than .01). Bulls also had higher concentrations of testosterone (P less than .01) and total estrogens (P less than .01). Triiodothyronine concentration was greater in ovariectomized heifers than in bulls (P less than .01) or steers (P less than .05). Females had higher concentrations of thyroxine than males (P less than .01). Concentrations of triiodothyronine in the cattle were greater (P less than .01) during the winter and early spring as compared with the summer. Concentrations of insulin and glucose were not influenced by sex or castration; however, insulin increased in all cattle with age (P less than .01). The mean increase in IGF-I concentration with age within each of the four groups was associated with an increase in concentration of plasma insulin but the differences due to sex were not related to differences in insulin.(ABSTRACT TRUNCATED AT 250 WORDS)