Effect of androgens on plasma somatomedin-C/insulin-like growth factor I responses to growth hormone.

In search of the mechanism for the relatively high plasma somatomedin-C (Sm-C) concentrations in pubertal boys and girls, we have measured the plasma Sm-C responses to exogenous growth hormone (GH) in prepubertal hypopituitary boys before and after testosterone administration. Sm-C responses were determined in 5 hypopituitary boys (9-14 years of age) who were given two successive injections of GH (0.1 U/kg) 48 h apart. Eight days later, after administering 200 mg testosterone IM, their Sm-C responses to the same GH challenge were reassessed. There were no significant differences between the pre-testosterone Sm-C response to GH and the post-testosterone response, despite evidence for reductions in 24 h urinary nitrogen excretion and serum urea nitrogen concentrations in response to testosterone. The results provide no evidence that androgen augments the effect of GH to raise plasma Sm-C during puberty, or that androgen has a direct stimulatory effect on Sm-C production. By inference and from published reports, it appears more likely that a sex hormone-stimulated increase in GH secretion is responsible for the increased Sm-C observed during puberty.