Abstract.
Radioligand assays have been used to study the production of insulin-like growth factor (IGF) and its carrier (IGFCP) by rat liver in organ culture. Bound IGF released into the culture medium was dissociated and separated from its carrier by gel filtration in 1 m acetic acid. IGF was measured by a competitive protein-binding assay using human IGF and the specific IGFCP produced by rat liver. IGF CP concentrations were assessed in terms of IGFCP binding to labelled IGF, compared to a reference IGFCP preparation obtained from rat serum.
Without added hormone,the ratios IGF:total protein and IGFCP:total protein in the culture media after 3 days' culture were approximately 7 and 70 times higher, respectively, than those in serum.
GH and insulin both stimulated IGF production. The response was dose-dependent, and significant at physiological concentrations of hormone (10 ng/ml GH and 10 μU/ml insulin. With 1 μg/ml GH and 1 mU/ml insulin, the IGF concentrations in the media on average reached 2½ times the base-line level. These hormones had no significant effect on IGF CP production.
One ng/ml cortisol stimulated IGF production, but, in response to increasing concentrations, there was a dose-dependent decrease in IGF production. By contrast, IGFCP production was stimulated and there was a positive correlation between the carrier concentration in the culture media and the amount of cortisol added.
The results indicate that insulin and cortisol, in addition to GH, have a direct influence on IGF production by the liver in vitro. They also suggest that the biosyntheses of IGF and its carrier are subject to different systems of regulation.
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