Somatomedin production by rat liver in organ culture

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Abstract
Radioligand assays were used to study the production of insulin-like growth factor (IGF) and its carrier (IGF CP) 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 ICF CP produced by rat liver. IGF CP concentrations were assessed in terms of IGF CP binding to labeled IGF, compared to a reference IGF CP preparation obtained from rat serum. Without added hormone, the ratios IGF:total protein and IGF CP:total protein in the culture media after 3 days'' culture were .apprx. 7 and 70 times higher, respectively, than those in serum. GH [growth hormone] and insulin both stimulated IGF production. The response was dose-dependent, and significant at physiological concentrations of hormone (10 ng/ml GH and 10 .mu.U/ml insulin). With 1 .mu.g/ml GH and 1 mU/ml insulin, the IGF concentrations in the media on average reached 2 1/2 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, IGF CP production was stimulated and there was a positive correlation between the carrier concentration in the culture media and the amount of cortisol added. Insulin and cortisol, in addition to GH, apparently have a direct influence on IGF production by the liver in vitro. The biosyntheses of IGF and its carrier apparently are subject to different systems of regulation.