Insulin stimulation of somatomedin A production in monolayer cultures of rat hepatocytes.

The effect of insulin on the generation of somatomedin by a monolayer culture of rat hepatocytes was investigated. Release of somatomedin into the medium of cultured hepatocytes was observed for at least three days, as determined by a radioreceptor assay for somatomedin A. The addition of 1-100 micrograms/ml insulin to the medium caused dose-related increases in somatomedin A in the conditioned medium for up to three days. The conditioned medium also stimulated DNA synthesis in human fibroblasts. To investigate hormone degradation by hepatocytes, labelled somatomedin A or insulin was added to the medium and the degradation was examined by trichloroacetic acid (TCA) precipitation, and gel chromatography. Although [125I]somatomedin A was slightly degraded even after 24 h of incubation, [125I]insulin was degraded within 6 h and the TCA precipitation at 24 h was 15% only. This degradation of hormones was inhibited by 1 mM bacitracin. Insulin stimulation of somatomedin production was significantly increased to 131.2% as compared with cultures without bacitracin. The molecular size of the somatomedin released into the medium was greater than that of purified somatomedin, suggesting that this somatomedin is bound to carrier proteins.