The effects of insulin and growth hormone on the release of somatomedin by the isolated rat liver.

Livers from hypophysectomized (hypox) rats were perfused with oxygenated Waymouth's medium in a system which permitted continuous recirculation for separate 30 minute periods after which fresh medium was supplied. In most experiments 6 changes of medium were carried out over a 3 hour period. The somatomedin activity of each perfusate was determined by measuring its ability to stimulate sulfate uptake in hypox rat cartilage in vitro. For comparison between experiments the results are expressed as the per cent stimulation of sulfate uptake by the perfusate compared with the unperfused buffer. Without hormonal additions there was a progressive fall in the release of somatomedin activity during the 6 periods of study. When compared with the results without hormone, the addition of 1000 muU/ml of insulin per ml of medium during the 2nd to 6th period led to a significant increase in perfusate somatomedin activity at all periods. The addition of 100 muU/ml of insulin was without significant effect. The possible inter-relationship between insulin and growth hormone in the regulation of somatomedin release was studied with a dose of bGH of 250 ng/ml which had previously been shown to be insufficient by itsel to stimulate somatomedin release. When added to a medium containing 1000 muU/ml of insulin, this dose of bGH did not significantly stimulate somatomedin release beyond that obtained with insulin alone. However, when 250 ng/ml was added to a medium containing 100 muU/ml insulin, a significant stimulation of somatomedin release was observed while the addition of each hormone separately was without significant effect. These results support the hypothesis that insulin shares with GH the regulation of somatomedin release by the liver. Differences in insulin concentration may explain some clinical situations in which somatomedin concentrations cannot be correlated with GH levels.