Production of insulin-like growth factor I and its binding protein in rat hepatocytes cultured from diabetic and insulin-treated diabetic rats.

This study examines the effect of experimental diabetes on the release of rat insulin-like growth factor I (rIGF-I) and its binding protein (IGF-BP) by adult rat hepatocytes in primary culture. Rats treated with streptozotocin (75 mg/kg) had decreased serum rIGF-I values of 0.37 +/- 0.04 U/ml compared to 1.06 +/- 0.04 in age-matched untreated rats (1 U = 770 ng human IGF-I). Concomitant decreases in hepatocyte production rates for rIGF-I (15% of the rate in cells from normal rats) and IGF-BP (30% of normal) were also observed for hepatocytes isolated from diabetic rats. Insulin replacement therapy (1.2 U/day) for 3-4 days normalized serum rIGF-I levels (0.92 +/- 0.07 U/ml) and increased rIGF-I production by isolated hepatocytes to 67% the rate of normal cells and IGF-BP production to 70% normal. Treatment of streptozotocin-treated rats with rGH (150 micrograms/day) in vivo for 7 days failed to increase serum rIGF-I levels or hepatocyte production of rIGF-I. Insulin in vitro (3 X 10(-7) M) increased rIGF-I release by hepatocytes from nondiabetic rats, but had no effect on cells from diabetic animals, suggesting that factors other than insulin are required to maintain rIGF-I synthesis in diabetes. Serum rIGF-I levels showed a strong correlation with hepatocyte rIGF-I production in the animals used in this study. However, calculation of circulating rIGF-I half-life based on these values showed a 2-fold higher half-life in diabetic rats (7.91 +/- 1.58 h) and rGH-treated diabetic rats (7.52 +/- 1.25 h) than in nondiabetic (2.99 +/- 0.35 h) and insulin-treated diabetic animals (3.85 +/- 0.36 h). This suggests that the rate of clearance of circulating rIGF-I may be slower in diabetic animals.