Insulin stimulates amino acid and lipid metabolism in isolated fetal rat hepatocytes

Although fetal hyperinsulinemia is associated with excessive deposition of glycogen in liver, both in vivo and in vitro studies show little effect of insulin on glycogen synthesis from glucose or the activity of the enzyme glycogen synthase in the fetus. To investigate whether lack of insulin effect extends to other fetal metabolic processes, we compared the influence of insulin on amino acid uptake (¹⁴C-α-aminoisobutyric acid) and lipid synthesis (¹⁴C-acetate) in freshly isolated hepatocytes from 21-day fetal (F) and adult (A) rats. Viability of F and A hepatocytes was documented by trypan blue exclusion (>90%). In A, insulin stimulated ¹⁴C-α-aminoisobutyric acid uptake in a dose dependent manner with an apparent Km at 2 ng/ml and a Vmax at 10 ng/ml. When corrected for cell surface area, F cells responded to insulin in a similar dose response manner, although absolute values per 1 X 10⁶ cells always remained lower. In contrast, whereas A cells demonstrated a typical dose dependent response of ¹⁴C-acetate incorporation into lipid with a Km at 5 ng/ml and Vmax at 10 ng/ml of insulin, F cells remained totally unresponsive when the concentration of acetate was 5 mM or less. However, at higher medium acetate concentrations (15-30 mM) fetal responses were equal to or greater than that of adult, both basally and with insulin. These findings suggest differences in the maturation of insulin-mediated processes in fetal rat hepatocytes; effects on amino acid uptake appear earlier than those on lipid or glycogen synthesis. Copyright