To directly examine the relationship between insulin receptors and insulin action in fetal tissue, we compared insulin receptor characteristics and insulin-mediated 14C-glucose incorporation into glycogen, as well as glycogen synthase activity, in freshly isolated hepatocytes from 21-day fetal (F) and adult (A) rats. Viability of hepatocytes was documented by trypan blue exclusion (> 90%), time-dependent 14C-leucine incorporation into protein, and dose-related incorporation of glucose into glycogen. Percent specific binding of 125I-insulin per unit protein was significantly higher in F than A liver plasma membranes (32.2 ± 0.3 versus 18 ± 2.4; P < 0.01) and Scatchard plots revealed twice the number of receptors in F. Similarly, receptor number per cell surface area was threefold higher in F than in A (150 versus 50 sites/μm2). At a fixed medium glucose concentration of 11.2 mM, insulin stimulated 14C-glucose incorporation into glycogen in a dose-related manner in A with an apparent Km of 1.0 ng/ml and Vmax at 5–10 ng/ml corresponding to 30–40% of total receptor occupancy; no effect was obtained in F with insulin up to 100 ng/ml. Net glucose incorporation into glycogen (nmol/106 cells/h) increased progressively with increasing medium glucose concentrations ranging from 1.4 to 27.8 mM; incorporation by F was significantly greater than by A at each glucose concentration. However, whereas insulin at 100 ng/ml significantly augmented net glucose incorporation at each glucose concentration in A, no effect of insulin was apparent in F. Finally, insulin at 10 and 100 ng/ml significantly increased the active (a form) and total (a + b form) glycogen synthase activity in A; the ratio of active to inactive forms (% I) also increased significantly in A. In contrast, there was no demonstrable augmentation by insulin of glycogen synthase activity in F hepatocytes. Thus, insulin-mediated incorporation of glucose into glycogen and insulin-stimulated glycogen synthase activity was not demonstrable in fetal rat hepatocytes despite the presence of an increased number of insulin receptors.

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