Biologic Activity and Receptor Binding Properties of Biosynthetic Human Insulin in Isolated Rat Hepatocytes and Mouse Soleus Muscle In Vitro

Studies from our laboratory have shown that freshly isolated hepatocytes from the adult rat represent a suitable system to measure insulin binding to cell surface receptors and insulin biologic effects. Biosynthetic human insulin (BHI) binding to isolated rat hepatocytes was measured under steady-state conditions at 20°C and 37°C, using ¹²⁵I-BHI and varying concentrations of unlabeled BHI. Control binding experiments were run in parallel with 125I-pork insulin and varying concentrations of unlabeled pork insulin. No difference between BHI and pork insulin binding could be detected at either temperature. Similar results were obtained when BHI and pork insulin binding was studied using purified liver plasma membranes. These results indicate that BHI binds to hepatocyte receptors with the same characteristics as monocomponent pancreatic pork insulin. BHI's stimulation of α-amino ¹⁴C-isobutyric acid (AIB) influx in rat hepatocytes was found to be indistinguishable from that induced by pancreatic human insulin or pork insulin: for the three insulins, maximal responses (about a twofold increase above basal) and concentrations producing a half-maximal response (EC₅₀ ≅ 1 nM) were identical. The isolated mouse soleus is a slow twitch, red skeletal muscle, which binds insulin, and responds to the hormone, in a dose-related fashion as regards glucose transport and metabolism. BHI stimulated 2-deoxyglucose uptake to the same extent as pancreatic human insulin or pork insulin. Maximal stimulation (a fourfold increase) and concentrations which produced a half-maximal stimulation (EC₅₀ ≅ 0.5 nM) were identical for the three insulins. These in vitro studies show that BHI has the same intrinsic activity (maximal effectiveness) and biologic potency (EC₅₀) as pancreatic human insulin and pancreatic porcine insulin in two major target tissues of insulin, liver and skeletal muscle. Moreover, receptor-binding properties of BHI are identical to those of pork insulin in isolated rat hepatocytes and liver plasma membranes.