BIOLOGICAL ACTIONS OF INSULIN ARE DIFFERENTIALLY REGULATED BY GLUCOSEAND INSULIN IN PRIMARY CULTURED ADIPOCYTES - CHRONIC ABILITY TO INCREASE GLYCOGEN-SYNTHASE ACTIVITY

We have shown previously that prolonged exposure to insulin and glucos e impairs the insulin-responsive glucose transport system in primary c ultured adipocytes. To assess the ability of insulin and glucose to re gulate other cellular insulin actions, epididymal rat adipocytes were cultured in media containing 0-15 mM D-glucose and with or without ins ulin (50 ng/ml). After 24 h, cells were washed and basal and maximally insulin-stimulated rates of 2-deoxy-D-glucose uptake, L-leucine incor poration into protein, glucose oxidation to CO2, glucose incorporation into lipids, and glycogen synthase activity were measured. The result s confirmed that glucose potentiates insulin's chronic ability to decr ease basal and maximal glucose transport rates by similar to 50% at 5 mM glucose and by similar to 70% at 15 mM glucose compared with contro l cells. However, neither glucose nor insulin, alone or in combination , affected rates of leucine incorporation into protein. In addition, b asal and maximal rates of glucose oxidation and of glucose incorporati on into lipids were not regulated by glucose, and maximal responses de clined similar to 50% over 24 h only when insulin was not present duri ng preincubation (i.e., chronic insulin exposure was necessary to main tain full maximal responses). Glycogen synthase activity was measured in a cell-free system (0.5 mM UDP-glucose, with 10 or 0.01 mM glucose- 6-phosphate) after exposing intact cells to glucose and insulin. Both short-term (1 h) and long-term (24 h) exposure to glucose alone led a dose-dependent increase in I-form and D-form glycogen synthase activit y. Chronic exposure to insulin also increased total glycogen synthase activity (I- plus D-form) but did not affect absolute rates of maximal ly stimulated I-form activity. Glucose (but not insulin) increased the cellular content of immunoreactive glycogen synthase by 70% after 1 h . These results show that 1) chronic exposure to glucose and insulin i mpairs insulin responsiveness of the glucose transport system but does not affect rates of amino acid incorporation into protein; 2) the chr onic presence of insulin is necessary for the maintenance of normal ma ximally stimulated rates of glucose oxidation and of glucose incorpora tion into lipids in cultured cells; and 3) glucose increases both D-fo rm and I-form glycogen synthase activity, in part by increasing the am ount of synthase protein, whereas chronic insulin exposure increases t otal glycogen synthase activity without altering maximal absolute rate s of I-form activity. In conclusion, glucose and insulin differentiall y regulate the multiple biological actions of insulin in cultured adip ocytes, indicating that underlying mechanisms involve the hormone effe ctor systems or components of signal transduction unique to individual action pathways. In NIDDM, defects in glucose transport but not other insulin actions may be acquired as a result of high circulating blood glucose and insulin concentrations.