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The long exposure times required to observe stimulatory effects of insulin on steroidogenesis and protein synthesis in granulosa cells suggested that these effects might be secondary to stimulation of another metabolic process. The present studies examined the effects of insulin, the insulin-like growth factor somatomedin-C (Sm-C), human chorionic gonadotropin (hCG), and forskolin, a compound that activates adenylyl cyclase independently of a receptor, on glucose metabolism. Granulosa cells from preovulatory porcine ovarian follicles were incubated at 37 degrees C in Dulbecco's phosphate-buffered saline supplemented with bovine serum albumin, vitamins, amino acids, and glucose (0.01-20 mM). Cells were incubated with [14C]glucose for up to 23 h with or without a prior 20-h preincubation. Oxidation of glucose, assessed by quantitation of 14CO2 produced, was dependent on time and concentration of glucose. Optimal glucose concentrations for glucose oxidation were 3 mM in the absence or presence of insulin and correlated well with the measured glucose concentrations in follicular fluid (3 mM). After a 20-h preincubation in the absence or presence of insulin (1 microM), the rates of CO2 production were 10.6 and 21.6 pmol/micrograms DNA/h for control and insulin-treated cells, respectively. Insulin had an EC50 of 164 nM. Sm-C and hCG were more potent stimulators than insulin with EC50s of 768 pM and 161 pM, respectively. The greater sensitivity of granulosa cells to Sm-C than to insulin supports the concept that insulin exerts its effect via reactivity with the Sm-C receptor. The effect of hCG may have been mediated by cyclic adenosine 3',5'-monophosphate (cAMP), since forskolin also enhanced 14CO2 production.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin, Somatomedin-C, Human Chorionic Gonadotropin, and Forskolin Enhance Glucose Oxidation by Granulosa Cells1