To develop a technique for examining short term insulin effects in cultured human fibroblasts, the effect of insulin on glycogen synthase was determined. Fibroblasts contain detectable glycogen synthase activity which can be converted from the D form, dependent on glucose-6-phosphate for activity, to the I form, which is independent of glucose-6-phosphate, in the presence of insulin. In the basal state, about 6–20% of the glycogen synthase is independent activity. This increases to between 30 and 60 %l activity after insulin stimulation. Stimulation Is seen with insulin concentrations as low as 10−9 M, although maximal stimulation requires 10−7 – 10−6 M insulin. The effect of insulin is rapid, reaching a maximum within 20 min of incubation. Incubating the cells in fresh media without serum and glucose for up to 24 h before assay enhances the cellular response to insulin. Glucose has only a small, transient effect on the conversion of the enzyme from the D to the I form in the absence of insulin. These data demonstrate that human fibroblasts possess insulin-sensitive glycogen synthase, which may be used as a marker of metabolic response in disease states.

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