After cultured human fibroblasts had been maintained in a serum-free medium for twenty-four hours, a biologic effect of physiologic concentrations of insulin was demonstrated. Insulin at physiologic concentrations (0.1 mil./ml.) stimulated glucose uptake and uridine incorporation into RNA by cultured human fibroblasts. Incorporation of leucine into protein was a less sensitive index of insulin action. Fetal fibroblasts were more sensitive than adult fibroblasts to the insulin effect on RNA metabolism. This may be in part the reason for the faster growth rate of fetal fibroblasts.

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