Rats were infused for brief periods with buffer, glucose, or insulin. L-[4,5-3H] leucine (2.5 mCl) or L-[2,3-3H]-tryptophan (0.5 mCl) was quickly injected intravenously 30 min after the onset of the infusion, when marked hyperglycemia or hypoglycemia had been established. Rats remained connected to the infusion system and were killed 30 min after the injection of the labeled amino acid. Pancreatic islets were isolated by enzymatic digestion of the pancreas. They were processed for radioautography or for the measurement of [3H] proinsulin and [3H] insulin by immunoprecipitation and of other islet [3H]proteins by TCA precipitation. Various tissues of the rats were also removed to measure TCA-precipitable-labeled proteins. Incorporation of [3H]-leucine into proinsulin and insulin was 9 to 10 times greater in the hyperglycemic than in the hypoglycemic rats. Incorporation of [3H]-tryptophan into sedentary beta-cell proteins, measured by the density of silver grain in radioautographs, showed a sixfold difference. The great sensitivity of hormonal and nonhormonal protein biosynthesis of the pancreatic beta cell to plasma glucose was unique among tissues and among other pancreatic islet cells we studied.

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