Effect of fasting on the incorporation of [3H]-L-phenylalanine into proinsulin-insulin and total protein in isolated rat pancreatic islets.

Pancreatic islets of fed, 24 hr and 72 hr fasted rats were incubated with [3H]-L-phenylalanine at verious concentrations of glucose. Total islet protein was isolated by TCA-precipitation, proinsulin-insulin by polyacrylamide gel electrophoresis. The incorporation of label was expressed per mug dry islet weight. Fasting for 24 hr reduced the incorporation of label into (pro)insulin at glucose 3 mg/ml, but not at glucose 1 mg/ml. After 72 hr of fasting the incorporation into (pro)insulin was decreased both at glucose 1 and 3 mg/ml and the slope of the dose-response curve for glucose stimulation was reduced by 31%. In contrast, fasting caused the incorporation into total islet protein to increase. A similar tendency was observed in an unidentified protein fraction, which failed to migrate from the spacer gel on disc electrophoresis. Fasting did not affect the islet DNA content per mug dry weight. These results suggest that fasting reduces the rate of insulin biosynthesis by decreasing the glucose sensitivity of this process. Fasting may stimulate, however, the biosynthesis of an unidentified islet protein component.