1. Rat pancreatic islets were isolated and then maintained in culture for 2-4 days before being incubated in groups of 100 in the presence of different glucose (0-20 mM) or CaCl2 (1.2-4.2 mM) concentrations, or with uncoupler. 2. Increases in extracellular glucose concentration resulted in increases in the amount of active, non-phosphorylated, pyruvate dehydrogenase in the islets, with half-maximal effects around 5-6 mM-glucose. Increasing extracellular glucose from 3 to 20 mM resulted in a 4-6-fold activation of pyruvate dehydrogenase within 2 min. 3. The total enzyme activity was unchanged, and averaged 0.4 m-unit/100 islets at 37 degrees C. 4. These changes in active pyruvate dehydrogenase were broadly similar to changes in insulin secretion by the islets. 5. Increasing extracellular Ca2+ or adding uncoupler also activated pyruvate dehydrogenase to a similar degree, but only the former was associated with increased insulin secretion.

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