Export of metabolites from pancreatic islet mitochondria as a means to study anaplerosis in insulin secretion

Abstract

Recent evidence suggests that the mitochondrial synthesis (anaplerosis) of α-ketoglutarate or intermediates that can be converted to α-ketoglutarate, such as citrate or glutamate, are important for insulin secretion stimulated by metabolizable secretagogues. In order to focus on the productive role of mitochondria (anaplerosis) separate from the consumptive role of the cytosol (cataplerosis) in insulin secretion, pyruvate and other metabolites of insulin secretagogues were added to microgram amounts of mitochondria obtainable from rat or mouse pancreatic islets and the export of metabolites was surveyed. Cellular levels of metabolites in rat islets were also measured. The export of malate from mitochondria was the most responsive to various substrates. The export of citrate did not increase in the presence of pyruvate alone or pyruvate plus glutamate, but malate plus pyruvate caused citrate to be exported. Citrate levels in intact cells did not change with glucose. Glutamate levels did not increase in intact islets in the presence of glucose, thus not providing evidence for glutamate acting as a messenger in glucose-induced insulin secretion. The citrate level may not need to increase in order to provide increased malonyl-coenzyme A for signaling insulin secretion. Unlike many cells, insulin cells probably obtain cytosolic NADPH equivalents by exporting them from mitochondria to the cytosol via a pyruvate malate shuttle or an isocitrate shuttle. The current results suggest that the reason for anaplerosis in insulin secretion is quite complex and not fully explained by current knowledge.