Histochemical evidence for pathways insulin cells use to oxidize glycolysis-derived NADH

The activity of lactate dehydrogenase is known to be low in the pancreatic beta cell, and the activity of the mitochondrial glycerol phosphate dehydrogenase (mGPD), the key enzyme of the glycerol phosphate shuttle, is known to be high in this cell. Lactate dehydrogenase was demonstrated histochemically in insulin positive cells of the rat pancreas, and its activity was semiquantified densitometrically; activity in these cells was estimated to be about 8% of that in the surrounding acinar tissue. mGPD histochemical activity was extremely high in cells exhibiting insulin immunofluorescence, while activity in surrounding pancreas tissue was negligible. When the activity was measured in situ at a physiologic concentration of substrate, this enzyme was inactive in the absence of free calcium. These results are consistent with the idea that glucose, the most potent physiologic insulin secretagogue, stimulates insulin secretion via aerobic glycolysis. If glycolysis-derived NADH, instead of being reoxidized in a mitochondrial hydrogen shuttle, is reoxidized to NAD by pyruvate via the reaction catalyzed by lactate dehydrogenase with the resulting formation of lactate, there would be little or no pyruvate available for mitochondrial metabolism. Consequently adenosine triphosphate formation would be about 5% to 7% of that formed by the complete combustion of glucose to carbon dioxide via mitochondrial metabolism. The low lactate dehydrogenase and high mGPD emphasize the importance of mitochondrial hydrogen shuttles for reoxidation of glycolysis-derived NADH in insulin secretion.