Oscillations in citric acid cycle intermediates have never been previously reported in any type of cell. Here we show that adding pyruvate to isolated mitochondria from liver, pancreatic islets, and INS-1 insulinoma cells or adding glucose to intact INS-1 cells causes sustained oscillations in citrate levels. Other citric acid cycle intermediates measured either did not oscillate or possibly oscillated with a low amplitude. In INS-1 mitochondria citrate oscillations are in phase with NAD(P) oscillations, and in intact INS-1 cells citrate oscillations parallel oscillations in ATP, suggesting that these processes are co-regulated. Oscillations have been extensively studied in the pancreatic beta cell where oscillations in glycolysis, NAD(P)/NAD(P)H and ATP/ADP ratios, plasma membrane electrical activity, calcium levels, and insulin secretion have been well documented. Because the mitochondrion is the major site of ATP synthesis and NADH oxidation and the only site of citrate synthesis, mitochondria need to be synchronized for these factors to oscillate. In suspensions of mitochondria from various organs, most of the citrate is exported from the mitochondria. In addition, citrate inhibits its own synthesis. We propose that this enables citrate itself to act as one of the cellular messengers that synchronizes mitochondria. Furthermore, because citrate is a potent inhibitor of the glycolytic enzyme phosphofructokinase, the pacemaker of glycolytic oscillations, citrate may act as a metabolic link between mitochondria and glycolysis. Citrate oscillations may coordinate oscillations in mitochondrial energy production and anaplerosis with glycolytic oscillations, which in the beta cell are known to parallel oscillations in insulin secretion.The importance of oscillations to biological organisms can be judged from the fact that almost all cells exhibit some kind of oscillations. Oscillations have been extensively studied in the pancreatic beta cell where oscillations in glycolysis, NAD(P)/ NAD(P)H and ATP/ADP ratios, plasma membrane electrical activity, calcium levels, and insulin secretion have been well documented (1-5). However, oscillations in citric acid cycle intermediates have never been studied in any system. The beta cell is a unique fuel sensing organ in which mitochondria transduce a metabolic stimulus into multiple pharmacologic stimuli that activate the movement of insulin granules to the plasma membrane and granule extrusion into the circulation (6 -9). Glucose, the most potent insulin secretagogue, is metabolized via aerobic glycolysis. Anaplerosis, the synthesis of cycle intermediates, and cataplerosis, the export of cycle intermediates from mitochondria (10), are most likely very important for insulin secretion. In the beta cell about one-half of pyruvate, the terminal product of aerobic glycolysis in the cytosol, enters mitochondrial metabolism via carboxylation through the reaction catalyzed by pyruvate carboxylase, and the other half enters mitochondrial metabolism via decarboxylation catalyzed by pyru...
