Nitrogen-13 flux from L-[13N]glutamate in the isolated rabbit heart: effect of substrates and transaminase inhibition

Dynamic positron tomographic imaging with nitrogen-13 glutamate in patients with coronary artery disease: comparison with nitrogen-13 ammonia and fluorine-18 fluorodeoxyglucose imaging

This study was designed to test the usefulness of nitrogen-13 (N-13) glutamate imaging with positron emission tomography in defining myocardial ischemia in humans. Seventeen patients who had undergone coronary arteriography were studied with N-13 glutamate at peak supine exercise using a bicycle ergometer, as well as with the flow tracer N-13 ammonia at peak exercise during a second similar exercise test. Six of the patients also underwent imaging with N-13 glutamate at rest before exercise testing; in the remaining 11 patients imaging with fluorine-18 (F-18) fluorodeoxyglucose was performed to assess glucose metabolism after the second exercise test. Seven patients had classic metabolism-flow mismatches consistent with ischemia (that is, decreased N-13 ammonia uptake in a region with relatively increased F-18 fluorodeoxyglucose uptake). There was no evidence of increased N-13 glutamate uptake in the ischemic mismatched regions in any of these patients. In all 17 patients, the uptake of N-13 glutamate during exercise paralleled the uptake of N-13 ammonia during exercise, suggesting that N-13 glutamate behaves as a flow tracer rather than as a metabolic marker of ischemia in humans.

Association of myocardial glutamate and aspartate pool and functional recovery of postischemic heart

The effect of low flow ischemia and subsequent reperfusion with 5.5 mM glucose or 5 mM acetate on energy metabolism and catabolism of myocardial glutamate and aspartate was studied in isolated perfused guinea pig hearts. Reperfusion with acetate was followed by low recovery of the cardiac contractile function associated with a great rise in isovolumic end-diastolic pressure. It was combined with more profound losses of tissue adenine nucleotides and the total Cr compared to reperfusion with glucose. The total glutamate and aspartate pool decreased more than two-fold compared to the initial one regardless of substrate. However, glutamate content was reduced by 58 and 38% with acetate and glucose, respectively. The expenditure of both amino acids was caused by alanine formation stimulated by glycolysis/glycogenolysis. The remaining glutamate and aspartate pool in the reperfused hearts positively correlated with adenine nucleotides (r = 0.62), the total creatine (r = 0.65), and the recovery of contractile function (r = 0.64). The results suggest that the glutamate and aspartate pool may be of critical importance for postischemic functional and metabolic recovery of the heart.

Effects of anoxia on kinetics of [13N] glutamate and 13NH3 metabolism in rabbit myocardium

Positron emission tomography is a unique noninvasive imaging technique that provides cross-sectional images of radiotracer concentrations in myocardium and permits measurement of blood flow as well as metabolism. Ammonia and glutamate have been labeled with the positron-emitter 13N (half-life 10 minutes) for use with positron emission tomography as tracers of flow and metabolism, respectively. In order to characterize the fate of these 13N-labelled compounds in myocardium, isolated rabbit interventricular septa were used to study the kinetics of [13N] glutamate ([13N]glu) and 13NH3 under aerobic and anoxic conditions. Tissue analyses 6 minutes after injection of a [13N]glu bolus into myocardium revealed that 70% of the 13N-label was present in [13N]glu 12%, 11%, and 4% in [13N]alanine ([13N]ala), [13N]aspartate ([13N]asp), and [13N]glutamine ([13N]gln), respectively. The corresponding relative specific activities were 1.0:0.4:0.5:0.01. Anoxia resulted in a significant increase in [13N]ala with a reduction in [13N]glu. This was consistent with increased pyruvate production due to increased anaerobic glycolysis and transamination of pyruvate with [13N]glu to yield [13N]ala. In support of this, addition of 2 mM pyruvate to the perfusate under control conditions produced a tissue distribution of 13N similar to that with anoxia. Six minutes after a bolus of 13NH3 during both control and anoxic conditions, 60% of the tissue 13N-label was in [13N]gln with no detectable amounts in other amino acids. The rest of the 13N-label was in 13NH3. Time-activity curve analyses demonstrated that anoxia significantly reduced the tissue retention of 13N-label from 13NH3 but not from [13N]glu. Thus, 13N from 13NH3 and [13N]glu was retained in tissue by different mechanisms involving glutamate, which were affected differentially by anoxia. These results suggest that positron emission tomography imaging with 13NH3 and [13N]glu in combination may be useful in identifying ischemic myocardium.