Changes in citric acid cycle flux and anaplerosis antedate the functional decline in isolated rat hearts utilizing acetoacetate

Modeling enrichment kinetics from dynamic 13C-NMR spectra: theoretical analysis and practical considerations

Measurements of oxidative metabolism in the heart from dynamic 13C nuclear magnetic resonance (NMR) spectroscopy rely on 13C turnover in the NMR-detectable glutamate pool. A kinetic model was developed for the analysis of isotope turnover to determine tricarboxylic acid cycle flux (VTCA) and the interconversion rate between alpha-ketoglutarate and glutamate (F1) by fitting the model to NMR data of glutamate enrichment. The results of data fitting are highly reproducible when the noise level is within 10%, making this model applicable to single or grouped experiments. The values for VTCA and F1 were unchanged whether obtained from least-squares fitting of the model to mean experimental enrichment data with standard deviations in the cost function (VTCA = 10.52 mumol.min-1.g dry wt-1, F1 = 10.67 mumol.min-1.g dry wt-1) or to the individual enrichment values for each heart with the NMR noise level in the cost function (VTCA = 10.67 mumol.min-1.g dry wt-1, F1 = 10.18 mumol.min-1.g dry wt-1). Computer simulation and theoretical analysis indicate that glutamate enrichment kinetics are insensitive to the fractional enrichment of acetyl-CoA and changes in small intermediate pools (< 1 mumol/g dry wt). Therefore, high-resolution NMR analysis of tissue extracts and biochemical assays for intermediates at low concentrations are unnecessary. However, a high correlation between VTCA and F1 exists, as anticipated from competition for alpha-ketoglutarate, which indicates the utility of introducing independent experimental constraints into the data fitting for accurate quantification.

Contribution of various substrates to total citric acid cycle flux and anaplerosis as determined by 13C isotopomer analysis and O2 consumption in the heart

A simple relationship between parameters derived from a 13C NMR isotopomer analysis and O2 consumption is presented that allows measurement of the absolute rate of acetyl-CoA oxidation and anaplerotic flux in tissues oxidizing a mixture of four substrates. The method was first applied in a study of the effects of work state and beta-adrenergic stimulation on net acetate oxidation and anaplerosis in the isolated working rat heart. The results demonstrate that the anticipated ratio of 2 between O2 consumption and TCA cycle flux for hearts oxidizing only acetate holds at low workload when anaplerosis is low, but deviates toward a factor of 3 under high workload conditions when anaplerosis is increased. This analysis was also extended to hearts that oxidize a more physiological mixture of substrates including long-chain fatty acids, acetoacetate, lactate, pyruvate, and glucose. We show that the contribution each substrate makes to total TCA cycle flux can be determined by combined 13C NMR and O2 consumption measurements. The present study also demonstrates that stimulation of anaplerosis (by addition of propionate) can significantly alter the relative contribution each substrate makes to total TCA cycle flux. We conclude if 13C labeling patterns are selected appropriately, a comprehensive picture of flux through all major metabolic pathways feeding the cycle can be determined in a single experiment even when complex physiological mixtures of substrates are provided.

Kinetic analysis of dynamic 13C NMR spectra: metabolic flux, regulation, and compartmentation in hearts

Control of oxidative metabolism was studied using 13C NMR spectroscopy to detect rate-limiting steps in 13C labeling of glutamate. 13C NMR spectra were acquired every 1 or 2 min from isolated rabbit hearts perfused with either 2.5 mM [2-13C]acetate or 2.5 mM [2-13C]butyrate with or without KCl arrest. Tricarboxylic acid cycle flux (VTCA) and the exchange rate between alpha-ketoglutarate and glutamate (F1) were determined by least-square fitting of a kinetic model to NMR data. Rates were compared to measured kinetics of the cardiac glutamate-oxaloacetate transaminase (GOT). Despite similar oxygen use, hearts oxidizing butyrate instead of acetate showed delayed incorporation of 13C label into glutamate and lower VTCA, because of the influence of beta-oxidation: butyrate = 7.1 +/- 0.2 mumol/min/g dry wt; acetate = 10.1 +/- 0.2; butyrate + KCl = 1.8 +/- 0.1; acetate + KCl = 3.1 +/- 0.1 (mean +/- SD). F1 ranged from a low of 4.4 +/- 1.0 mumol/min/g (butyrate + KCl) to 9.3 +/- 0.6 (acetate), at least 20-fold slower than GOT flux, and proved to be rate limiting for isotope turnover in the glutamate pool. Therefore, dynamic 13C NMR observations were sensitive not only to TCA cycle flux but also to the interconversion between TCA cycle intermediates and glutamate.

Differing protection with aspartate and glutamate cardioplegia in the isolated rat heart

Aspartate and glutamate each have been shown to improve cardiac recovery after hypoxia or ischemia under normothermic conditions, but whether their effects are additive and to what extent they are modified by hypothermia has not been studied systematically. We set out to compare the individual and combined protective effects of aspartate and glutamate during cardioplegic arrest under normothermic and hypothermic conditions in the rat. Using isolated working rat hearts, functional and metabolic recovery was assessed after 0.5 hours of potassium arrest at 37 degrees C or 5 hours at 2 degrees C in control hearts (C) and in hearts in which 20 mmol/L glutamate (G), 20 mmol/L aspartate (A), or both (A + G) was added to the cardioplegic solution. Under normothermic conditions, percentage recovery of prearrest work (mean +/- standard error of the mean) was as follows: C = 31.7 +/- 2.8, G = 34.8 +/- 0.2, A = 49.6 +/- 2.8*, A + G = 53.7 +/- 2.3*. Under hypothermic conditions, the values were as follows: C = 40.4 +/- 4.0, G = 45.2 +/- 2.3, A = 59.4 +/- 1.8*, A + G = 54.1 +/- 1.2* (*p < 0.01 versus C and G). Recovery of postischemic high-energy phosphate content followed the same pattern: A = A + G > G or C. Measurement of postischemic myocardial content of amino acids showed that recovery of function and energy status correlated with maintenance of myocardial levels of aspartate (r = 0.9; p < 0.01) but not glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)

Estimation of TCA cycle flux, aminotransferase flux, and anaplerosis in heart: validation with syntactic model

Weiss et al. (Circ. Res. 70: 392-408, 1992) proposed a model of the citric acid cycle (CAC) in myocytes and a system of 17 differential equations that can be used to describe the changes over time in enrichment of carbons C-2 and C-4 of glutamate under conditions of metabolic steady state. They also proposed an empirical measure (KT) of flux through the CAC, which has been shown to be correlated to O2 consumption in rat hearts perfused with acetate or a mixture of glucose and acetate. We report a new method for estimation of the absolute rate of the flux through the CAC in heart (vTCA), without the numerical solution of differential equations. Unlike KT, our estimate is equal to the rate of flux catalyzed by the alpha-ketoglutarate dehydrogenase complex (vTCA), not merely correlated with it. We also estimate the rate of flux catalyzed by aspartate aminotransferase (vTA) and by NADP(+)-dependent malic enzyme (an anaplerotic reaction). The formula for vTCA during administration of [2-13C]acetate is as follows: vTCA = M[(C-2ssLC-4)/[C-4ss(LC-4-LC-2)]], where C-2ss and C-4ss represent steady-state fractional enrichment, LC-2 and LC-4 represent dominant rate constants of C-2 and C-4 of glutamate, respectively, and M is the sum of concentrations of aspartate, glutamate, and intermediates of the CAC. The assumptions underlying our formula are as follows: 1) metabolic steady state is maintained, 2) exchange of molecules between cytosolic and mitochondrial compartments is rapid, 3) 13C enters pools of the CAC only from acetyl CoA via citrate synthase, 4) [citrate]/[glutamate] < 1 + (vTCA/vTA), and 5) (m-[glutamate])/M < C-2ss/C-4ss.

Coenzyme A sequestration in rat hearts oxidizing ketone bodies

Previous studies have indicated that ketone body-mediated contractile failure in rat hearts is due to inhibition of 2-oxoglutarate dehydrogenase, and it has been speculated that this inhibition is due to the sequestration of intramitochondrial CoA as acetoacetyl-CoA and acetyl-CoA. These studies were performed to determine whether oxidation of acetoacetate by isolated rat heart mitochondria results in a fall in intramitochondrial nonesterified CoA [CoASH] and whether increasing the available CoA improves contractile performance in hearts oxidizing acetoacetate. The oxidation of acetoacetate by isolated rat heart mitochondria resulted in depressed state 3 respiration as well as in a decrease in [CoASH]. Increasing the tissue content of CoASH in perfused hearts by providing the precursors for CoA relieved inhibition of 2-oxoglutarate dehydrogenase and improved the contractile performance of isolated working hearts. In contrast, the addition of carnitine increased the tissue content of CoASH but did not improve function. These findings suggest the presence of two different pools of CoASH. We conclude that ketone body-mediated inhibition of 2-oxoglutarate dehydrogenase is due to decreased intramitochondrial CoASH and that this inhibition of the citric acid cycle is a plausible mechanism for concomitant contractile failure.

Nuclear magnetic resonance evaluation of metabolic and respiratory support of work load in intact rabbit hearts

Pre-steady-state 13C nuclear magnetic resonance (NMR) spectra can provide a nondestructive probe of metabolic events associated with the physiology of intact organs. Therefore, the relation between phosphorylation state and intermediary metabolism in rabbit hearts, oxidizing [2-13C]acetate, was examined with a combination of 31P and 13C NMR. Multiple enrichment of the tissue glutamate pool with 13C as an index of metabolic turnover within the tricarboxylic acid cycle was readily observed as a function of work load. Dynamic changes in pre-steady-state 13C spectra evolved according to work load and correlated closely to respiratory rate in rabbit hearts perfused 1) under normal conditions (n = 7), 2) at basal metabolic rates (20 mM KCl arrest, n = 5), 3) and at heightened contractile state (10(-7) M isoproterenol, n = 7). The ratio of signal intensity arising from the secondary labeling sites within glutamate (C-2 and C-3) to that of the initial labeling site (C-4) reached steady state within 8.5 minutes in isoproterenol-treated hearts versus 18.5 minutes in control hearts. Work load did not affect glutamate concentration or fractional enrichment at the C-4 position, although an unlabeled fraction of glutamate persisted. Arrested hearts displayed slowed evolution of steady-state 13C enrichment with increased contributions from anaplerotic sources for tricarboxylic acid intermediate formation (32%) as compared with control (9%). Thus, the response of mitochondrial dehydrogenase activity to the demands of cardiac performance is likely to influence the recruitment of anabolic sources supplying the tricarboxylic acid cycle.(ABSTRACT TRUNCATED AT 250 WORDS)