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J Appl Physiol 100: 76-82, 2006. First published September 1, 2005; doi:10.1152/japplphysiol.00255.2005
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Differential effects of heptanoate and hexanoate on myocardial citric acid cycle intermediates following ischemia-reperfusion

Isidore C. Okere,1 Tracy A. McElfresh,1 Daniel Z. Brunengraber,1 Wenjun Martini,1,2 Joseph P. Sterk,1 Hazel Huang,1 Margaret P. Chandler,1 Henri Brunengraber,2 and William C. Stanley1,2

Departments of 1Physiology and Biophysics and 2Nutrition, Case Western Reserve University, Cleveland, Ohio

Submitted 4 March 2005 ; accepted in final form 30 August 2005


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
In the normal heart, there is loss of citric acid cycle (CAC) intermediates that is matched by the entry of intermediates from outside the cycle, a process termed anaplerosis. Previous in vitro studies suggest that supplementation with anaplerotic substrates improves cardiac function during myocardial ischemia and/or reperfusion. The present investigation assessed whether treatment with the anaplerotic medium-chain fatty acid heptanoate improves contractile function during ischemia and reperfusion. The left anterior descending coronary artery of anesthetized pigs was subjected to 60 min of 60% flow reduction and 30 min of reperfusion. Three treatment groups were studied: saline control, heptanoate (0.4 mM), or hexanoate as a negative control (0.4 mM). Treatment was initiated after 30 min of ischemia and continued through reperfusion. Myocardial CAC intermediate content was not affected by ischemia-reperfusion; however, treatment with heptanoate resulted in a more than twofold increase in fumarate and malate, with no change in citrate and succinate, while treatment with hexanoate did not increase fumarate or malate but increased succinate by 1.8-fold. There were no differences among groups in lactate exchange, glucose oxidation, oxygen consumption, and contractile power. In conclusion, despite a significant increase in the content of carbon-4 CAC intermediates, treatment with heptanoate did not result in improved mechanical function of the heart in this model of reversible ischemia-reperfusion. This suggests that reduced anaplerosis and CAC dysfunction do not play a major role in contractile and metabolic derangements observed with a 60% decrease in coronary flow followed by reperfusion.

anaplerosis; fatty acids; heart; metabolism; mitochondria; pyruvate dehydrogenase

THE ATP FORMATION THAT IS required to support cardiac function is dependent on sufficient citric acid cycle (CAC) flux. Normal CAC flux is sustained by a small pool of intermediates with a rapid turnover. Under normal aerobic conditions, there is a physiological loss of some CAC intermediates, i.e., citrate, {alpha}-ketoglutarate ({alpha}-KG), succinate, fumarate, and malate (5, 8, 16, 21, 25, 26, 42). This loss of CAC intermediates is matched by the entry of intermediates from outside the cycle, a process termed anaplerosis (14), from pyruvate, glutamate, or propionyl-CoA (Fig. 1) (6, 8, 22, 26, 28). The necessity of constant anaplerosis in the normal heart is evident by the decrease in mechanical work by hearts perfused with buffer containing acetoacetate as the only fuel. When an anaplerotic substrate such as pyruvate, lactate, or propionate was added to the perfusate, in addition to acetoacetate, the mechanical function of the heart improved immediately (30, 31).



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  		Fig. 1. Schematic depiction of the metabolism of odd- and even-chain fatty acids in the heart. CPT, carnitine palmatoyl transferase; {alpha}-KG, {alpha}-ketoglutarate.

 
In vitro studies suggest that, during postischemic reperfusion, there is inhibition of the activity of the CAC enzymes isocitrate dehydrogenase, aconitase, and {alpha}-KG dehydrogenase, causing impairment in CAC flux and reduction in mitochondrial ATP generation (12, 19, 24, 32). Additional evidence suggests that there is excessive loss of CAC intermediates in the reperfused heart and that infusion of anaplerotic substrates improves the mechanical function of the heart (11, 31, 40, 41). Elevating arterial pyruvate concentrations reduces infarct size and improves contractile recovery following ischemia, possibly through stimulation of anaplerosis via pyruvate carboxylation (20, 36). It has been suggested that treatment with propionate improves cardiac metabolic and contractile function with ischemia and/or reperfusion (9, 17). Our laboratory recently demonstrated that propionate is efficiently converted to the CAC intermediate succinyl-CoA in perfused rat hearts and in live pig hearts (22, 28).

In patients with deficiencies in the {beta}-oxidation of long-chain fatty acids, supplementation of the diet with the triglyceride of heptanoate [which generates propionyl-CoA after two cycles of {beta}-oxidation (Fig. 1)] resulted in a dramatic clinical improvement in cardiac and skeletal muscle function that is not observed with octanoate supplementation (29). As these patients often present with high activities of plasma creatine kinase (reflecting increased cell permeability), it was postulated that the beneficial effect of heptanoate resulted from its anaplerotic property. One can thus hypothesize that acute treatment with heptanoate during and after myocardial ischemia could be cardioprotective due to enhanced anaplerotic flux. On the other hand, treatment with the medium-chain fatty acids hexanoate or octanoate results in an increase in the myocardial content of some CAC intermediates (26, 39), despite the absence of a direct anaplerotic effect. The differential effects of even-numbered and odd-numbered medium-chain fatty acids on cardiac function, substrate metabolism, and the myocardial content of the various CAC intermediates during ischemia-reperfusion have not been addressed.

The goal of the present investigation was to assess whether treatment with the anaplerotic medium-chain fatty acid heptanoate would improve contractile function during ischemia and reperfusion. Studies were performed in anesthetized pigs subjected to a 60% reduction in coronary blood flow for 1 h followed by 30 min of reperfusion, with heptanoate treatment administered during the last 30 min of ischemia and during reperfusion. Heptanoate-treated pigs were compared with nonischemic hearts, ischemic-reperfused hearts treated with saline, and ischemic-reperfused hearts treated with hexanoate, a nonanaplerotic medium-chain fatty acid. We hypothesized that treatment with heptanoate would selectively increase the myocardial content of four carbon CAC intermediates (reflective of greater anaplerosis from propionyl-CoA) and improve cardiac function during ischemia and reperfusion. In addition, we assessed the incorporation of heptanoate into the CAC through anaplerosis and {beta}-oxidation (Fig. 1) using an intracoronary infusion of [13C]heptanoate under normal flow condition.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Domestic pigs (30–40 kg) were randomly assigned to four groups: a saline-treated control group (Con) (n = 9), a heptanoate group (Hep) (n = 10), a hexanoate group (Hex) (n = 7), and a nonischemic group (n = 7/group). All personnel involved in performance of the animal experiments or biochemical analysis were blinded to treatment. The experimental solutions were prepared freshly each day by an independent investigator not involved in the animal experimentation. Studies were conducted in accordance with the Guide for the Care and Use of Laboratory Animals (NIH publication Number 85–23) and the Institutional Animal Care and Use Committee at Case Western Reserve University.

Surgical Preparation

The experimental preparation has been previously described in detail (3, 26). Briefly, animals were sedated with Telazol (6 mg/kg im), anesthetized with isoflurane (5%), intubated, and maintained on isoflurane (0.75–1.5%) and ketamine (3 mg·kg–1·h–1 iv). The heart was exposed via a midline sternotomy, and the animal was heparinized (200 U/kg bolus + 100 U·kg–1·h–1 iv). A cannula was placed in the anterior interventricular vein to collect venous blood samples from the perfusion zone of the left anterior descending coronary artery (LAD). An extracorporeal perfusion circuit via roller pumps controlled blood flow to the LAD, with blood supplied from the femoral artery. The LAD pump flow was adjusted to give an interventricular venous oxygen saturation of 35–45%. Arterial blood gases were maintained in the normal range (PO2 ≥ 100 Torr, PCO2 35–45 Torr, and pH 7.35–7.45). Left ventricular (LV) pressure was measured with a high-fidelity manometer-tipped catheter (Millar Instruments). Regional segment length was measured in duplicate in the LAD bed using sonomicrometry, and anterior wall contractile function was assessed from the LV pressure-segment length loop area (3).

Experimental Protocols

We first performed studies by using the swine preparation described above to assess the incorporation of heptanoate into the CAC through anaplerosis via propionyl-CoA formation from carbons 5, 6, and 7 of heptanoate and subsequent carboxylation to methylmalonyl-CoA and isomerization to succinyl-CoA, and to acetyl-CoA through {beta}-oxidation (Fig. 1). An infusion of unlabeled heptanoate (50 mM as a sodium salt in NaCl at 308 mosmol/kgH2O) was started into the coronary bypass circuit at a rate that imposed an inflowing concentration in the LAD artery of 0.25 mM. After 20 min, the infused substrate was switched to either [5,6,7-13C3]heptanoate (n = 2) or [1-13C]heptanoate from Isotec (Miamisburg, OH) (n = 6) at the same concentration, which was infused for an additional 60 min. At the end of the experiment, a biopsy of the LAD perfusion bed was rapidly freeze clamped and subsequently analyzed by GC-MS for the enrichment of citrate, succinate, fumarate, and malate with 13C, as previously described (22). The raw data were corrected for natural enrichment of heavy isotopomers using GC-MS data from pigs not receiving 13C isotope infusion (1, 10).

Ischemia-Reperfusion Protocol

After the instrumentation was completed, LAD blood flow was adjusted to give interventricular venous oxygen saturation of 35–45%. A continuous infusion of [U-14C]glucose (0.2 µCi/min) was initiated 30 min before ischemia into the proximal end of the coronary perfusion line at a rate of 0.1 ml/min. Regional myocardial ischemia-reperfusion was induced in the LAD perfusion bed by reducing the LAD flow by 60% for 60 min, followed by 30 min of reperfusion at the preischemic LAD flow. Arterial and interventricular venous samples were drawn 5 and 3 min before ischemia; at 5, 20, 26, 40, 50, and 57 min of ischemia; and at 7, 20, and 30 min of reperfusion. From 30 min of ischemia to the end of the protocol, animals were treated with an infusion of either 1) saline (Con); 2) sodium heptanoate (Hep) (50 mM; Sigma-Aldrich); or 3) sodium hexanoate (Hex) (50 mM; Sigma-Aldrich) in NaCl at 308 mosmol/kgH2O into the perfusion circuit at 8 µl/min for every milliliter of blood flow in the LAD perfusion circuit. This infusion rate was aimed to result in concentration of hexanoate and heptanoate of 0.4 mM in LAD arterial blood. Dilution of anterior interventricular venous blood with blood not derived from the LAD was measured by using a constant infusion of indocyanide green dye (0.3 mg/min for 5 min) into the LAD perfusion line under aerobic conditions (0.15 mg/ml for 5 min) during ischemia with substrate perfusion and (0.3 mg/ml for 5 min) during reperfusion (3). All blood samples were analyzed for the concentrations of oxygen, lactate, glucose, plasma free fatty acids, heptanoate, hexanoate concentrations, and 14CO2. Heart rate, LV pressure, and segment length were recorded online (3). At the end of the protocol, a transmural myocardial biopsy was taken from the anterior LV free wall, rapidly freeze-clamped in large steel tongs precooled with liquid nitrogen, and stored at –80°C for subsequent analysis for the myocardial content of lactate, glycogen, ATP, and CAC intermediates.

For the nonischemic group, the animals were anesthetized and surgically prepared as described above but with no flow restriction or substrate perfusion during the protocol. Transmural myocardial punch biopsies were taken after 55 min from the LV free wall and freeze-clamped as above.

Analytic Methods

Blood levels of glucose and lactate were measured by using enzymatic spectrophotometric assays. Plasma-free fatty acids were assayed by using enzymatic spectrophotometric assays (Wako Chemicals, Richmond, VA), which measures both medium- and long-chain fatty acids, and thus includes heptanoate and hexanoate concentrations. The concentration of 14CO2 and the specific radioactivity of [14C]glucose in blood were measured as previously described (3). Tissue lactate and glycogen content were measured by using enzymatic spectrophotometric methods; ATP content of the tissue was determined by the luciferase assay (3). The myocardial activity of pyruvate dehydrogenase (PDH) was measured in the terminal biopsy by using [2-14C]pyruvate, as previously described (38).

CAC intermediates were analyzed on an Agilent 5973 mass spectrometer, equipped with an Agilent 6890 gas chromatograph, using a HP-5MS 5% phenyl methyl siloxane fused silica capillary column (60 m, 250-µm inner diameter, 0.25-µm film thickness), according to previously published methods (7, 34). The internal standards were [2,2,4,4-2H4]citrate, [2,2,3,3-2H4]succinate, [U-13C4]fumarate, [2,3,3-2H3]malate, and {alpha}-keto-[U-13C5]glutarate ({alpha}-KG). The latter standard was generated by the transamination of [U-13C5]glutamate, as previously described (15). For the studies with [5,6,7-13C3]heptanoate or [1-13C]heptanoate, the enrichment on CAC intermediates was corrected for the background natural abundance of 13C, as previously described (26). The enrichment of CAC intermediates is expressed relative to the natural abundance, with M set to the natural abundance, and M+1 and M+3 meaning 1 or 3 additional mass units. Free CoA and acetyl-CoA levels were measured by using a radioenzymatic method (2).

Calculations

The myocardial net uptake (µmol·g–1·min–1) of lactate and free fatty acids was calculated as the product of the arterial and coronary venous substrate concentration difference and myocardial blood flow. The rate of glucose oxidation (µmol·g–1·min–1) was calculated as the product of myocardial blood flow (ml·g–1·min–1) and the release of either 14CO2 (disintegrations·min–1·ml–1) into the coronary vein, divided by the arterial specific radioactivity of glucose (disintegrations·min–1·µmol–1) (3, 43, 44). The interventricular venous concentration of 14CO2 was corrected for dilution of blood (~ 10%) derived from coronary arteries other than the LAD by dividing the measured values by the ratio of venous to arterial plasma green dye concentration (3). Myocardial blood flow (ml·g–1·min–1) was measured from the calibrated pump flow of the coronary perfusion line and the weight of the LAD perfusion bed (3). The LV pressure segment length loop area times heart rate was used as an index of anterior wall external power (3).

Statistical Analysis

A three-way ANOVA was used to compare all changes in hemodynamic function, plasma fatty acids, and glucose and lactate concentrations and uptakes over time and among treatment groups. A one-way ANOVA was used for the comparison of tissue contents measured in the terminal myocardial biopsy. All values are reported as means ± SE, with a 0.05 level of significance.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Preliminary Studies with [13C]Heptanoate

In the pigs perfused with [1-13C]heptanoate (which enters the CAC with the conversion of 1-13C-acetyl-CoA to citrate), there was clear incorporation of label into citrate, and the expected fall in enrichment from citrate to succinate clearly demonstrating that carbon-1 of heptanoate enters the CAC via {beta}-oxidation and not through anaplerosis. An estimate of the enrichment of mitochondrial acetyl-CoA can be calculated by deducting the M1 enrichment of oxaloacetate from that of M1 citrate and multiplying the result by two [since one heptanoate molecule yields two acetyl-CoA groups: (6.2 – 2.3) x 2 = 7.8%]. In the experiments with [5,6,7-13C3]heptanoate, there was a clear enrichment of M3 succinate, which is the approximate percentage of the succinate molecules derived from anaplerosis from heptanoate through the propionyl-CoA pathway (Table 1) (22). Taken together, these results demonstrate that, even at low arterial concentrations (0.25 mM), heptanoate is readily metabolized by the myocardium and enters the CAC via both {beta}-oxidation and anaplerosis.


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  		Table 1. Mass isotopomer distribution of citric acid cycle intermediates in the left anterior descending coronary artery bed myocardium following 60 min of infusion with [5,6,7- 13C3]heptanoate or [1-13C]heptanoate

 
Ischemia-Reperfusion Studies

Hemodynamic measurements and regional contractile function.   Myocardial ischemia resulted in decreased regional contractile function without a significant change in heart rate or LV peak systolic pressure (Table 2). Contractile function improved during reperfusion but did not return to preischemic values. There were no significant differences among treatment groups.


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  		Table 2. Cardiac function

 
Metabolic measurements.   Myocardial oxygen consumption was not different among groups under nonischemic conditions (2.41 ± 0.21, 2.87 ± 0.18, and 2.23 ± 0.28 µmol·g–1·min–1 in Con, Hep, and Hex groups, respectively), decreased by 49% during ischemia, and was restored to 78% of nonischemic values by 30 min of reperfusion. There were no differences among groups in myocardial oxygen consumption during ischemia or reperfusion. Arterial free fatty acid concentration was not different among groups under nonischemic condition or during the first 30 min of ischemia but was significantly elevated in the Hep and Hex groups compared with the Con group during the ischemic treatment period (0.649 ± 0.078, 0.816 ± 0.149, and 0.479 ± 0.096 mM, respectively) and during reperfusion (0.849 ± 0.126, 0.983 ± 0.115, and 0.496 ± 0.122 mM, respectively). The net uptake of fatty acids was not different among groups during ischemia before treatment; however, it was significantly higher in the Hep and Hex groups during ischemia and reperfusion (Fig. 2).



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  		Fig. 2. Net rate of fatty acid uptake by the myocardium. Values are means ± SE. *P < 0.05, and {dagger}P < 0.01 compared with the saline group.

 
There were no differences among treatment groups in myocardial net lactate uptake. Ischemia resulted in a switch from positive to negative lactate uptake (–0.225 ± 0.073, –0.365 ± 0.087, and –0.291 ± 0.169 µmol·g–1·min–1 in Con, Hep, and Hex groups, respectively), which persisted into the ischemic treatment period (–0.175 ± 0.084, –0.152 ± 0.064, and –0.250 ± 0.163 µmol·g–1·min–1, respectively). Reperfusion resulted in a reversal back to net lactate uptake (0.160 ± 0.063, 0.164 ± 0.057, and 0.106 ± 0.015 µmol·g–1·min–1, respectively). The rate of glucose oxidation was not affected by ischemia or reperfusion within each treatment group, and it was not different among the three treatment groups under aerobic conditions (0.048 ± 0.014, 0.059 ± 0.016, and 0.068 ± 0.036 µmol·g–1·min–1 for the Con, Hep, and Hex groups, respectively) during ischemia before treatment (0.056 ± 0.013, 0.077 ± 0.027, and 0.053 ± 0.021 µmol·g–1·min–1), during ischemia with treatment (0.068 ± 0.021, 0.091 ± 0.024, and 0.040 ± 0.016 µmol·g–1·min–1), and during reperfusion (0.048 ± 0.012, 0.064 ± 0.014, and 0.045 ± 0.011 µmol·g–1·min–1, respectively).

Myocardial lactate content was not different between the normal nonischemic group and the Con ischemic/reperfused group, and there were no differences among the Con, Hep, and Hex groups (Table 3). Myocardial glycogen content was significantly reduced in the Con ischemic/reperfused compared with the normal nonischemic group, and there were no differences among the Con, Hep, and Hex groups. Myocardial ATP content was significantly reduced in the Con ischemic/reperfused compared with the normal nonischemic group, but there were no differences among the Con, Hep, and Hex groups. There was no difference in myocardial ADP content (Table 3). The myocardial acetyl-CoA content was increased by ischemia-reperfusion compared with normal aerobic conditions and was significantly reduced by heptanoate treatment (Table 3). Free CoA content was not different among groups.


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  		Table 3. Myocardial metabolite content and pyruvate dehydrogenase activity in the nonischemic control group and in the groups subjected to ischemia followed by reperfusion

 
Ischemia-reperfusion resulted in a significant increase in the percent activation of PDH in the Con group compared with the nonischemic group (Table 3); however, there were no differences among the Con, Hep, and Hex groups. In addition, there was no difference in the maximal activity of PDH among treatment groups (data not shown).

The myocardial content of CAC intermediates was not significantly different between ischemia-reperfusion Con group compared with normal aerobic myocardium (Table 4, Fig. 3), as we have previously reported in a swine model with 1 h of total LAD occlusion with 2-h reperfusion (36). Treatment with heptanoate during ischemia and reperfusion resulted in a significant reduction in {alpha}-KG and increases in fumarate and malate content, but no change in citrate or succinate. On the other hand, compared with the Con group, treatment with hexanoate resulted in a significant decrease in {alpha}-KG and an increase in succinate, with no change in citrate, fumarate, and malate contents compared with the Con group (Table 4). The sum of the concentrations of the measured CAC intermediates was increased significantly in the Hep group relative to the Con group (Table 4), whereas there was no difference between the Hep and Hex groups.


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  		Table 4. Myocardial citric acid cycle intermediate content

 


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  		Fig. 3. Myocardial content of citric acid cycle intermediates expressed as a percentage of the nonischemic group. I/R, subjected to ischemia and reperfusion; CIT, citrate; SUC, succinate; FUM, fumarate; MAL, malate. Values are means ± SE, expressed as nmol/g. *P < 0.05 compared with saline group; **P < 0.05 compared with saline and heptanoate groups; {dagger}P < 0.05 compared with saline and hexanoate groups.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
The present study found that, despite exerting significant effects on myocardial CAC intermediate content, treatment with heptanoate did not improve contractile function during ischemia and reperfusion. Myocardial uptake of heptanoate resulted in more than twofold increase in fumarate and malate but did not alter myocardial lactate exchange or glucose oxidation or improve regional anterior wall contractile power. On the other hand, the nonanaplerotic medium-chain fatty acid hexanoate did not affect fumarate and malate levels but increased succinate by 1.8-fold. Thus, although the heart readily took up either heptanoate or hexanoate, either one differentially increased CAC intermediate content but did not result in improved contractile function or energetics. These findings suggest that, with a 60% flow reduction followed by reperfusion, impaired CAC function or depletion of CAC intermediates does not contribute to ischemic or postischemic dysfunction.

The present findings support the concept that mitochondrial substrate oxidation, specifically the function of the CAC, effectively adapts to short-term ischemia. As our laboratory has previously reported in a swine model with 1 h of total LAD occlusion and 2 h of reperfusion (36), in the present study there were no significant changes in the levels of CAC intermediates in myocardium subjected to ischemia-reperfusion compared with normal flow conditions. It is important to note, however, that there can be a disconnection between changes in anaplerotic flux through the propionyl-CoA-generating pathway without affecting the myocardial content of CAC intermediates (22, 26, 36). The results of the present investigation show that increasing anaplerotic flux and increasing CAC intermediate content do not provide a clear benefit in ischemia-reperfusion.

There were several differences in the effects of hexanoate and heptanoate in the profile of myocardial metabolites. The lower concentration of acetyl-CoA and the trend to lower free CoA with heptanoate vs. hexanoate treatment probably reflect the trapping of CoA in the intermediates in the propionyl-CoA pathway (13). In regard to the CAC, we observed a significant reduction in {alpha}-KG content in the Hep and Hex groups compared with the normal group or the saline-treated ischemia-reperfusion group. In contrast, Comte et al. (5) observed higher myocardial {alpha}-KG concentrations in isolated rat hearts perfused with octanoate compared with oleate under aerobic conditions, but no difference when the hearts were stressed with low-flow ischemia. While the data from the present study demonstrate a clear decrease in {alpha}-KG under treatment with medium-chain fatty acids during ischemia-reperfusion, the cause of this effect is not clear. Among the four carbon intermediates of the CAC, heptanoate had no effect on succinate concentration but significantly increased fumarate and malate (Fig. 3). This is surprising since one would expect that stimulation of the propionyl-CoA pathway would also raise succinate concentration. On the other hand, hexanoate elevated only succinate content. The present study would have been strengthened if isotopically labeled heptanoate and hexanoate were used to trace their route of metabolism and quantify their contribution to CAC flux. In any case, while the mechanisms for the changes in the concentration of CAC intermediates are not clear, one must keep in mind that we assessed whole tissue and not the mitochondrial compartment. While it is likely that most of the myocardial CAC intermediate pool resides in the mitochondria, it is possible that the changes we observed reflect alterations in the extramitochondrial pool.

Glucose oxidation was unaffected by ischemia or reperfusion in all groups, and PDH activity was elevated in myocardium subjected to ischemia-reperfusion compared with nonischemic hearts (Table 3). Previous in vivo studies in dogs and pigs found no impairment in glucose oxidation or PDH activity with a 50–70% decrease in LAD blood flow or with reperfusion compared with normal flow conditions (23, 27, 33, 35). The present investigation extends these findings to show that supplementation with medium-chain fatty acids does not affect either glucose oxidation or PDH activity during reperfusion. These results run counter to work from isolated buffer perfused rodent hearts, where there is a consistent reduction in glucose oxidation during reperfusion following global no-flow ischemia in the presence of fatty acids (18, 37). Churchill et al. (4) recently found reduced PDH activity following no-flow ischemia in isolated rat hearts perfused in the absence of insulin with 10 mM glucose as the sole substrate. The discrepancies between the ex vivo studies and the present investigation are likely due to the severity of ischemia and the lack of physiological conditions (e.g., normal arterial concentrations of substrates, oxygen, and hormones, and high coronary buffer flow during reperfusion) in the in vitro studies. In addition, we measured PDH activity in rapidly frozen tissue, while Churchill et al. assayed activity following tissue homogenization and centrifugation to isolated mitochondria, which would likely affect the phosphorylation state of the PDH complex.

In conclusion, we observed that acute treatment with the anaplerotic medium-chain fatty acid heptanoate resulted in a significant increase in CAC intermediates but did not improve contractile function during ischemia or reperfusion. In addition, a 60% reduction in coronary flow followed by reperfusion had no effect on the myocardial content of CAC intermediates. Taken together, these results suggest that depletion of CAC intermediates does not play a major role in the functional and metabolic derangements observed in acute ischemia and reperfusion.


   GRANTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
ACKNOWLEDGMENTS
 REFERENCES
 
This research was supported by National Institutes of Health Grants HL-074237 and DK-35543.


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
REFERENCES
 
The authors thank Dr. Radu Iancu and Julie Rennison for assistance.


   FOOTNOTES
 

Address for reprint requests and other correspondence: W. C. Stanley, Dept. of Physiology and Biophysics, School of Medicine, Case Western Reserve Univ., 10900 Euclid Ave., Cleveland, OH 44106–4970 (e-mail: wcs4{at}case.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
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 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 

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