Pyruvate Dehydrogenase Influences Postischemic Heart Function

Abstract

Background The pyruvate dehydrogenase (PDH) enzyme complex determines the extent of carbohydrate oxidation in the myocardium. PDH is in a largely inactive state during early reperfusion of postischemic myocardium. The resultant decrease in pyruvate oxidation in postischemic hearts has been documented with 13 C nuclear magnetic resonance (NMR) spectroscopy. This study demonstrates that counteracting depressed pyruvate oxidation can enhance contractile recovery in the absence of increases in either glycolytic activity or glucose oxidation. The findings indicate that increased incorporation of carbon units from pyruvate into the intermediates of the oxidative pathways by PDH influences the metabolic efficiency and mechanical work of postischemic hearts. Methods and Results Isolated rabbit hearts were situated in an NMR magnet and perfused or reperfused (10 minutes of ischemia) with 2.5 mmol/L [3- 13 C]pyruvate as sole substrate to target PDH directly and bypass the glycolytic pathway. Hearts were observed with or without activation of PDH with dichloroacetate. Mechanical function and oxygen consumption (MV̇ o 2 ) were monitored. 13 C and 31 P NMR spectroscopy allowed observations of pyruvate oxidation and bioenergetic state in intact, functioning hearts. Metabolite content and 13 C enrichment levels were then determined with in vitro NMR spectroscopy and biochemical assay. PDH activation did not affect performance of normal hearts. Postischemic hearts with augmented pyruvate oxidation (dichloroacetate-treated) sustained improved mechanical function throughout 40 minutes of reperfusion. Rate-pressure-product (RPP) increased from 8300±1800 (mean±SEM) in untreated postischemic hearts to 21 300±2400 in hearts treated with dichloroacetate ( P <.05). Oxygen use per unit work [MV̇ o 2 multiplied by 10 4 divided by RPP] was improved from 1.50±0.13 to 1.14±0.11 ( P <.05) without differences in high-energy phosphate content between treated and untreated hearts. Values of dP/dt were also consistently higher, by as much as 185%, during reperfusion with dichloroacetate. Postischemic hearts displayed reduced pyruvate oxidation from the incorporation of 13 C into the tissue glutamate pool. With the tissue alanine level as a marker of 13 C-enriched pyruvate availability in the cell, the ratio of labeled glutamate to alanine was only 58% of the control value during early reperfusion. With dichloroacetate, that ratio was 167% greater than that of untreated hearts ( P <.05). By the end of the reperfusion period, the 13 C enrichment of the tissue glutamate pool by pyruvate oxidation was elevated from dichloroacetate treatment (untreated, 62±7%; DCA-treated, 81±6%; P <.05), but glycogen content was similar in both groups and 13 C enrichment of tissue alanine remained unchanged, near 60%, indicating no increases in glycolytic end-product formation. Conclusions Metabolic reversal of contractile dysfunction was achieved in isolated hearts by counteracting depressed PDH activity in the postischemic myocardium. Improved cardiac performance did not result from, nor require, increased glycolysis secondary to the activation of PDH. Rather, restoring carbon flux through PDH alone was sufficient to improve mechanical work by postischemic hearts.