Is lactate-induced myocardial ischaemic injury mediated by decreased pH or increased intracellular lactate?

The detrimental effect of exogenous lactate during ischaemia on post-ischaemic contractile function may be mediated either by a lactate-induced intracellular H+ load or by an increase in intracellular lactate. To distinguish between these two mechanisms, isolated rat hearts were perfused with lactate or pyruvate during low flow ischaemia, the rationale being that both would decrease H+ efflux via lactate/H+ cotransport and lead to decreased pH, but only exogenous lactate would decrease lactate efflux and lead to increased intracellular lactate. 31P NMR spectra were acquired sequentially while hearts were subjected to 32 min low flow (0.5 ml/min) ischaemia and 32 min reperfusion. During ischaemia, hearts were perfused with Krebs-Henseleit buffer containing 11 mM glucose (controls) or 11 mM glucose plus either 10 mM lactate or 10 mM pyruvate. Reperfusion of all hearts was with buffer containing only glucose. Intracellular volume, estimated to be 0.52 ml/heart using 31P NMR spectroscopy with phosphonate space markers, did not change under any of the ischaemic conditions during the protocol. Control and pyruvate hearts recovered approximately 85% of pre-ischaemic contractile function, but there was no recovery of function in lactate hearts. This lack of recovery correlated with a 57% loss of ATP during ischaemia, which was significantly greater (P < 0.001) than the 41% loss of ATP in control and pyruvate-perfused hearts. End-ischaemic intracellular pH was 6.60 in both lactate-perfused and control hearts, but significantly lower (P < 0.05) at pH 6.43 in pyruvate-perfused hearts. Both exogenous pyruvate and lactate should have decreased H+ efflux, however the higher pH in the lactate-perfused hearts could be explained by a 60% inhibition of glycolysis, determined by measurement of myocardial lactate production. Thus, the intracellular pH during ischaemia does not necessarily predict the extent of myocardial injury. We propose that lactate-induced damage is a consequence of increased intracellular lactate leading to inhibition of glycolysis, presumably via an increased NADH/NAD ratio. This study highlights the important role of glycolysis in the ischaemic rat heart.