The cardioprotective effect of sildenafil is mediated by the activation of malate dehydrogenase and an increase in the malate-aspartate shuttle in cardiomyocytes

Recent evidence has shown the cardioprotective effect of PDE5 inhibition in myocardial ischemia/reperfusion injury, heart failure and cardiac hypertrophy. To investigate the biochemical changes that occur during PDE5 inhibition in cardiac cells, this study assessed the metabolic profile of the HL1 cell line, a murine atrial cell line with adult cardiomyocyte properties. After one hour of treatment with sildenafil, glycolysis was moderately but selectively stimulated, unlike the pentose phosphate pathway and the Krebs cycle. Moreover, malate and a-Ketoglutarate accumulated, paralleled by a decrease in aspartate and glutamate. Interestingly, increased activity of malate dehydrogenase (MDH) was also detected in these cells after sildenafil treatment. Thus, we hypothesized that sildenafil stimulates the malate-aspartate shuttle (MAS) with the final effect of transferring electrons and protons from glycolysis-derived cytosolic NADH into the matrix for use by the electron transport chain, using malate as an electron carrier. Through this metabolic modification, sildenafil may counteract what is often observed in ischemia, i.e. reduced MAS flux as well as a dramatic acceleration of glycolysis, which switches to lactate production. Additionally, the results observed in HL1 cells were also found in isolated mouse hearts. The documented metabolic alteration in cardiomyocytes upon treatment with sildenafil occurred by stimulating cGMP production, which did not activate PKG (cGMP-PKG signaling), since the addition of DT-2, a PKG inhibitor, did not block malate accumulation and increased MDH activity. Conversely, the addition of chelerythrine, a PKC inhibitor, counteracted both malate accumulation and MAS activation, supporting previous evidence that, upon the addition of sildenafil, some PKC isoforms may be implicated in cardioprotection (cGMP-PKC signaling). Interestingly, an increase in cGMP, driven by sildenafil, another cGMP stimulator such as nitroprusside (SNP), or a C-type natriuretic peptide (CNP) which does not inhibit PDE5, led to MAS stimulation and increased MDH activity.