Abnormal Sodium Handling and Mitochondrial Metabolism in Cardiac Dystrophy

Abstract

Lack of dystrophin in striated muscle results in the activation of several stretch-induced transplasmalemmal ion influx pathways. Recently we demonstrated that mechanical challenges produce substantial inward currents in dystrophic (mdx) but not in WT cardiomyocytes. We also detected a significant increase in cytosolic [Na+] following stretch in mdx cells. We suggested that beat-to-beat mechanical activity of dystrophic heart might lead to accumulation of Na+ inside the cardiomyocytes (Na+ overload). Here we measured resting [Na+]i in mdx and WT cells using the ratiometric Na+ indicator SBFI. The averaged value of [Na+]i was indeed significantly greater in mdx than in WT myocytes (24.2 ± 3.1, n=13 vs. 14.0 ± 1.7 mM, n=9). Na+ overload can have a profound effect on cellular functions. E.g. it can change the reversal potential of NCXsl, reducing its ability to remove Ca2+. This is in agreement with our recent report that the reverse mode of NCXsl contributes to cytosolic Ca2+ overload following mechanical stress, despite little change in the resting potential. Elevated [Na+]i can also eventually affect mitochondrial metabolism as it could enhance Ca2+ extrusion from the mitochondria via NCXmt. Therefore we measured NADH autofluorescence. Maximal oxidation of NADH by FCCP/oligomycin was taken as 0% reduced NADH, whereas maximal reduction by rotenone and β-hydroxy-butyrate was defined as 100% reduced NADH. The averaged values show that mitochondrial matrix is significantly more oxidized in resting mdx myocytes compared with WT cells (35% ±3.1, n=27 vs. 53 ± 5.2 %, NADH reduction n=10). The oxidation of mitochondrial matrix can contribute to the cellular oxidative stress and favor the opening of mPTP and cell death -observations that we previously reported for dystrophic cardiomyocytes. Taken together, our findings suggest that elevated [Na+]i may contribute to the development of dystrophic cardiomyopathy.