Evidence for Altered Communication Between the L-Type Ca2+ Channel and Mitochondria in a Model of Cardiomyopathy

Abstract

Progression of cardiac hypertrophy to failure and development of many cardiomyopathies involves myocyte remodeling, disorganisation of cytoskeletal proteins and reduced energy metabolism. The mechanisms by which cytoskeletal disruption leads to mitochondrial dysfunction are poorly understood.Calcium influx through the L-type Ca2+ channel (LTCC) is a requirement for contraction in the heart. The LTCC can influence mitochondrial superoxide production, NADH production and metabolic activity in a calcium-dependent manner. Activation of the channel can also increase mitochondrial membrane potential (Ψm) in a calcium-independent manner. This response is dependent upon the cytoskeleton. We hypothesized that disruption of normal cytoskeletal architecture will result in altered communication between the LTCC and mitochondria. We investigated this hypothesis in the murine model of Duchenne Muscular Dystrophy(mdx). Myocytes from 8 week old mdx mice that exhibit disorganised cytoskeletal protein networks but not yet overt cardiomyopathy, demonstrated significantly slower LTCC inactivation rate (26.2±1.8ms, n=13 vs 21.1±1.3ms, n=16; p<0.05) and significantly greater calcium influx (681±40nM, n=7 vs 432±51nM, n=5). However activation of the LTCC did not increase Ψm (n=6) or metabolic activity (measured as formation of formazan from tetrazolium salt) in mdx myocytes (n=8). Application of 4,4′diisothiocyano-2,2′-stilbenedisulfonic acid, that blocks anion transport mimicked the response of BayK(-) in control myocytes (n=4) and “restored” the increase in Ψm in mdx myocytes (n=4).The activities of the mitochondrial respiratory complexes were normal in mitochondria isolated from 8 week old mdx hearts and from 40 week old mdx hearts that had developed cardiomyopathy. We conclude that the communication between the L-type Ca2+ channel and mitochondria is altered in the dystrophin-deficient cardiomyocyte. This appears to involve an alteration in the association between the channel protein, actin filaments and mitochondrial anion transport and precedes the development of cardiomyopathy.