Myofilament mutations alter calcium channel and mitochondrial functional communication

H. Viola,¹ V. Johnstone,¹ H. Cserne Szappanos,¹ T. Richman,² T. Tsoutsman,^3,4 A. Filipovska,² C. Semsarian,^3,4,5 J. Seidman,⁶ C. Seidman⁶ and L. Hool,^1,7 ¹School of Anatomy, Physiology and Human Biology, The University of Western Australia, Crawley, WA 6009, Australia, ²The Harry Perkins Institute for Medical Research, The University of Western Australia, Crawley, WA 6009, Australia, ³Agnes Ginges Centre for Molecular Cardiology, Centenary Institute, Sydney, NSW 2050, Australia, ⁴Sydney Medical School, University of Sydney, NSW 2006, Australia, ⁵Department of Cardiology, Royal Prince Alfred Hospital, Sydney, NSW 2050, Australia, ⁶Harvard Medical School, Harvard University, Boston, MA 02115, USA and ⁷Victor Chang Cardiac Research Institute, Sydney, NSW 2010, Australia.

Hypertrophic cardiomyopathy (HCM) affects 1: 200 of the general population. It is associated with myocyte remodeling, disorganization of cytoskeletal proteins and altered energy metabolism. Some patients are responsive to L-type calcium channel (LTCC) antagonists as therapy. However the role of LTCC in development of the cardiomyopathy is unknown. Since mitochondrial function can be regulated by alterations in LTCC activity, we investigated the role of LTCC in regulating mitochondrial function in mice overexpressing the human HCM causing mutation Arg403Gln (αMHC^403/+). We examined LTCC kinetics in cardiomyocytes from pre- and post-cardiomyopathic αMHC^403/+ mice using whole cell patch-clamp technique, and the effect of LTCC activation on mitochondrial membrane potential (Ψ_m, JC-1 fluorescence) and mitochondrial oxygen consumption (flavoprotein autofluorescence). Cardiomyocytes isolated from cardiomyopathic αMHC^403/+ mice demonstrated similar LTCC current density compared to age-matched wt cardiomyocytes. However the inactivation rate of the current was faster in αMHC^403/+ cardiomyocytes (32.8 ± 2.0, n=14 versus 40.7 ± 2.5, n=8; mean ± SEM; P<0.05). Application of BayK(-) caused a significantly greater increase in Ψ_m in αMHC^403/+ versus wt cardiomyocytes (28.7±3.5%, n=9 versus 14.7±2.0%, n=10; P<0.05), that could be attenuated by LTCC antagonists nisoldipine or diltiazem. BayK(-) also caused a greater increase in flavoprotein oxidation in MHC^403/+ versus wt cardiomyocytes (24.6±3.8%, n=7 versus 8.8±1.0%, n=15; P<0.05). Similar results were recorded in cardiomyocytes isolated from pre-cardiomyopathic αMHC^403/+ mice. Our data indicate that αMHC^403/+ mice exhibit altered cardiac LTCC kinetics and a hypermetabolic mitochondrial state following LTCC activation. This may contribute to the development of the cardiomyopathy because the responses occur in pre-cardiomyopathic mice. LTCC antagonists may be effective in reducing the cardiomyopathy by “normalizing” mitochondrial function.