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 (αMHC403/+). We examined LTCC kinetics in cardiomyocytes from pre- and post-cardiomyopathic αMHC403/+ mice using whole cell patch-clamp technique, and the effect of LTCC activation on mitochondrial membrane potential (Ψm, JC-1 fluorescence) and mitochondrial oxygen consumption (flavoprotein autofluorescence). Cardiomyocytes isolated from cardiomyopathic αMHC403/+ mice demonstrated similar LTCC current density compared to age-matched wt cardiomyocytes. However the inactivation rate of the current was faster in αMHC403/+ 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 Ψm in αMHC403/+ 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 MHC403/+ 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 αMHC403/+ mice. Our data indicate that αMHC403/+ 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.