Introduction/Background: The muscular dystrophies (MDs) are a group of inherited muscular disorders, which are characterized by progressive muscle weakening and wasting. Six different groups can be classified, among which are the dystrophinopathies (e.g. Duchenne muscular dystrophy, DMD) and the limb-girdle muscular dystrophies (e.g.
LGMD2B). DMD is the most common and severe form among the MDs. Both DMD (severe) and LGMD2B (mild) are characterized by cardiac pathology and dysfunction. The cardiac phenotype may be related to impairments in the expression and function of voltage-gated ion channels. One of the most important ion channels in the heart is the L-type voltage-gated Ca2+ channel, which is involved in action potential shaping and in the inition of contraction. The main aim of this study was to identify potential abnormalities in L-type Ca2+ channel function in mouse models of DMD and LGMD2B. Methods and Results: To study ion channel abnormalities in the dystrophic heart, isolated single ventricular cardiomyocytes from adult DMD (mdx, dystrophin-deficient; mdx-utr, dystrophin- and utrophin-deficient) and LGMD2B (dysf, dysferlin deficient) mouse models were compared to wild type (wt) cells. In dystrophin-deficient cardiomyocytes currents through L-type Ca2+ channels were increased and channel inactivation was reduced. To study the functional effects of abnormal Ca2+ channels, action potentials (APs) of dystrophin-deficient cardiomyocytes and electrocardiograms (ECGs) from anaesthetised dystrophin-deficient mice were recorded.
Although AP duration was unchanged, Ca2+ current dependent ECG parameters in dystrophin-deficient hearts were significantly altered.
Western blot and immunofluorescence experiments showed no differences in Ca2+ channel protein levels or localization between dystrophin-deficient and wt cardiomyocytes. To identify the reasons for abnormal Ca2+ channel properties in dystrophin-deficient cardiomyocytes, electrophysiological measurements modulating protein kinase A (PKA) and neuronal nitric oxide synthase (nNOS) were performed. There was no difference in the response of Ca2+ currents to stimulation or inhibition of the PKA-mediated pathway in wt and mdx cardiomyocytes, but there was a difference in the channel's response to nNOS pathway modulation. Furthermore nNOS levels seemed to be reduced in mdx cardiomyocytes. Finally, there were no significant differences between wt and dysf cardiomyocytes regarding Ba2+, Ca2+ and Na+ currents and APs, as well as normal ECG parameters in dysferlin deficient mice. Conclusions: (1) Significant abnormalities of L-type Ca2+ channels in dystrophin-deficient cardiomyocytes, but no alterations in cardiac voltage-gated ion channels in dysferlin deficient cardiomyocytes suggest that dystrophin regulates cardiac ion channels, whereas dysferlin does not. Thus, abnormalities in voltage-gated ion channels do not represent a common feature of all types of MDs. (2) The enhanced Ca2+ currents in dystrophin-deficient cardiomyocytes may be caused by a reduced nNOS activity in these cells. (3) The observed gain of L-type Ca2+ channel function in dystrophin-deficient cardiomyocytes may disturb the electrophysiology of the dystrophic heart and thereby contribute to the cardiac pathology observed in DMD patients.