Pathways of abnormal stress-induced Ca 2+ influx into dystrophic mdx cardiomyocytes

Abstract

In Duchenne muscular dystrophy, deficiency of the cytoskeletal protein dystrophin leads to well-described defects in skeletal muscle, but also to dilated cardiomyopathy, accounting for about 20% of the mortality. Mechanisms leading to cardiomyocyte cell death and cardiomyopathy are not well understood. One hypothesis suggests that the lack of dystrophin leads to membrane instability during mechanical stress and to activation of Ca 2+ entry pathways. Using cardiomyocytes isolated from dystrophic mdx mice we dissected the contribution of various putative Ca 2+ influx pathways with pharmacological tools. Cytosolic Ca 2+ and Na + signals as well as uptake of membrane impermeant compounds were monitored with fluorescent indicators using confocal microscopy and photometry. Membrane stress was applied as moderate osmotic challenges while membrane current was quantified using the whole-cell patch-clamp technique. Our findings suggest a major contribution of two primary Ca 2+ influx pathways, stretch-activated membrane channels and short-lived microruptures. Furthermore, we found evidence for a secondary Ca 2+ influx pathway, the Na +–Ca 2+ exchange (NCX), which in cardiac muscle has a large transport capacity. After stress it contributes to Ca 2+ entry in exchange for Na + which had previously entered via primary stress-induced pathways, representing a previously not recognized mechanism contributing to subsequent cellular damage. This complexity needs to be considered when targeting abnormal Ca 2+ influx as a treatment option for dystrophy.