RESUMO
Muscle necrosis in Duchenne muscle dystrophy (DMD) and in the mdx mouse has been related to abnormal calcium homeostasis associated with the lack of dystrophin. We have previously shown that the testosterone-dependent levator ani (LA) muscle of the mdx mouse develops a mild muscle wasting and fiber degeneration compared to the less hormone sensitive diaphragm (DIA) muscle, suggesting a protective effect of androgens. This study assessed the calcium handling mechanisms and cytosolic calcium concentration ([Ca2+]i) in LA muscles of mdx mice at critical stages of muscle disease. Muscle contractures induced by caffeine and 4-chloro-m-cresol (4-CmC), two activators of ryanodine channels, were recorded in LA and DIA muscles of prepubertal (1 month-old), adult (4 month-old) and aged (18 month-old) wild-type (wt) and mdx mice. [Ca2+]i was estimated with the fura-2 fluorescent dye in enzymatically dissociated LA muscle fibers of the same wt and mdx groups. Tetanus tension (TT) in the LA increased proportionately to the muscle weight (4 to 5-fold), but specific TT (TT/mg) did not differ among age-matched wt and mdx groups. Muscle contractures induced by caffeine (3-100 mM) or 4-CmC (0.1-5.0 mM) in the LA were greater in prepubertal than in adult and aged mice, but they did not differ among age-matched wt and mdx groups. The resting [Ca2+]i in mdx LA muscle fibers was not significantly affected at any age. Comparatively, dystrophic DIA presented reduced muscle strength in adult (40%) and aged (45%) mice, whereas the muscle responses to caffeine increased with age (63 to 82%), indicating changes in the Ca2+ handling mechanisms. The results indicated that muscle strength and calcium homeostasis in dystrophic LA muscle fibers were not significantly altered, confirming previous evidence of androgens beneficial effects on hormone-sensitive skeletal muscles.
