Sarcoplasmic reticulum function in slow- and fast-twitch skeletal muscles from mdx mice

Abstract

The aim of the present study was to establish whether alterations in sarcoplasmic reticulum function are involved in the abnormal Ca(2+) homeostasis of skeletal muscle in mice with muscular dystrophy ( mdx). The properties of the sarcoplasmic reticulum and contractile proteins of fast- and slow-twitch muscles were therefore investigated in chemically skinned fibres isolated from the extensor digitorum longus (EDL) and soleus muscles of normal (C57BL/10) and mdx mice at 4 and 11 weeks of development. Sarcoplasmic reticulum Ca(2+) uptake, estimated by the Ca(2+) release following exposure to caffeine, was significantly slower in mdx mice, while the maximal Ca(2+) quantity did not differ in either type of skeletal muscle at either stage of development. In 4-week-old mice spontaneous sarcoplasmic reticulum Ca(2+) leakage was observed in EDL and soleus fibres and this was more pronounced in mdx mice. In addition, the maximal Ca(2+)-activated tension was smaller in mdx than in normal fibres, while the Ca(2+) sensitivity of the contractile apparatus was not significantly different. These results indicate that mdx hindlimb muscles are affected differently by the disease process and suggest that a reduced ability of the Ca(2+)-ATPase to load Ca(2+) and a leaky sarcoplasmic reticulum membrane may be involved in the altered intracellular Ca(2+) homeostasis.

Depolarization-induced contraction and SR function in mechanically skinned muscle fibers from dystrophic mdx mice

“…Our finding of normal SR Ca 2ϩ leak in fibers from mdx mice also confirms previous observations from saponin-skinned single muscle fibers (8). The results support the notion that although dystrophin-deficient fibers have an increased sarcolemmal membrane permeability to divalent ions (35), SR membrane permeability is not affected.…”

“…The decrease in caffeine-induced contraction is not likely to be mediated by an effect on the myofilaments because caffeine has similar effects in permeabilized fibers from mdx and control mice, i.e., decreased maximum Ca 2ϩ -activated tension and increased Ca 2ϩ sensitivity of the myofilaments (8). Rather, the effect could be attributed to differences in the saturation of SR Ca 2ϩ loading in fibers from mdx mice.…”

“…Data on caffeine-induced contraction in saponinskinned single fibers from mdx mice are conflicting, with either an increase or decrease in the amplitude of caffeine-induced contraction being reported (8,33). It should be noted that a consequence of using saponin to permeabilize the sarcolemmal membrane is an increase in SR permeability to Ca 2ϩ and reduced SR Ca 2ϩ loading, due to the activation of Ca 2ϩ release channels (21).…”

“…Experiments on chemically skinned fibers prepared from the tibialis anterior muscles of mdx mice indicated a "more prominent" caffeine-induced contracture and a greater leakage of Ca 2ϩ from the SR of mdx than control mice (33). In contrast, single fibers from the EDL muscles of 11-wk-old mdx mice had a decreased amplitude of caffeine-induced contraction and slower SR Ca 2ϩ uptake but no difference in SR Ca 2ϩ leak (8). To address these contradictory findings, we examined the contractile properties of mechanically skinned single fibers from mdx mice.…”

Increased Ca2+ storage capacity of the skeletal muscle sarcoplasmic reticulum of transgenic mice over‐expressing membrane bound calcium binding protein junctate

“…Chemical skinning was carried out immediately after dissection, using Triton X-100. Skinned fibers were transferred to a chamber and mounted between two stainless-steel tubes that supported a metal target facing the sensor of a transducer, as previously described (Divet and Huchet-Cadiou, 2002;Divet et al, 2005). At the beginning of each experiment, the fiber was adjusted to a length where it was slack, and then progressively stretched at pCa 4.5 until the tension developed reached its peak (which was generally equals to the resting length plus 20%).…”