Calcium-sensitive cross-bridge transitions in mammalian fast and slow skeletal muscle fibers

The fundamental mechanism underlying the differing rates of tension development in fast and slow mammalian skeletal muscle is still unknown. Now, in skinned (membrane-permeabilized) single fibers it has been shown that the rate of formation of the strongly bound, force-producing cross-bridge between actin and myosin is calcium-sensitive in both fast and slow fibers and that the rate is markedly greater in fast fibers. The transition rates obtained at high calcium concentrations correlated with myosin isoform content, whereas at low calcium concentrations the thin filament regulatory proteins appeared to modulate the rate of tension development, especially in fast fibers. Fiber type-dependent differences in rates of cross-bridge transitions may account for the characteristic rates of tension development in mammalian fast and slow skeletal muscles.     

Increased phosphorylation of myosin light chain kinase after an increase in cyclic AMP in intact smooth muscle

The role of cyclic adenosine monophosphate-mediated phosphorylation of myosin light chain kinase in relaxing smooth muscle was examined. The kinase was immunoprecipitated from tissue extracts and the phosphate content was determined. The addition of forskolin to resting or methacholine-contracted muscles resulted in an increase in myosin light chain kinase phosphorylation of myosin light chain kinase is one of the reactions in the process by which cyclic adenosine monophosphate causes relaxation of smooth muscle.     

Phosphorylation of myosin light chains in mouse fast-twitch muscle associated with reduced actomyosin turnover rate

Phosphorylation of the 18,000-dalton light chains of the fast-twitch myosin in mouse extensor digitorum longus muscles was correlated with reduction in the rate of the actomyosin adenosinetriphosphatase in vivo, but neither of these changes occurred in the soleus muscle. These results suggest that actomyosin interactions can be down-regulated by a reversible covalent modification of myosin light chains, that a mechanism for thick-filament regulation occurs in vertebrate skeletal muscle, and that the expression of this regulation may be limited to a specific fiber type.     

Ribonuclease-inhibitor system abnormality in dystrophic mouse skeletal muscle

Skeletal muscle extracts from mice with muscular dystrophy contain severalfold higher than normal levels of free alkaline ribonuclease II activity and none of the free ribonuclease inhibitor normally present. This abnormal pattern is not seen in heart or liver extracts from dystrophic mice.     

Continued expression of neonatal myosin heavy chain in adult dystrophic skeletal muscle

The expression of myosin heavy chain isoforms was examined in normal and dystrophic chicken muscle with a monoclonal antibody specific for neonatal myosin. Adult dystrophic muscle continued to contain neonatal myosin long after it disappeared from adult normal muscle. A new technique involving western blotting and peptide mapping demonstrated that the immunoreactive myosin in adult dystrophic muscle was identical to that found in neonatal normal muscle. Immunocytochemistry revealed that all fibers in the dystrophic muscle failed to repress neonatal myosin heavy chain. These studies suggest that muscular dystrophy inhibits the myosin gene switching that normally occurs during muscle maturation.     

DNA synthesis in differentiating skeletal muscle cells: initiation by ultraviolet light

As skeletal muscle cells differentiate, they fail to initiate DNA synthesis. This rigid regulation, which persists even after cells are fully developed, does not extend to "repair" DNA synthesis, in that ultraviolet light initiates DNA synthesis in 99 percent of the muscle nuclei exposed. The rate of "repair" DNA synthesis in these nuclei, however, drops over 50 percent at the time of cell differentiation.     

红景天苷对运动疲劳大鼠骨骼肌线粒体自由基代谢及呼吸链功能的影响

为研究红景天苷对运动疲劳大鼠骨骼肌线粒体自由基代谢及呼吸链功能的影响,将健康雄性Wistar大鼠随机分为空白对照组(A组)、运动模型组(B组)、红景天苷低剂量运动组[50 mg·(kg·d)-1,C组]、中剂量运动组[100 mg· (kg· d)-1,D组]、高剂量运动组[200mg· (kg· d)-1,E组].除A组外,其余各组大鼠进行递增负荷跑台运动建立运动疲劳模型,记录大鼠体重及末次跑台力竭时间,检测骨骼肌线粒体锰超氧化物歧化酶(Mn-SOD)、谷胱甘肽过氧化物酶(GSH-Px)、丙二醛(MDA)水平及呼吸链酶复合体Ⅰ-Ⅳ(CⅠ-CⅣ)活性,采用蛋白免疫印迹法检测骨骼肌Kelch样ECH相关蛋白1(Keap1)、核呼吸因子2(Nrf2)及血氧红素加氧酶(HO-1)表达水平.结果 表明:①与A组相比,B组大鼠跑台力竭运动时间,Mn-SOD、GSH-Px、C Ⅰ-CⅣ活性及Keap1、Nrf2、HO-1蛋白表达均显著降低,MDA含量显著升高(P＜0.01).②与B组相比,D、E组大鼠跑台力竭时间、Mn-SOD活性显著增加(P＜0.01),E组大鼠MDA含量显著降低(P＜0.01);C、D、E组GSH-Px、CⅢ、CⅣ活性及Keap1、Nrf2、HO-1蛋白表达量显著增加(P＜0.01或0.05),D、E组CⅠ活性显著增加(P＜0.01或0.05),E组CⅡ活性显著增加(P＜0.05).红景天苷可通过提升机体抗氧化防御能力及线粒体呼吸链中相关酶活性,缓解运动性疲劳的产生,其机制可能是通过激活Keap1-Nrf2-ARE通路来实现的.