Intrinsic contractile dysfunction due to impaired sarcoplasmic reticulum Ca2+ release in compensatory hypertrophied muscle fibers following synergist ablation

Abstract

Synergist ablation (SA) is an experimental procedure for the induction of hypertrophy. However, SA causes a decrease in specific force (i.e., force per cross-sectional area), likely due to excessive muscle use. Here, we investigated the mechanisms behind the SA-induced intrinsic contractile dysfunction, especially focusing on the excitation-contraction (EC) coupling. Male Wistar rats had unilateral surgical ablation of gastrocnemius and soleus muscles to induce compensatory hypertrophy in the plantaris muscles. Two weeks after SA, plantaris muscle was dissected from each animal and used for later analyses. SA significantly increased the mean fiber cross-sectional area (+18%). On the other hand, the ratio of depolarization-induced force to the maximum Ca²⁺-activated specific force, an indicator of sarcoplasmic reticulum (SR) Ca²⁺ release, was markedly reduced in mechanically skinned fibers from the SA group (-51%). These functional defects were accompanied by an extensive fragmentation of the SR Ca²⁺ release channel, the ryanodine receptor 1 (RyR1), and a decrease in the amount of other triad proteins (i.e., DHPR, STAC3, and junctophilin1). SA treatment also caused activation of calpain-1 and increased the amount of NADPH oxidase 2, endoplasmic reticulum (ER) stress proteins (i.e., Grp78, Grp94, PDI, and Ero1), and lipid peroxidation [i.e., 4-hydroxynonenal (4-HNE)] in SA-treated muscles. Our findings show that SA causes skeletal muscle weakness due to impaired EC coupling. This is likely to be induced by Ca²⁺-dependent degradation of triad proteins, which may result from Ca²⁺ leak from fragmented RyR1 triggered by increased oxidative stress.NEW & NOTEWORTHY Synergist ablation (SA) has widely been used to understand the mechanisms behind skeletal muscle hypertrophy. However, compensatory hypertrophied muscles display intrinsic contractile dysfunction, i.e., a hallmark of overuse. Here, we demonstrate that SA-induced compensatory hypertrophy is accompanied by muscle weakness due to impaired sarcoplasmic reticulum Ca²⁺ release. This dysfunction may be caused by the degradation of triad proteins due to the reciprocal amplification of reactive oxygen species and Ca²⁺ signaling at the junctional space microdomain.

Keywords: excitation-contraction coupling; overload; overuse; specific force; synergist ablation.