Studying the Super-Relaxed (SRX) state in cardiac and skeletal muscle: a protocol-based perspective from fibers to fibrils.

Abstract

The super-relaxed state (SRX) of myosin plays a key role in maintaining muscle efficiency and regulating metabolic rate. In this state, myosin heads exhibit significantly reduced ATPase activity, resulting in lower energy consumption when muscles are at rest. SRX is in dynamic equilibrium with the more disordered relaxed state (DRX), allowing shifts between these states to profoundly influence muscle function and energy expenditure. This deeper understanding of SRX, and its ability to be modulated has important implications for treating cardiac and skeletal muscle disorders where contractile function is impaired. Here, we present a detailed experimental protocol for studying SRX in cardiomyocytes and skeletal muscle fibers developed by Roger Cooke for different species, and adapted for our understanding of SRX biophysics. The protocol encompasses muscle dissection, data analysis, key reagents, auxiliary lab tools, notes, and tips that help with reproducibility and troubleshooting challenges. We hope this contribution will serve as a resource and inspiration for the next generation of scientists to explore muscle function and apply their findings toward improving human health.