Dual-filament regulation of relaxation in mammalian fast skeletal muscle

• DOI: 10.1073/pnas.2416324122
• Publication Date: 2025-03-12T17:39:51Z
• Authors: Cameron Hill; Michaeljohn Kalak...; Alice Arcidiacono; Flair Paradine Cu...; Yanhong Wang; Atsuki Fukutani; Theyencheri Naray...; Elisabetta Brunello; Luca Fusi; Malcolm Irving
• Keywords: Myofibril; Muscle relaxation; Meromyosin

Muscle contraction is driven by myosin motors from the thick filaments pulling on actin-containing thin of sarcomere, and it regulated structural changes in both filaments. Thin are activated an increase intracellular calcium concentration [Ca 2+ ] i binding to actin. Thick direct sensing filament load. However, these mechanisms cannot explain muscle relaxation when decreases at high load attached There is, therefore, a fundamental gap our understanding relaxation, despite its importance for function vivo, example, rapid eye movements or, slower timescales, efficient control posture. Here, we used time-resolved small-angle X-ray diffraction (SAXD) determine how switch OFF extensor digitorum longus (EDL) muscles mouse response either or describe distribution sarcomere lengths (SLs) during relaxation. We show that reducing more effective switching than normal In latter case, initially remain fully ON, although number bound actin force per motor increases. That initial slow phase abruptly terminated yielding one population sarcomeres, triggering redistribution SLs leads completion mechanical