Assessment of Myofilament Ca²⁺ Sensitivity Underlying Cardiac Excitation-contraction Coupling

Heart failure and cardiac arrhythmias are the leading causes of mortality and morbidity worldwide. However, the mechanism of pathogenesis and myocardial malfunction in the diseased heart remains to be fully clarified. Recent compelling evidence demonstrates that changes in the myofilament Ca²⁺ sensitivity affect intracellular Ca²⁺ homeostasis and ion channel activities in cardiac myocytes, the essential mechanisms responsible for the cardiac action potential and contraction in healthy and diseased hearts. Indeed, activities of ion channels and transporters underlying cardiac action potentials (e.g., Na⁺, Ca²⁺ and K⁺ channels and the Na⁺-Ca²⁺ exchanger) and intracellular Ca²⁺ handling proteins (e.g., ryanodine receptors and Ca²⁺-ATPase in sarcoplasmic reticulum (SERCA2a) or phospholamban and its phosphorylation) are conventionally measured to evaluate the fundamental mechanisms of cardiac excitation-contraction (E-C) coupling. Both electrical activities in the membrane and intracellular Ca²⁺ changes are the trigger signals of E-C coupling, whereas myofilament is the functional unit of contraction and relaxation, and myofilament Ca²⁺ sensitivity is imperative in the implementation of myofibril performance. Nevertheless, few studies incorporate myofilament Ca²⁺ sensitivity into the functional analysis of the myocardium unless it is the focus of the study. Here, we describe a protocol that measures sarcomere shortening/re-lengthening and the intracellular Ca²⁺ level using Fura-2 AM (ratiometric detection) and evaluate the changes of myofilament Ca²⁺ sensitivity in cardiac myocytes from rat hearts. The main aim is to emphasize that myofilament Ca²⁺ sensitivity should be taken into consideration in E-C coupling for mechanistic analysis. Comprehensive investigation of ion channels, ion transporters, intracellular Ca²⁺ handling, and myofilament Ca²⁺ sensitivity that underlie myocyte contractility in healthy and diseased hearts will provide valuable information for designing more effective strategies of translational and therapeutic value.