Direct measurement of myofilament calcium in living cardiomyocytes

Visualising when and where calcium appears and disappears in cardiomyocytes is a major goal of cardiovascular research. Surprisingly we find that the chemical dyes widely used for this purpose disrupt cell contractility, due at least in part due to direct inhibition of the acto-myosin ATPase required to generate force. In order to improve calcium detection methods, we have developed a genetically encoded indicator that sits within the myofilament to directly visualise the changes occurring at the sarcomere. This tool improves on established chemical dyes and untargeted genetically encoded indicators for analysing small molecule modulators of myofilament-based calcium signalling. Importantly this is achieved without any measurable change in contractile function.