To truly understand excitation–contraction coupling at the molecular level, calcium dynamics and sarcomere motion must be recorded in the same coordinate system. This study achieved that for the first time by targeting a FRET‑based Ca²⁺ sensor, yellow Cameleon‑Nano, to the Z‑disk, enabling real‑time, simultaneous imaging of local Ca²⁺ concentration and single‑sarcomere length in rat neonatal cardiomyocytes.
Methodological advances
Four YC‑Nano variants were tested; α‑actinin–YC‑Nano140 (K_d = 140 nM) offered the best combination of dynamic range and fluorescence intensity.
At a scan speed of 33 fps, the system resolved sarcomere length changes with 17 nm precision.
Key observations
Ca²⁺ transients propagated synchronously along entire myofibrils, whereas contraction timing of adjacent sarcomeres was slightly offset—meaning traditional “average‑length” analyses had underestimated displacement by ~50 %.
The set‑up remained stable during electrical stimulation (5 Hz), spontaneous beating, and elevated temperature (37 °C). β‑adrenergic stimulation (isoproterenol) accelerated Ca²⁺ kinetics and doubled sarcomere‑lengthening velocity.
Using the myosin activator omecamtiv mecarbil and observing local Ca²⁺ waves, the study detailed how pharmacological and thermal interventions affect excitation–contraction coupling at the single‑sarcomere level.
Significance
The technique provides a new platform for visualizing subcellular events throughout cardiac development—from embryonic stages to adulthood—and for probing early pathologies in heart failure and arrhythmia. Adenoviral‑vector delivery paves the way for in vivo applications, potentially yielding a “nano‑imaging electrocardiogram” inside the beating heart for disease research and drug screening.
Article information & citation
Seiichi Tsukamoto, Teruyuki Fujii, Kotaro Oyama, Seine A. Shintani, Togo Shimozawa, Fuyu Kobirumaki‑Shimozawa, Shin’ichi Ishiwata, Norio Fukuda. Simultaneous imaging of local calcium and single sarcomere length in rat neonatal cardiomyocytes using yellow Cameleon‑Nano140. Journal of General Physiology 148 (4), 341‑355 (2016).