BS10 Breaking the limitations of one-dimensional isometric assays: a novel technique for generating cardiac work-loops on adult cardiomyocytes to enhance our understanding of heart failure and myocardial recovery

Background

In vitro contractility studies on cardiomyocytes provide mechanistic insight into the pathophysiology of heart failure (HF) and the phenomenon of myocardial recovery with mechanical unloading from ventricular assist devices. However, they often rely on unloaded isometric twitches, which do not reflect the dynamic movements of the heart. In vivo contractility is characterised by 4 distinct phases, as reflected in pressure-volume (PV) loops: isovolumic contraction, ejection, isovolumic relaxation and diastolic filling (figure 1A). Isometric assays also fail to capture the pathological loads seen in HF. To address this limitation, we present a research platform that can generate cardiac work-loops (WL) in cardiomyocytes, reflecting the cardiac cycle and potentially replicating pressure and volume-overloaded states.

Methods

Adult rat ventricular myocytes were electrically stimulated and suspended in solution with one end connected to a piezo-translator and the other end to a force transducer (IonOptix, Milton, USA). Mechanical WLs were generated using a force-length feedback algorithm at different cell lengths, pacing frequency, and afterload (i.e. force opposing myocyte shortening as a percentage of the isometric developed force) (figure 2A). Stroke work (SW), cardiac power and end-systolic force-length relation (marker of intrinsic contractility) can be derived from these WLs (figure 1). Parameters to generate pathological loads were investigated.

Results

During work-loop contractions, a high afterload (e.g. 90% of the developed isometric force) generates a narrower work-loop similar to a pressure-overloaded state, e.g. in hypertensive heart disease or severe aortic stenosis. This is associated with a significantly lower mechanical SW and cardiac power (e.g. SW 8.6pJ vs. 15.6pJ of a normal WL in figure 2B). In contrast, a higher preload and lower afterload resulted in larger length changes and significantly greater SW, similar to a volume-overloaded state (e.g. in chronic severe aortic regurgitation). Based on skeletal muscle fatigue experiments using similar WLs, a burst of fatigue-inducing repetitive work-loops at pressure or volume-overloaded states can be applied to a cardiomyocyte to test the hypothesis of fatigue and recovery (1). However, the occurrence of force and length oscillations beyond 3Hz (figure 1B) limits their usefulness as rat hearts have a high physiological heart rate of around 300-500bpm (equivalent to 5-8Hz). Isolated human ventricular myocytes are a better alternative.

Conclusion

We describe a novel work-loop contractility assay that can mimic the physiological and pathological loads seen in cardiac dysfunction and has the potential to translate research on skeletal muscle fatigue to the human myocardium to enhance our understanding of HF and myocardial recovery. A study to address this gap is currently underway (POWER study NCT04899635).

Conflict of Interest

None