Evidence for intermittent coupling of intramyocardial small, engineered heart tissues acutely implanted into rabbit myocardium.

Aims: Electrical integration of human iPSC-derived cardiomyocyte (hiPSC-CM)-based tissue with the host myocardium is a requirement of successful regeneration therapy. This study was designed to identify electrical coupling in the acute phase (1-2h) post-grafting using an ex vivo model.

Methods And Results: Small, engineered heart tissues (mini-EHTs) consisting of ∼50,000 hiPSC-CMs on a hydrogel (spontaneous rate 0.34±0.05 Hz), were loaded with Cal520-AM. EHTs were implanted sub-epicardially into a Langendorff-perfused rabbit heart after blebbistatin treatment. For up to 100 mins, a continuous pseudo-ECG was recorded during sinus rhythm (rate 2.0-3.5 Hz). At 25 min intervals, EHT calcium transients (CaTs) were recorded for 10-20 s (No Contraction group). To study the influence of mechanical activity, blebbistatin was washed off after implantation (Contraction Recovery group).Periodic entrainment of EHTs with the myocardium was detected less often (p=0.011) in the No Contraction group (1/9 hearts) than in the Contraction Recovery group (5/6 hearts). The average coupling delay (QRS-CaT) and the difference in consecutive delays (Δdelay) were 89±50 ms and 10±3 ms respectively (n=12 traces; N=6 hearts). Coupling ratios (QRS:CaT) varied from 2:1 to 4:1. These coupling parameters were not significantly different in the two experimental groups. Modelling of hiPSC-CM tissue separated by a 25 μm saline gap from the myocardium demonstrated field-effect coupling with similarly variable activation delays. Importantly, coupling failed with a gap of 100 μm.

Conclusions: In conclusion, EHT entrainment is possible immediately after grafting and has features compatible with field-effect coupling. Sensitivity to the gap dimensions may explain why entrainment is more common in actively contracting myocardium.