Hypoxia has been suggested to be both beneficial and harmful to the proliferation of cardiomyocytes. This controversy remains unresolved, and the underlying mechanism by which hypoxia exerts its effects remains unclear. We here hypothesize that cardiomyocyte developmental stage may play a role.

: The embryonic ventricular myocyte cell line H9C2, primary isolated fetal cardiomyocytes, and neonatal cardiomyocytes were cultured with normal O₂ (21% O₂) or under hypoxic conditions (10% O₂) for 7 days, and then harvested for Western blotting, qRT-PCR, and immunostaining. When cultured under hypoxic conditions, proliferating marker-Ki67, mRNA level, and the percentage of Ki67-positive cardiomyocytes were significantly lower in H9C2 and fetal cardiomyocytes but higher in neonatal cardiomyocytes. Consistently, the mRNA and protein levels and induced nuclear localization of yes associated protein 1(YAP1), one of the most important regulators of cardiomyocyte proliferation, were significantly lower in H9C2 and fetal cardiomyocytes but up-regulated in neonatal cardiomyocytes when treated with hypoxia. Compared to neonatal cardiomyocytes, there was a lower level of troponin T mRNA and protein expression in H9C2 and fetal cardiomyocytes. When H9C2 or fetal cardiomyocytes overexpressing troponin T in were cultured under hypoxic condition, their ability to proliferate increased.

The effect of hypoxia on the proliferation of cardiomyocyte is associated with their developmental stage. YAP1 expression is positively correlated with the change in cardiomyocyte proliferation in response to hypoxia. Developmental stage- specific sarcomere component troponin T may partly account for the underlying mechanism.