Calcium signaling regulates ventricular hypertrophy during development independent of contraction or blood flow

In utero interventions aimed at restoring left ventricular hemodynamic forces in fetuses with prenatally diagnosed hypoplastic left heart syndrome failed to stimulate ventricular myocardial growth during gestation, suggesting chamber growth during development may not rely upon fluid forces. We therefore hypothesized that ventricular hypertrophy during development may depend upon fundamental Ca2 +-dependent growth pathways that function independent of hemodynamic forces. To test this hypothesis, zebrafish embryos were treated with inhibitors or activators of Ca2 + signaling in the presence or absence of contraction during the period of chamber development. Abolishment of contractile function alone in the setting of preserved Ca2 + signaling did not impair ventricular hypertrophy. In contrast, inhibition of L-type voltage-gated Ca2 + influx abolished contraction and led to reduced ventricular hypertrophy, whereas increasing L-type voltage-gated Ca2 + influx led to enhanced ventricular hypertrophy in either the presence or absence of contraction. Similarly, inhibition of the downstream Ca2 +-sensitive phosphatase calcineurin, a known regulator of adult cardiac hypertrophy, led to reduced ventricular hypertrophy in the presence or absence of contraction, whereas hypertrophy was rescued in the absence of L-type voltage-gated Ca2 + influx and contraction by expression of a constitutively active calcineurin. These data suggest that ventricular cardiomyocyte hypertrophy during chamber formation is dependent upon Ca2 + signaling pathways that are unaffected by heart function or hemodynamic forces. Disruption of Ca2 +-dependent hypertrophy during heart development may therefore represent one mechanism for impaired chamber formation that is not related to impaired blood flow.