Exposure of immature rat heart to antenatal glucocorticoid results in cardiac proliferation

Background ATP synthesis and cardiac contraction‐related protein production are accelerated in the immature fetal heart by antenatal glucocorticoids (GC). This study investigated the structural maturity of the myocardium and underlying signal pathway associated with cardiac growth in fetal rats that received antenatal GC. Methods and Results Dexamethasone (DEX) was given to pregnant rats for 2 days from day 17 or day 19 of gestation, and the hearts of 19 and 21 day fetuses and 1‐day‐old neonates were analyzed. Although irregular myofibril orientation was observed morphologically in 19 day fetal hearts, the myofibril components were organized in fetuses after DEX. The cross‐sectional area of the myocardium and Ki‐67‐positive cells were significantly increased in fetal DEX groups, suggesting that cardiac enlargement resulted from myocyte proliferation. Glycogen synthase kinase‐3β (GSK‐3β) protein was significantly decreased in fetal DEX groups. β‐Catenin and vascular endothelial growth factor protein were also significantly increased. Furthermore, increased cardiomyocyte proliferation appeared to be mediated by GC receptors after culture with DEX in vitro. Conclusions Antenatal DEX induces structural maturity accompanying cardiomyocyte proliferation in the premature fetal rat heart, and GSK‐3β and β‐catenin are thought to contribute to cardiac growth.