Rapid Conversion fromβ-MHC toα-MHC mRNA Expression in Embryonic Rat Ventricle Culturedin oculois not Dependent on Thyroid Hormone or Testosterone

We investigated the expression of myosin heavy chain (MHC) isoenzymes in embryonic rat ventricles cultured in the anterior eye chamber of an adult rat.In oculo, these grafts beat and mature in an environment where the hormonal milieu can be manipulated. S1 nuclease protection assays were performed on pooled samples of ventricle grafts and compared to normally growing ventricles. At the time of grafting (embryonic day 12, E-12), 23±4% of the MHC mRNA was of theαisoform. While the proportion of ventricularα-MHC mRNA did not increasein utero, embryonic ventricles culturedin oculoshowed a rapid increase in the relative amount ofα-MHC mRNA expression (to 84±10% by 3 days and 86±5% by 8 daysin oculo).α-MHC mRNA expression predominated through 8 weeks of culturein oculo, being 76% at 8 weeksin oculo. Additional experiments were performed to determine whether the rapid conversion toα-MHC expression resulted from exposure to adult levels of testosterone or thyroid hormone. Reduction of testosterone exposure to non-detectable levels by host orchiectomy did not affect the rapid conversion toα-MHC mRNA expression. Exposure to a hypothyroid milieu (i.e., PTU-treated hosts) decreased but did not prevent the conversion fromβ- toα-MHC mRNA expression at 8 daysin oculo; with 83% of the MHC mRNA being of theαisoform in hypothyroid hosts compared to 95% in euthyroid hosts. After 8 weeks of culture in hypothyroid hosts, however,α-MHC mRNA expression was undetectable in grafted ventricles. These data suggest that E-12 myocardial grafts respond to the hormonal milieu of an adult rat with rapid conversion fromβ- toα-MHC mRNA expression and thatα-MHC expression in early developing heart may show reduced sensitivity to downward modulation by a hypothyroid hormonal milieu.