Myocardial hypertrophy in vivo is associated with reexpression of contractile protein isogenes characteristic of fetal and neonatal development. The molecular signals for hypertrophy and isogene switching are unknown. We studied α (sarcomeric)-actin messenger RNA (mRNA) expression in cultured cardiac myocytes from the neonatal rat. In the cultured cells, as in the adult heart in vivo, expression of cardiac α-actin (cACT) predominated over that of skeletal α-actin (sACT) mRNA, the fetal/neonatal isoform. α1-Adrenergic receptor stimulation induced hypertrophy of these cells, increasing total RNA and cytoskeletal actin mRNA by 1.8-fold over control, and total α-actin mRNA by 4.3 fold. This disproportionate increase in total α-actin mRNA was produced by a preferential induction of sACT mRNA, which increased by 10.6-fold over control versus only 2.6-fold for cACT mRNA. The α1-adrenoceptor is the first identified molecular mediator of early developmental isogene reexpression in cardiac myocyte hypertrophy.
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