Thyroid hormone regulation of Na,K-ATPase subunit-mRNA expression in neonatal rat myocardium.

Regulation of Na,K-ATPase mRNA alpha isoform and mRNA beta expression by thyroid hormone (T3) in neonatal rat myocardium was examined. In euthyroid neonates between ages of 2 and 5 days, mRNA alpha 1, mRNA alpha 3, and mRNA beta 1 abundances were nearly constant while mRNA alpha 2 was undetectable. During the interval between postnatal days 5 and 15, mRNA alpha 3 decreased to negligible levels and mRNA alpha 2 became expressed and increased in abundance to account for approximately 20% of the mRNA alpha pool by the 15th postnatal day. To examine the effect of T3 on this developmental program, neonates were injected with 75 micrograms T3/100 g body weight or diluent alone on the second and third postnatal days and myocardial Na,K-ATPase subunit-mRNA abundances were determined on the third and fourth postnatal days. Because T3 treatment increased the RNA/DNA ratios of myocardial tissue, the subunit-mRNA abundances were normalized per unit DNA. Following 24 and 48 hr of T3 treatment, the abundances of mRNA alpha 1, mRNA alpha 3, and mRNA beta 1 increased, while mRNA alpha 2 continued to remain undetectable during the 2-day interval between the second to fourth postnatal days. It is concluded that T3 augments the abundance of Na,K-ATPase subunit mRNAs that are already being expressed in the neonatal rat myocardium. The results further suggest that T3 does not act as a "molecular switch" in the developmental expression of the mRNA alpha isoforms in rat myocardium during the first four postnatal days.