Thyroid hormone up-regulates Na+/K+ pump α2 mRNA but not α2 protein isoform in cultured skeletal muscle.

Thyroid hormone (T3) is known to up-regulate the physiological expression of the Na+/K+ pump in cultured skeletal muscle. We recently reported that primary cultured rat skeletal muscle expresses only the α1, β1 and β2 protein isoforms of Na+/K+ pump. Interestingly, α2 mRNA is detectable while the α2 protein isoform is not. We therefore examined whether T3 might up-regulate the expression of Na+/K+ pump α2 isoform at the protein and mRNA level. We also examined the regulation by this hormone of the other isoforms of the pump. Primary cultures were treated with T3 for 48 h from day 4 to day 6 of differentiation. Protein and mRNA isoforms of Na+/K+ pump were identified by Western blotting and Northern blotting, respectively. T3 induced a marked increase in the β1 protein and a slight increase in the α1 protein. T3 did not affect expression of the β2 protein. The α2 protein was not detected in either untreated or T3-treated cells. In contrast, α2 mRNA was highly up-regulated by T3 treatment compared to the other isoforms. The lack of expression of the α2 protein isoform following T3 treatment suggests that posttranscriptional events related to this isoform may be dependent on other growth factors or hormones.