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

Abstract

Thyroid hormone (T 3) 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 α 1, β 1 and β 2 protein isoforms of Na +/K + pump. Interestingly, α 2 mRNA is detectable while the α 2 protein isoform is not. We therefore examined whether T 3 might up-regulate the expression of Na +/K + pump α 2 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. T 3 induced a marked increase in the β 1 protein and a slight increase in the α 1 protein. T 3 did not affect expression of the β 2 protein. The α 2 protein was not detected in either untreated or T 3-treated cells. In contrast, α 2 mRNA was highly up-regulated by T 3 treatment compared to the other isoforms. The lack of expression of the α 2 protein isoform following T 3 treatment suggests that posttranscriptional events related to this isoform may be dependent on other growth factors or hormones.