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.