Differential expression of the Na,K-ATPase alpha 1 and alpha 2 subunit genes in a murine myogenic cell line. Induction of the alpha 2 isozyme during myocyte differentiation.

Abstract

Expression of Na,K-ATPase catalytic alpha isoform (alpha 1, alpha 2, and alpha 3) and beta subunit genes in rodent muscle was investigated using the murine C2C12 myogenic cell line. RNA blot analyses of myoblasts revealed expression primarily of the alpha 1 mRNA and low levels of alpha 2 mRNA. Fusion of the proliferating myoblasts to form myotubes was accompanied by an approximate 12-fold induction of the alpha 2 mRNA. In contrast, expression of alpha 1 mRNA remained constant throughout myogenesis. The alpha 3 mRNA was not detected in either myoblasts or myotubes. The beta mRNA abundance also increased 2-3-fold during myotube formation. In rodent tissues, low and high affinity cardiac glycoside (e.g. ouabain) receptors have been shown to be associated with the Na,K-ATPase catalytic alpha 1 and alpha 2 isoform subunits, respectively. The existence of these two functional classes of Na,K-ATPase in myoblasts and myotubes correlated with the biphasic ouabain inhibition of Na,K-ATPase activity. Confluent myoblasts expressed primarily the alpha 1 isozyme (IC50 = 3.6 X 10(-5) M; 95% of total activity) and lesser amounts of the alpha 2 isozyme (IC50 = 1.1 X 10(-7) M; 5% of total activity). In contrast, the myotubes showed significant levels of the alpha 1 isozyme (IC50 = 4.0 X 10(-5) M; 68% of total activity) and, in addition, showed a 6-fold increase in the relative levels of the alpha 2 isozyme (IC50 = 1.1 X 10(-7) M; 32% of total activity). To quantitate further the expression of the high affinity, ouabain-sensitive alpha 2 isozyme, a whole cell [3H]ouabain-binding assay was used. Results revealed that myotubes have an approximately 6-fold greater concentration of [3H]ouabain-binding sites than myoblasts with an apparent dissociation constant (Kd) of 1.4 X 10(-7) M. The results indicate that muscle cells can express multiple isozymes of Na,K-ATPase and that expression of the alpha 2 isozyme is developmentally regulated during myogenesis.