Na+-K+-ATPase in rat skeletal muscle:: content, isoform, and activity characteristics

The purpose of this study was to investigate the hypothesis that muscle Na+-K+-ATPase activity is directly related to Na+-K+-ATPase content and the content of the alpha2-catalytic isoform in muscles of different fiber-type composition. To investigate this hypothesis, tissue was sampled from soleus ( Sol), red gastrocnemius (RG), white gastrocnemius (WG), and extensor digitorum longus (EDL) muscles at rest from 38 male Wistar rats weighing 413 +/- 6.0 g (mean +/- SE). Na+- K+-ATPase activity was determined in homogenates (Hom) and isolated crude membranes ( CM) by the regenerating ouabain-inhibitable hydrolytic activity assay (ATPase) and the 3-O-methylfluorescein K+-stimulated phosphatase (3-O-MFPase) assay in vitro. In addition, Na+-K+-ATPase content (B-max) and the distribution of alpha(1)-, alpha(2)-, beta(1)-, and beta(2)-isoforms were determined by [H-3] ouabain binding and Western blot, respectively. For the ATPase assay, differences ( P < 0.05) in enzyme activity between muscles were observed in Hom ( EDL > WG) and in CM ( Sol > EDL = WG). For the 3-O-MFPase assay, differences ( P < 0.05) were also found for Hom ( Sol > RG = EDL > WG) and CM (Sol = WG > RG). For B-max, differences in the order of RG = EDL > Sol = WG ( P < 0.05) were observed. Isoform distribution was similar between Hom and CM and indicated in CM, a greater density (P < 0.05) of alpha(1) in Sol than WG and EDL (P < 0.05), but more equal distribution of alpha(2) between muscles. The beta(1) was greater (P < 0.05) in Sol and RG, and the beta(2) was greater in EDL and WG (P < 0.05). Over all muscles, the correlation ( r) between Hom 3-O-MFPase and B-max was 0.45 (P < 0.05) and between Hom alpha(2) and B-max, 0.59 (P < 0.05). The alpha(1) distribution correlated to Hom 3-O-MFPase (r = 0.79, P < 0.05) CM ATPase (r = 0.69, P < 0.005) and CM 3-O-MFPase activity ( r = 0.32, P < 0.05). The alpha(2) distribution was not correlated with any of the Na+-K+-ATPase activity measurements. The results indicate generally poor relationships between activity and total pump content and alpha(2) isoform content of the Na+- K+-ATPase. Several factors, including the type of preparation and the type of assay, appear important in this regard.