Previous studies in expression systems have found different ion activation of the Na+/K+-ATPase isozymes, which suggest that different muscles have different ion affinities. The rate of ATP hydrolysis was used to quantify Na+,K+-ATPase activity, and the Na+ affinity of Na+,K+-ATPase was studied in total membranes from rat muscle and purified membranes from muscle with different fiber types. The Na+ affinity was higher (K m lower) in oxidative muscle compared with glycolytic muscle and in purified membranes from oxidative muscle compared with glycolytic muscle. Na+,K+-ATPase isoform analysis implied that heterodimers containing the β1 isoform have a higher Na+ affinity than heterodimers containing the β2 isoform. Immunoprecipitation experiments demonstrated that dimers with α1 are responsible for approximately 36% of the total Na,K-ATPase activity. Selective inhibition of the α2 isoform with ouabain suggested that heterodimers containing the α1 isoform have a higher Na+ affinity than heterodimers containing the α2 isoform. The estimated K m values for Na+ are 4.0, 5.5, 7.5 and 13 mM for α1β1, α2β1, α1β2 and α2β2, respectively. The affinity differences and isoform distributions imply that the degree of activation of Na+,K+-ATPase at physiological Na+ concentrations differs between muscles (oxidative and glycolytic) and between subcellular membrane domains with different isoform compositions. These differences may have consequences for ion balance across the muscle membrane.
The Journal of Membrane Biology – Springer Journals
Published: Feb 23, 2010
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