Na + ,K + -ATPase Na + Affinity in Rat Skeletal Muscle Fiber Types

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 β 1 isoform have a higher Na + affinity than heterodimers containing the β 2 isoform. Immunoprecipitation experiments demonstrated that dimers with α 1 are responsible for approximately 36% of the total Na,K-ATPase activity. Selective inhibition of the α 2 isoform with ouabain suggested that heterodimers containing the α 1 isoform have a higher Na + affinity than heterodimers containing the α 2 isoform. The estimated K m values for Na + are 4.0, 5.5, 7.5 and 13 mM for α 1 β 1 , α 2 β 1 , α 1 β 2 and α 2 β 2 , 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.