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Abstract
Previous studies in expression systems have found different ion activation of the Na^sup +^/K^sup +^-ATPase isozymes, which suggest that different muscles have different ion affinities. The rate of ATP hydrolysis was used to quantify Na^sup +^,K^sup +^-ATPase activity, and the Na^sup +^ affinity of Na^sup +^,K^sup +^-ATPase was studied in total membranes from rat muscle and purified membranes from muscle with different fiber types. The Na^sup +^ affinity was higher (K ^sub m^ lower) in oxidative muscle compared with glycolytic muscle and in purified membranes from oxidative muscle compared with glycolytic muscle. Na^sup +^,K^sup +^-ATPase isoform analysis implied that heterodimers containing the β^sub 1^ isoform have a higher Na^sup +^ affinity than heterodimers containing the β^sub 2^ isoform. Immunoprecipitation experiments demonstrated that dimers with α^sub 1^ are responsible for approximately 36% of the total Na,K-ATPase activity. Selective inhibition of the α^sub 2^ isoform with ouabain suggested that heterodimers containing the α^sub 1^ isoform have a higher Na^sup +^ affinity than heterodimers containing the α^sub 2^ isoform. The estimated K ^sub m^ values for Na^sup +^ are 4.0, 5.5, 7.5 and 13 mM for α^sub 1^β^sub 1^, α^sub 2^β^sub 1^, α^sub 1^β^sub 2^ and α^sub 2^β^sub 2^, respectively. The affinity differences and isoform distributions imply that the degree of activation of Na^sup +^,K^sup +^-ATPase at physiological Na^sup +^ 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.[PUBLICATION ABSTRACT]