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J Membrane Biol (2010) 234:3545 DOI 10.1007/s00232-010-9237-6
Na+,K+-ATPase Na+ Afnity in Rat Skeletal Muscle Fiber Types
Michael Kristensen Carsten Juel
Received: 18 November 2009 / Accepted: 1 February 2010 / Published online: 23 February 2010 Springer Science+Business Media, LLC 2010
Abstract Previous studies in expression systems have found different ion activation of the Na?/K?-ATPase isozymes, which suggest that different muscles have different ion afnities. The rate of ATP hydrolysis was used to quantify Na?,K?-ATPase activity, and the Na? afnity of Na?,K?-ATPase was studied in total membranes from rat muscle and puried membranes from muscle with different ber types. The Na? afnity was higher (Km lower) in oxidative muscle compared with glycolytic muscle and in puried membranes from oxidative muscle compared with glycolytic muscle. Na?,K?-ATPase isoform analysis implied that heterodimers containing the b1 isoform have a higher Na? afnity than heterodimers containing the b2 isoform. Immunoprecipitation experiments demonstrated that dimers with a1 are responsible for approximately 36%
of the total Na,K-ATPase activity. Selective inhibition of the a2 isoform with ouabain suggested that heterodimers containing the a1 isoform have a higher Na? afnity than heterodimers containing the a2 isoform. The estimated Km values for Na? are 4.0, 5.5, 7.5 and 13 mM for a1b1, a2b1, a1b2 and a2b2, respectively. The afnity 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.
Keywords Na,K pump Na,K-ATPase isoform
Ion activation
Introduction
Ion gradients across muscle membranes are important for muscle function. Activity-induced changes in ion distribution affect muscle excitability and may lead to impairment of force development.
Na?,K?-ATPase (Na,K pump) maintains normal trans-membrane gradients for Na? and K? and counteracts changes in ion gradients due to muscle activity. Therefore, regulation of the pump is important for muscle function. The minimal functional pump is a heterodimer comprising one a- and one b-subunit. Each subunit is expressed in a number of isoforms, of which a1, a2, b1 and b2 are important in rat skeletal muscle (Clausen 2003). The a1 and a2 isoforms are found in all muscle ber types; however, they have different cellular localizations (Kristensen and Juel...