Altered expression and insulin-induced trafficking of Na+-K+-ATPase in rat skeletal muscle: effects of high-fat diet and exercise

Abstract

Skeletal muscle Na+-K+-ATPase plays a central role in the clearance of K+from the extracellular fluid, therefore maintaining blood [K+]. Na+-K+-ATPase activity in peripheral tissue is impaired in insulin resistant states. We determined effects of high-fat diet (HFD) and exercise training (ET) on skeletal muscle Na+-K+-ATPase subunit expression and insulin-stimulated translocation. Skeletal muscle expression of Na+-K+-ATPase isoforms and transcription factor DNA binding was determined before or after 5 days of swim training in Wistar rats fed chow or HFD for 4 or 12 wk. Skeletal muscle insulin resistance was observed after 12 wk of HFD. Na+-K+-ATPase α1-subunit protein expression was increased 1.6-fold ( P < 0.05), whereas α2- and β1-subunits and protein expression were decreased twofold ( P < 0.01) in parallel with decrease in plasma membrane Na+-K+-ATPase activity after 4 wk of HFD. Exercise training restored α1-, α2-, and β1-subunit expression and Na+-K+-ATPase activity to control levels and reduced β2-subunit expression 2.2-fold ( P < 0.05). DNA binding activity of the α1-subunit-regulating transcription factor ZEB (AREB6) and α1mRNA expression were increased after HFD and restored by ET. DNA binding activity of Sp-1, a transcription factor involved in the regulation of α2- and β1-subunit expression, was decreased after HFD. ET increased phosphorylation of the Na+-K+-ATPase regulatory protein phospholemman. Phospholemman mRNA and protein expression were increased after HFD and restored to control levels after ET. Insulin-stimulated translocation of the α2-subunit to plasma membrane was impaired by HFD, whereas α1-subunit translocation remained unchanged. Alterations in sodium pump function precede the development of skeletal muscle insulin resistance. Disturbances in skeletal muscle Na+-K+-ATPase regulation, particularly the α2-subunit, may contribute to impaired ion homeostasis in insulin-resistant states such as obesity and type 2 diabetes.