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.