Prolonged submaximal exercise induces isoform-specific Na+-K+-ATPase mRNA and protein responses in human skeletal muscle

Abstract

This study investigated effects of prolonged submaximal exercise on Na⁺-K⁺-ATPase mRNA and protein expression, maximal activity, and content in human skeletal muscle. We also investigated the effects on mRNA expression of the transcription initiator gene, RNA polymerase II (RNAP II), and key genes involved in protein translation, eukaryotic initiation factor-4E (eIF-4E) and 4E-binding protein 1 (4E-BP1). Eleven subjects (6 men, 5 women) cycled at 75.5% (SD 4.8%) peak O₂uptake and continued until fatigue. A vastus lateralis muscle biopsy was taken at rest, fatigue, and 3 and 24 h postexercise. We analyzed muscle for Na⁺-K⁺-ATPase α₁, α₂, α₃, β₁, β₂, and β₃, as well for RNAP II, eIF-4E, and 4E-BP1 mRNA expression by real-time RT-PCR and Na⁺-K⁺-ATPase isoform protein abundance using immunoblotting. Muscle homogenate maximal Na⁺-K⁺-ATPase activity was determined by 3 -O-methylfluorescein phosphatase activity and Na⁺-K⁺-ATPase content by [³H]ouabain binding. Cycling to fatigue [54.5 (SD 20.6) min] immediately increased α₃( P = 0.044) and β₂mRNA ( P = 0.042) by 2.2- and 1.9-fold, respectively, whereas α₁mRNA was elevated by 2.0-fold at 24 h postexercise ( P = 0.036). A significant time main effect was found for α₃protein abundance ( P = 0.046). Exercise transiently depressed maximal Na⁺-K⁺-ATPase activity ( P = 0.004), but Na⁺-K⁺-ATPase content was unaltered throughout recovery. Exercise immediately increased RNAP II mRNA by 2.6-fold ( P = 0.011) but had no effect on eIF-4E and 4E-BP1 mRNA. Thus a single bout of prolonged submaximal exercise induced isoform-specific Na⁺-K⁺-ATPase responses, increasing α₁, α₃, and β₂mRNA but only α₃protein expression. Exercise also increased mRNA expression of RNAP II, a gene initiating transcription, but not of eIF-4E and 4E-BP1, key genes initiating protein translation.