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

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 O-2 uptake 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 alpha(1), alpha(2), alpha(3), alpha(1), alpha(2), and alpha(3), 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 [H-3] ouabain binding. Cycling to fatigue [54.5 (SD 20.6) min] immediately increased alpha(3) (P = 0.044) and beta(2) mRNA (P = 0.042) by 2.2- and 1.9-fold, respectively, whereas alpha(1) mRNA was elevated by 2.0-fold at 24 h postexercise (P = 0.036). A significant time main effect was found for alpha(3) protein 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 alpha(1), alpha(3), and beta(2) mRNA but only alpha 3 protein 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.