Changes in mitogen-activated protein kinase phosphorylation and inorganic phosphate induced by skeletal muscle contraction

Skeletal muscles adjust to the level of contractile activity by different biochemical alterations. Contraction is a multifactorial stimulus with short-term metabolic effects and long-term responses due to changes in protein synthesis. A central issue in exercise physiology is to elucidate the underlying signaling mechanisms behind adaptations to exercise. The mitogenactivated protein (MAP) kinase signaling pathway is suggested to be involved in exerciseinduced changes in gene expression in skeletal muscle. We studied short- and long-term adaptations to skeletal muscle contractions using both human and animal models, and by employing different exercise intensities and contraction regimens. An increased tetanic force production and reduced inorganic phosphate (Pi) concentration were observed within minutes after a brief bout of repeated contractions. Increases in ATP, phosphocreatine (PCr) or glucose-6-phosphate (G-6-P) could not account for the reduction in Pi. Intensity-dependent increases in the phosphorylation of two MAP kinases, MAPK erk 1/2 and MAPKK mek1/2, were identified in response to one-leg exercise in humans. In isolated rat muscle, isometric contractions induced a similar increase in phosphorylation of MAPKerk1/2 in slow- and fast-twitch muscles, while a significant increase in MAPK p38 phosphorylation was observed in fast- twitch skeletal muscles only. Since increased MAPK phoshporylation was observed in isolated muscles and in the exercised leg only in humans, the main stimulating factor(s) appears to be local. The total amount of MAPK erk 1/2 and MAPK p38 was higher in slow-twitch than in fast-twitch skeletal muscle. Mild passive stretches and concentric contrations induced phosphorylation of MAPK erk l/2 , but not MAPK p38. During concentric contractions, reactive oxygen species (ROS) was the key triggering factor behind the activation. Eccentric contractions resulted in an augmented phosphorylation of MAPKerk1/2 and MAPK p38 , whereas severe stretches mainly induced MAPKerk1/2 phosphorylation. Thus, mechanical perturbations, which are known to affect protein synthesis, stimulate phosphorylation of these kinases differently. In conclusion, repeated contractions result in marked adaptations in skeletal muscle: a reduced Pi may cause a transient increase in force production while increased MAP kinase signaling may induce long-term changes in protein expression

Effects of concentric and eccentric contractions on phosphorylation of MAPKerk1/2 and MAPKp38 in isolated rat skeletal muscle

Exercise and contractions of isolated skeletal muscle induce phosphorylation of mitogen-activated protein kinases (MAPKs) by undefined mechanisms. The aim of the present study was to determine exercise-related triggering factors for the increased phosphorylation of MAPKs in isolated rat extensor digitorum longus (EDL) muscle.Concentric or eccentric contractions, or mild or severe passive stretches were used to discriminate between effects of metabolic/ionic and mechanical alterations on phosphorylation of two MAPKs: extracellular signal-regulated kinase 1 and 2 (MAPKerk1/2) and stress-activated protein kinase p38 (MAPKp38).Concentric contractions induced a 5-fold increase in MAPKerk1/2 phosphorylation. Application of the antioxidants N-acetylcysteine (20 mm) or dithiothreitol (5 mm) suppressed concentric contraction-induced increase in MAPKerk1/2 phosphorylation. Mild passive stretches of the muscle increased MAPKerk1/2 phosphorylation by 1.8-fold, whereas the combination of acidosis and passive stretches resulted in a 2.8-fold increase. Neither concentric contractions, nor mild stretches nor acidosis significantly affected phosphorylation of MAPKp38.High force applied upon muscle by means of either eccentric contractions or severe passive stretches resulted in 5.7- and 9.5-fold increases of phosphorylated MAPKerk1/2, respectively, whereas phosphorylation of MAPKp38 increased by 7.6- and 1.9-fold (not significant), respectively.We conclude that in isolated rat skeletal muscle an increase in phosphorylation of both MAPKerk1/2 and MAPKp38 is induced by mechanical alterations, whereas contraction-related metabolic/ionic changes (reactive oxygen species and acidosis) cause increased phosphorylation of MAPKerk1/2 only. Thus, contraction-induced phosphorylation can be explained by the combined action of increased production of reactive oxygen species, acidification and mechanical perturbations for MAPKerk1/2 and by high mechanical stress for MAPKp38