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Effect of exercise on interstitial K+, fatigue development and Na+, K+ pump mRNA expression in human muscle

Nikolai Nordsbord

Summary

The [K+]i was shown to increase to more than 10‐12 mM during the final part of exhaustive exercise lasting ~ 3‐6 min in Study I and II. When extracellular K+ concentrations are raised to this high level around skeletal muscles in vitro, it causes a severe depolarization of Em and a reduction of membrane excitability, which is likely to be causing the simultaneous fatigue development. Thus, based on the findings in Study I and II, extracellular K+ accumulation may be suggested to cause fatigue development in human skeletal muscle. However, in Study I the [K+]i at exhaustion after intense one‐legged knee‐extensions, preceded by bilateral arm‐cranking, was found to be higher than at exhaustion after leg exercise without prior arm‐exercise. This finding, together with the finding of a reduction in [K+]i at exhaustion with repeated intense one‐legged knee‐extensions in Study II, clearly shows that a critical [K+]i at which fatigue develops does not exist. It seems more appropriate to evaluate the effect of extracellular K+ accumulation on muscle fatigue development by determining the effect of intense exercise on skeletal muscle Em. However, the effect of intense exercise on skeletal muscle Em in humans is not easily determined. By use of the values for [K+]i provided in Study I and II as well as literature values, it can be calculated that the human skeletal muscle Em at exhaustion may approach the critical level of around ‐60 mV observed in vitro. However, a precise calculation of human skeletal muscle Em at exhaustion requires knowledge of the intra‐ and extracellular distribution of K+, Na+ and Cl‐ as well as their respective conductances in addition to knowledge of the contribution of the electrogenic Na+, K+ pump. Of these variables intracellular Cl‐ concentration, the conductances for K+, Na+ and Cl‐ as well as the electrogenic contribution of the Na+, K+ pump are all unknown in exhausted human skeletal muscle. Thus, further research is warranted in order to clarify the role of extracellular K+ accumulation in fatigue development. 

In addition to the findings regarding the magnitude of K+ accumulation and the possible effect on Em and fatigue development, Study I and II also provided further insight into the regulation of K+ efflux from the exercising muscle. In Study I, a higher K+ efflux was found concurrent with an expected increase in muscle acidity, and in Study II a reduction of K+ efflux occurred at the same time as a reduction of muscle acidity with repeated intense exercise. Thus, a link between H+ homeostasis and K+ homeostasis is suggested. 

Na+, K+ pump mRNA alterations – relation to exercise and protein expression Study III, IV, V and VI all showed that substantial variability should be expected when analyzing mRNA expression. Study V and provided previously unpublished data, further showed that both technical and biological variability should be expected in analysis of mRNA. It was suggested that normalization to the amount of cDNA in the sample used for PCR could be an appropriate alternative to the use of reference genes. If using this approach for normalization and applying duplicate RNA extraction and triplicate PCR analysis in a study involving eight subjects, like Study IV, changes of ~0.5‐1.0 fold in Na+, K+ pump mRNA expression are detectable.

 In Study IV, it was demonstrated that the Na+, K+ pump α2 and β1 subunits are the most highly expressed at the mRNA level and that the increase after exercise of these subunits mRNA are the quantitatively most important. This apparently contradicts the result of Study III, in which only α1 mRNA was found to increase after high‐intensity exercise. However, due to the observed variability in Study III, it is suggested that highintensity exercise in general results in an increase of α1, α2, β1, β3 and possibly of β2 mRNA as showed in Study IV. 

Study III further showed that α1 mRNA was increased after high‐intensity exercise performed with an untrained leg, whereas performance of the same exercise after a period of high‐intensity intermittent training did not result in an increase of α1 mRNA. Furthermore, when comparing the resting level of α1 mRNA in the trained and untrained muscle, the trained muscle tended (P<0.10) to have a higher α1 mRNA level. These findings lead to the suggestion that exercise training may increase the mRNA resting level and reduce the responsiveness of the investigated muscle to a given stimulus with respect to increases in mRNA. 

Study IV showed that the increase in Na+, K+ pump mRNA after exercise is most likely caused by intracellular events, since activation of additional muscle mass during intense exercise, which increased the hormonal response to exercise, did not result in an amplified Na+, K+ pump mRNA increase after exercise. Furthermore, Study VI showed that changes in Na+, K+ pump mRNA in response to a given stimulus may be muscle specific, since treatment with dexamethasone only resulted in increased Na+, K+ pump mRNA levels in the deltoid, and not the vastus lateralis muscles 11 hrs after the last treatment. Nevertheless, an increase in Na+, K+ pump content and maximal activity was observed in both muscles in Study VI, indicating that Na+, K+ pump protein expression is not only regulated by the prevailing mRNA level. However, in the case of exercise it seems reasonable to suggest that exercise increases both Na+, K+ pump α1, α2 and β1 protein and mRNA expression, which implies that increases in Na+, K+ pump mRNA could be responsible for the exercise induced increase in Na+, K+ pump protein expression.