However, the effect of muscle contractions on the T-system Na +–K +-ATPase currently remains unclear, because it is very difficult to biochemically purify T-system Na +–K +-ATPases (Clausen, 2013). Specifically, Na +–K +-ATPases in the T-system are thought to contribute, at least in part, to the alteration of membrane excitability during and after high-intensity contractions, because the T-system is a very narrow space, owing to its high surface-to-volume ratio (Dulhunty, 1984 Allen et al. Additionally, Na +–K +-ATPase plays a crucial role in maintaining the normal electrochemical gradients for K + and Na +. Contraction-induced alteration of K + and Cl − conductance has been shown to cause a decrease in membrane excitability, which is highly mediated by muscle-specific Cl − channels and ATP-dependent K + channels (Allen et al. Of these, the membranes are adequately polarized by K + and Cl − gradients in the resting state. The membrane potential in skeletal muscle is largely dependent on the electrochemical gradients for K +, Na + and Cl −. 2008), a fatigue-induced change in T-system excitability is particularly important for skeletal muscle fatigue but has yet to be investigated. the rate of AP propagation) are quite different from those of the surface membrane (Allen et al. As the properties of the T-system membrane (e.g. However, the M-wave mostly depends on the propagation of the AP on the surface membrane, but not the T tubular-system (T-system) membrane although T-system depolarization directly elicits muscle contraction. To date, numerous studies on muscle fatigue in humans have examined the effect of fatigue on a compound surface AP, i.e. It has been argued that muscle fatigue resulting from high-intensity exercise is caused, at least in part, by an impairment of the membrane excitability (Overgaard et al. High-intensity exercise causes significant force depression in skeletal muscles, i.e. A muscle action potential (AP) is initiated at the neuromuscular junction, propagates along the sarcolemma and subsequently moves throughout the transverse (T) tubule (Posterino et al. Normal muscle contraction relies on electrical excitation of the muscle fibre. These results suggest that (i) T-system excitability is decreased during contractions, in part due to a downregulation of T-system Na +–K +-ATPase, (ii) S-glutathionylation contributes to the fatigue-induced decline of the T-system Na +–K +-ATPase function, and (iii) ATP depression throughout contractions may enhance S-glutathionylation of T-system Na +–K +-ATPase. In line with this, the α2- and β-subunits of Na +–K +-ATPase were more S-glutathionylated in stimulated than in rested muscles. ![]() The skinned fibres from rested muscles exhibited slower repriming only when treated with oxidized glutathione (GSSG) under very low ATP (≤1 m m) conditions, whereas the RP in stimulated fibres was not altered after GSSG treatment without ATP. 8.0 ± 1.5 ms in stimulated) however, this increase in RP was reversed by sequential treatment with dithiothreitol. ![]() ![]() The RP under partially depolarized conditions was increased in stimulated fibres (5.9 ± 1.0 ms in rested vs. Rat gastrocnemius (GAS) muscles were subjected to repetitive contractions until the force was decreased to ∼50% of initial force, and then the muscles were very quickly excised and used for skinned fibre and biochemical experiments. The RP under partially depolarized conditions was highly dependent on the function of Na +–K +-ATPase. T-system excitability was estimated by measuring the repriming period (RP) required for double action potentials in mechanically skinned fibres where the sarcolemma was removed but the T-system still functioned. The purpose of this study was to investigate transverse tubular system (T-system) excitability after skeletal muscle contractions in vivo, and the contribution of S-glutathionylation of Na +–K +-ATPase. In conclusion, T-system excitability was decreased after high-intensity exercise due at least in part to the S-glutathionylation of Na +–K +-ATPase, which may be enhanced by contraction-induced ATP depression.The S-glutathionylation of Na +–K +-ATPase in whole muscle was increased after muscle contractions and also occurred under very low ATP conditions in rested but not stimulated fibres.T-system excitability estimated by using skinned fibres, which is highly regulated by T-system Na +-K +-ATPase, was decreased after muscle contractions, but was fully restored by treatment with dithiothreitol.Using mechanically skinned rat muscle fibres, we investigated (i) transverse tubular-system (T-system) excitability after high-intensity contractions, and (ii) the mechanisms underlying the fatigue-induced alteration of the T-system excitability.
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