Fatigue-induced change in T-system excitability and its major cause in rat fast-twitch skeletal muscle in vivo

Abstract

Key points: 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. 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. 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. 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. Abstract: 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. 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 RP under partially depolarized conditions was highly dependent on the function of Na+–K+-ATPase. 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 increased in stimulated fibres (5.9 ± 1.0 ms in rested vs. 8.0 ± 1.5 ms in stimulated); however, this increase in RP was reversed by sequential treatment with dithiothreitol. The skinned fibres from rested muscles exhibited slower repriming only when treated with oxidized glutathione (GSSG) under very low ATP (≤1 mm) conditions, whereas the RP in stimulated fibres was not altered after GSSG treatment without ATP. In line with this, the α2- and β-subunits of Na+–K+-ATPase were more S-glutathionylated in stimulated than in rested muscles. 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.