Effects of high-intensity interval exercise on muscle fatigue and SR function in rats: a comparison with moderate-intensity continuous exercise.

In this study, we compared muscle fatigue induced by high-intensity interval exercise (HIIE) and moderate-intensity continuous exercise (MICE), with a focus on changes in the function of sarcoplasmic reticulum (SR) and myofibril. To achieve the aim of this study with mechanically skinned fibers with sealed transverse tubules and intact SR membrane, myofibrillar Ca2+ sensitivity, depolarization-induced force, and action potential-induced force were evaluated. Rat gastrocnemius muscles were subjected to HIIE-mimicking or MICE-mimicking stimulation in situ. The number of contractions was the same for MICE- and HIIE-mimicking stimulation (total of 360 contractions). Three hours after cessation of stimulation, the superficial regions of gastrocnemius muscles were dissected and used for biochemical and skinned fiber analyses. At 3 h of recovery, forces at 20 and 100 Hz in whole muscles had returned to resting levels in MICE but not HIIE muscles. The reduced glutathione content was decreased only in HIIE muscles. Both MICE- and HIIE-mimicking stimulation resulted in an increase in myofibrillar Ca2+ sensitivity in skinned fibers. Only HIIE-mimicking stimulation led to a decrease in the ratio of force at 1 Hz to that at 50 Hz and the ratio of depolarization-induced force to the maximum Ca2+-activated force. These results reflect the properties of type IIX and IIB fibers (the latter is not expressed in human skeletal muscles) and suggest that HIIE requires longer recovery periods than those normally used with MICE, which is ascribable to long-lasting depressions in SR Ca2+ release.NEW & NOTEWORTHY Over the past decade, high-intensity interval exercise (HIIE) training has received attention as a more efficient training to improve endurance capacity. It is unclear, however, whether the extent of acute exercise-related muscle fatigue differs between HIIE and moderate-intensity continuous exercise, traditional endurance training. Here we provide evidence that restoration of force production takes a longer time after HIIE, which is ascribable to long-lasting depressions in Ca2+ release of the sarcoplasmic reticulum.

Treatment with EUK-134 improves sarcoplasmic reticulum Ca2+ release but not myofibrillar Ca2+ sensitivity after fatiguing contraction of rat fast-twitch muscle.

Skeletal muscles undergoing vigorous activity can enter a state of prolonged low-frequency force depression (PLFFD). This study was conducted to examine whether antioxidant treatment is capable of accelerating the recovery from PLFFD, with a focus on the function of the sarcoplasmic reticulum (SR) and myofibril. One hour before fatiguing stimulation (FS) was administered, rats received an intraperitoneal injection of Eukarion (EUK-134), which mimics the activities of superoxide dismutase and catalase. Intact muscles of the hindlimbs were electrically stimulated via the sciatic nerve until the force was reduced to ~50% of the initial force (FS). Thirty minutes after cessation of FS, the superficial regions of gastrocnemius muscles were dissected and used for biochemical and skinned-fiber analyses. Whole muscle analyses revealed that antioxidant alleviated the FS-induced decrease in the reduced glutathione content. Skinned-fiber analyses showed that the antioxidant did not affect the FS-induced decrease in the ratio of force at 1 Hz to that at 50 Hz. However, the antioxidant partially inhibited the FS-mediated decrease in the ratio of depolarization-induced force to the maximum Ca2+-activated force. Furthermore, the antioxidant completely suppressed the FS-induced increase in myofibrillar Ca2+ sensitivity. These results suggest that antioxidant treatment is ineffective in facilitating the restoration of PLFFD, probably due to its negative effect on myofibrillar Ca2+ sensitivity, which supersedes its positive effect on SR Ca2+ release.

Ingestion of soy protein isolate attenuates eccentric contraction-induced force depression and muscle proteolysis via inhibition of calpain-1 activation in rat fast-twitch skeletal muscle.

OBJECTIVE: Eccentric contraction (ECC) is a contraction in which skeletal muscles are stretched while contracting. The aim of this study was to determine how ingestion of soy protein isolate (SPI) or animal-based proteins affect force deficit, calpain activation, and proteolysis of calcium ion (Ca2+)-regulatory proteins in rat fast-twitch muscles subjected to ECC. METHODS: In the first experiment, male Wistar rats were randomly assigned to a control and an SPI group, which were fed a 20% casein and a 20% SPI diet, respectively, for 28 d before the ECC protocol. Anterior crural muscles underwent 200 repeated ECCs and were excised 3 d later. In the second experiment, half of the SPI rats were given water containing NG-nitro-l-arginine-methyl ester (L-NAME), an inhibitor of nitric oxide synthase, for 3 d of recovery after ECC. RESULTS: SPI ingestion attenuated ECC-induced force deficit, proteolysis of Ca2+-regulatory proteins, and autolysis of calpain-1. Co-ingestion of L-NAME inhibited SPI-associated increases in nitrite and nitrate levels and negated the force recovery effects of SPI. CONCLUSION: These results suggest that SPI ingestion inhibits ECC-elicited force deficit and proteolysis of Ca2+ regulatory proteins, which is caused by inhibited activation of calpain-1 via increased nitric oxide production.

Thermal pretreatment facilitates recovery from prolonged low-frequency force depression in rat fast-twitch muscle.

The aim of this study was to examine whether thermal pretreatment can accelerate recovery from prolonged low-frequency force depression. The hindlimbs of thermal treated (T-treated) rats were immersed in water heated to 42.0°C for 20 min (thermal pretreatment). The thermal pretreatment was performed once a day for 5 days before fatiguing stimulation. Intact gastrocnemius muscles were electrically stimulated via the sciatic nerve until force was reduced to ~50% of the initial and dissected immediately [recovery 0 (REC0)] or 60 min [recovery 60 (REC60)] following the cessation of stimulation. Using skinned fiber prepared from the superficial region, the ratio of force at 1 Hz to that at 50 Hz (low-to-high force ratio), the ratio of depolarization (depol)-induced force to maximum Ca2+ -activated force (depol/max Ca2+ force ratio), the steepness of force-Ca2+ concentration curves, and myofibrillar Ca2+ sensitivity were measured. At REC0, the low-to-high force ratio and depol/max Ca2+ force ratio decreased in stimulated muscles from both non- and thermal-treated rats. At REC60, these two parameters remained depressed in non-treated rats, whereas they reverted to resting levels in T-treated rats. Thermal pretreatment exerted no effect on myofibrillar Ca2+ sensitivity. The present results reveal that thermal pretreatment can facilitate recovery of submaximum force after vigorous contraction, which is mediated via a quick return of Ca2+ release from the sarcoplasmic reticulum to resting levels.

l-arginine ingestion inhibits eccentric contraction-induced proteolysis and force deficit via S-nitrosylation of calpain.

It has been shown that calpains are involved in the proteolysis of muscle proteins that occurs with eccentric contraction (ECC) and that exogenously applied nitric oxide decreases the calpain-mediated proteolysis. The aim of this study was to examine the effects of ingestion of l-arginine (ARG), a nitric oxide precursor, on ECC-related calpain activation. In the first and second experiments, male Wistar rats were given ARG in water for 7 days starting from 3 days before the ECC protocol (average ingestion, ~600 mg kg-body wt-1  day-1 ). Tibialis anterior muscles underwent 200 repeated ECCs and, subsequently, were excised 3 days later. Whole muscle analyses (the first experiment) revealed that ARG attenuated ECC-induced force deficit and autolysis of calpain-1, and increased the amounts of S-nitrosylated calpain-1. Regarding ryanodine receptor (RyR) and dihydropyridine receptor (DHPR), ECC-induced proteolysis was completely inhibited by ARG, whereas the inhibition was partial for junctophilin-1 (JP1). Skinned fiber analyses (the second experiment) showed that ARG also inhibited ECC-elicited reductions in the ratio of depolarization-induced to maximum Ca2+ -activated force. In the third experiment, homogenates of rested muscles were treated with S-nitrosylating agent, S-nitrosoglutathione (GSNO), and/or high Ca2+ concentration ([Ca2+ ]). Treatment with high [Ca2+ ] and without GSNO produced proteolysis of RyR, DHPR, and JP1. On the other hand, treatment with high [Ca2+ ] and GSNO caused complete inhibition of RyR and DHPR proteolysis and partial inhibition of JP1 proteolysis. These results indicate that ARG ingestion can attenuate ECC-induced proteolysis of Ca2+ regulatory proteins and force deficit by decreasing calpain activation via S-nitrosylation.