Abstract
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.
Highlights
Vigorous muscle activity eventually leads to fatigue development with decreased force production and slower contractions
At recovery 60 (REC60), the ratio remained depressed in non-treated rats, whereas it reverted to the resting level in T-treated rats (Fig. 1C)
If not all (Larkins et al 2012), studies have revealed that heat stress and/or vigorous muscle contraction result in the upregulation of HPS70 in fast- and slow-twitch muscles from mammals (Oishi et al 2002; Tupling et al 2007; Tamura et al 2015; Hafen et al 2018)
Summary
Vigorous muscle activity eventually leads to fatigue development with decreased force production and slower contractions. A specific type of delayed force recovery was originally observed by Edwards et al (1977) in human adductor pollicis muscles. In this type of muscle fatigue, tetanic force is more depressed at low than at high stimulation frequencies, which is called “prolonged lowfrequency force depression (PLFFD)” (Bruton et al 2008; Watanabe and Wada 2016). It is proposed that decreased myofibrillar Ca2+ (my-Ca2+) sensitivity and/or reduced Ca2+ release from the sarcoplasmic reticulum (SR) can in theory mediate depressions in force at low frequencies with little influence at high frequencies (Wada et al.2013). Treatment of muscle fibers with S-nitrosylation was reported to bring about decreased my-Ca2+ sensitivity (Dutka et al 2017)
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