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.