A mitochondrial‐targeted antioxidant improves myofilament Ca²⁺ sensitivity during prolonged low frequency force depression at low

Abstract

Skeletal muscle can develop a prolonged low frequency-stimulation force depression (PLFFD) following fatigue-inducing contractions. Increased levels of reactive oxygen species (ROS) have been implicated in the development of PLFFD. During exercise the skeletal muscle intracellular PO2 decreases to relatively low levels, and can be further decreased when there is an impairment in O diffusion or availability, such as in certain chronic diseases and during exercise at high altitude. Since ROS generation by mitochondria is elevated at very low PO2 in cells, we tested the hypothesis that treatment of muscle fibres with a mitochondrial-targeted antioxidant at a very low, near hypoxic, PO2 can attenuate PLFFD. We treated intact single fibres from mice with the mitochondrial-specific antioxidant SS31, and measured force development and intracellular [Ca ] 30 min after fatigue at an extracellular PO2 of ∼5 Torr. After 30 min following the end of the fatiguing contractions, fibres treated with SS31 showed significantly less impairment in force development compared to untreated fibres at submaximal frequencies of stimulation. The cytosolic peak [Ca ] transients (peak [Ca ] ) were equally decreased in both groups compared to pre-fatigue values. The combined force and peak [Ca ] data demonstrated that myofibrillar Ca sensitivity was diminished in the untreated fibres 30 min after fatigue compared to pre-fatigue values, but Ca sensitivity was unaltered in the SS31 treated fibres. These results demonstrate that at a very low PO2, treatment of skeletal muscle fibres with a mitochondrial antioxidant prevents a decrease in the myofibrillar Ca sensitivity, which alleviates the fatigue induced PLFFD.

Incubation with sodium nitrite attenuates fatigue development in intact single mouse fibres at physiological

“…); R min (0.24 ± 0.04) and R max (5.03 ± 1.88) are the fluorescence ratios at no cytosolic Ca 2+ and high Ca 2+ , respectively, and were determined using an internal in vivo calibration described by Gandra et al . ().…”

“…To determine myofilament Ca 2+ sensitivity, force development data at each peak [Ca 2+ ] c during the force‐frequency (FF) curves were fitted with a sigmoidal equation (Gandra et al . ) described in eqn (2): …”

“…Isolated single mouse fibres were electrically stimulated using a Grass S88X stimulator (Quincy, MA, USA) and signal was acquired and analysed as described previously (Gandra et al 2018). Force development (in mN) was normalized to the cross-sectional area (in mm 2 ) determined from the diameter of the fibre (32.8 ± 6.6 µm diameter; n = 22 fibres).…”

Cigarette smoke directly impairs skeletal muscle function through capillary regression and altered myofibre calcium kinetics in mice

“…The fibre was superfused with Tyrode solution (in mM: 121 NaCl, 5 KCl, 1.8 CaCl 2 , 0.5 MgCl 2 , 0.4 NaH 2 PO 4 , 24 NaHCO 3 , 5.5 glucose, 0.1 K 2 EGTA) that was constantly bubbled with 2% O 2 , 5% CO 2 (for solution pH 7.4), balance N 2 for the entire experimental procedure. The solution was measured with a fibre optic oxygen sensor (Oxymicro, World Precision Instruments, Sarasota, FL, USA) and maintained ∼15 Torr, which is not limiting to mitochondrial respiration during maximal work in mouse fibres (Gandra et al . 2018), as also determined in preliminary results from our laboratory in intact single mouse fibres.…”

“…R max was determined for each of the contracting fibres and was set to 132% of the peak ratio obtained when fibres were contracted at 120 Hz in the presence of 10 mM caffeine (R max was 3.54 ± 0.24; n = 12 fibres, mean ± SEM) (Westerblad & Allen, 1991). β was also determined for each of the contracting fibres (5.5 ± 0.6; n = 12 fibres, mean ± SEM) as described previously (Gandra et al 2018). Calculations of peak and basal [Ca 2+ ] c were previously described in Nogueira et al (2013).…”