Reactive oxygen/nitrogen species and contractile function in skeletal muscle during fatigue and recovery

Abstract

The production of reactive oxygen/nitrogen species (ROS/RNS) is generally considered to increase during physical exercise. Nevertheless, direct measurements of ROS/RNS often show modest increases in ROS/RNS in muscle fibres even during intensive fatiguing stimulation, and the major source(s) of ROS/RNS during exercise is still being debated. In rested muscle fibres, mild and acute exposure to exogenous ROS/RNS generally increases myofibrillar submaximal force, whereas stronger or prolonged exposure has the opposite effect. Endogenous production of ROS/RNS seems to preferentially decrease submaximal force and positive effects of antioxidants are mainly observed during fatigue induced by submaximal contractions. Fatigued muscle fibres frequently enter a prolonged state of reduced submaximal force, which is caused by a ROS/RNS-dependent decrease in sarcoplasmic reticulum Ca(2+) release and/or myofibrillar Ca(2+) sensitivity. Increased ROS/RNS production during exercise can also be beneficial and recent human and animal studies show that antioxidant supplementation can hamper the beneficial effects of endurance training. In conclusion, increased ROS/RNS production have both beneficial and detrimental effects on skeletal muscle function and the outcome depends on a combination of factors: the type of ROS/RNS; the magnitude, duration and location of ROS/RNS production; and the defence systems, including both endogenous and exogenous antioxidants.

Figure 1. Changes in myofibrillar Ca²⁺ sensitivity have a much larger effect on submaximal than on maximal force

Schematic representation of the effect of increased (blue lines) and decreased (red lines) myofibrillar Ca²⁺ sensitivity. (1) Stimulation at frequencies giving unfused tetani results in forces on the steep part of the force–[Ca²⁺]_i relationship and changes in sensitivity have a large effect (∼40% in the example) on force output. (2) Conversely, the same changes in sensitivity have little effect (here ∼10%) at higher stimulation frequencies and fused tetani. Similar changes occur with changes in tetanic [Ca²⁺]_i, i.e. larger effects in unfused contractions.

Figure 2. Transient and reversible effects of H₂O₂ on myofibrillar contractile function

Original [Ca²⁺]_i and force records from submaximal (50 Hz) contractions of single FDB muscle fibres exposed to H₂O₂ (300 μm) for up to 8 min (A) and H₂O₂ for 6 min followed by exposure to the reducing agent DTT (1 mm) for 10 min (B). Note that H₂O₂ causes major force changes whereas [Ca²⁺]_i is little affected, which means that H₂O₂ mainly acts at the myofibrillar level. The effects on force are time dependent with brief H₂O₂ exposure resulting in increased submaximal force, which is followed by a progressive force decline (A). Furthermore, the force depression caused by prolonged H₂O₂ exposure is reversed by reduction with DTT (B). Conversely, prolonged exposure to DTT results in depressed submaximal force that can be reversed by application of H₂O₂ (not shown). Figure adapted from Andrade et al. (1998).

Figure 3. The prolonged low‐frequency force depression (PLFFD) after fatiguing stimulation is the result of complex ROS/RNS effects on SR Ca²⁺ release and myofibrillar Ca²⁺ sensitivity

A, [Ca²⁺]_i (upper panel) and force (lower panel; mean data ± SEM) in 30 Hz contractions produced during PLFFD (red circles) initially in standard Tyrode solution, followed by addition of DTT (1 mm) or the non‐metabolizable analogue of H₂O₂, t‐BOOH (10 μm). The effect of the same exposures on unfatigued fibres are also shown (green circles). B, simplified model of ROS/RNS effects on SR Ca²⁺ release and myofibrillar Ca²⁺ sensitivity. Key proteins for SR Ca²⁺ release are the t‐tubular voltage sensors, the dihydropyridine receptors (blue boxes), and the SR Ca²⁺ release channels, RyR1 (green boxes). These proteins appear to be in an optimal redox state at rest and become overly oxidized during fatiguing stimulation resulting in decreased [Ca²⁺]_i, which is not affected by application of either t‐BOOH or DTT (see A). In the rested state, myofibrillar proteins are in a suboptimal reduced state. Some myofibrillar proteins become overly oxidized during induction of fatigue and the resulting force decrease is transiently counteracted by application of DTT. Intriguingly, other myofibrillar proteins apparently remain reduced during fatigue since application of the oxidizing agent t‐BOOH temporarily improves force generation, i.e. similar to the effect in the rested state (see A). Figure adapted from Cheng et al. (2015).