Thermal tolerance of contractile function in oxidative skeletal muscle: no protection by antioxidants and reduced tolerance with eicosanoid enzyme inhibition

Abstract

Mechanisms for the loss of muscle contractile function in hyperthermia are poorly understood. This study identified the critical temperature, resulting in a loss of contractile function in isolated diaphragm (thermal tolerance), and then tested the hypotheses 1) that increased reactive oxygen species (ROS) production contributes to the loss of contractile function at this temperature, and 2) eicosanoid metabolism plays an important role in preservation of contractile function in hyperthermia. Contractile function and passive force were measured in rat diaphragm bundles during and after 30 min of exposure to 40, 41, 42 or 43 degrees C. Between 40 and 42 degrees C, there were no effects of hyperthermia, but at 43 degrees C, a significant loss of active force and an increase in passive force were observed. Inhibition of ROS with the antioxidants, Tiron or Trolox, did not inhibit the loss of contractile force at 43 degrees C. Furthermore, treatment with dithiothreitol, a thiol (-SH) reducing agent, did not reverse the effects of hyperthermia. A variety of global lipoxygenase (LOX) inhibitors further depressed force during 43 degrees C and caused a significant loss of thermal tolerance at 42 degrees C. Cyclooxygenase (COX) inhibitors also caused a loss of thermal tolerance at 42 degrees C. Blockage of phospholipase with phospholipase A(2) inhibitors, bromoenol lactone or arachidonyltrifluoromethyl ketone failed to significantly prevent the loss of force at 43 degrees C. Overall, these data suggest that ROS do not play an apparent role in the loss of contractile function during severe hyperthermia in diaphragm. However, functional LOX and COX enzyme activities appear to be necessary for maintaining normal force production in hyperthermia.

High temperature does not alter fatigability in intact mouse skeletal muscle fibres

“…In fast-twitch extensor digitorum longus fibres the increased ROS production at high temperature was accompanied by a reduction in tetanic force, which was counteracted by application of antioxidant or reducing agents (van der Poel & Stephenson, 2002;Edwards et al 2007). Conversely, a force depression at high temperature, which was not prevented by antioxidants or reversed by reducing agents, has been described in slow-twitch peroneus longus fibres and strips of diaphragm muscles (van der Poel & Stephenson, 2002;Oliver et al 2008). In the present study, we observed no decrease in tetanic force in soleus fibres when the temperature was increased from 37 to 43 • C. Furthermore, tetanic force was not decreased in FDB when the temperature was increased from 31 to 37 • C. Thus, intact soleus and FDB fibres can tolerate a temperature ∼6 • C above the in situ temperature without displaying any decrease in tetanic force production.…”

Hyperthermia increases interleukin-6 in mouse skeletal muscle

“…Group 1 muscles were subjected to twitches at 1/min throughout the remaining 2 h protocol. This low level stimulation has previously been shown not to fatigue isolated muscles and was used to continually monitor contractile function during the heat stimulus, as described in previous studies from our laboratory (51,72). Following completion of the protocol the muscles were quickly blotted on tissue paper, weighed, flash-frozen in liquid nitrogen, and stored at Ϫ80°C for subsequent analysis.…”

Maximizing Cellular Adaptation to Endurance Exercise in Skeletal Muscle

“…Within muscle, phospholipases (PLAs) catalyze the metabolism of membrane phospholipids of several organelles, producing eicosanoids that function in an autocrine manner to activate the GTPase, Rac, and production of superoxide (O 2 .À ). This heat-induced production of ROS within myocytes appears to be functionally important in acute exercise responses via regulation of cell volume and force production (King et al, 2016;Oliver et al, 2008) and adaptively in stimulating upstream regulators of PGC1a (Tamura et al, 2014).…”