Chronology of Early Heat Stress Mediated Changes in Oxidative Skeletal Muscle

Abstract

We have previously reported that 24 hours of heat stress (HS) induced free radical injury in oxidative skeletal muscle. In addition, we found widespread aberrant intracellular signaling following 12h of HS. The aim of this investigation was to extend our knowledge regarding the early chronology of HS‐mediated intracellular signalling in oxidative skeletal muscle. We hypothesized that 2, 4, and 6 hours of HS would result in increased autophagy and decreased mitochondrial content in oxidative skeletal muscle compared to muscle maintained under thermoneutral (TN) conditions. To address this hypothesis, pigs were assigned to four groups (n=8/group) and housed under TN conditions (21°C) or exposed to HS (37°C) for 2, 4, or 6 hours. Next, animals were euthanized and the semitendinosus red (STR) was collected and analyzed. Compared to TN, relative protein abundance of autophagy initiators phosphoinositide 3‐kinase Class III and Beclin‐1 were increased by 57% and 56%, respectively (p<0.05), following 6 hours of HS. At the same time point, relative abundance of the pre‐autophagosomal structure stabilizer, autophagy‐related protein 16‐1 (ATG16), was increased 59% (p<0.05). Microtubule‐associated protein A/B light chain 3 (LC3)‐I was similar between all four groups but the autophagosome formation marker LC3‐II was increased by 94% (p<0.05) following 6 hours of HS compared to TN. This resulted in a 2‐fold increase in the LC3II/I ratio (p<0.05), which is indicative of increased autophagic flux, though sequstome‐1 (p62) protein abundance was similar between groups. In addition, the mitophagy marker, BCL2/adenovirus E1B protein‐interacting protein 3‐like (BNIP3), was increased by 41% (p<0.05) following 6 hours of HS compared to TN. Relative protein abundance of TCA enzymes, electron transport chain components, and mitochondrial markers was similar between groups. These data demonstrate that short‐term HS induced autophagy/mitophagy. Together with our previous work we also suggest thatbetween 6 and 12 hours of HS the intracellular environment of oxidative skeletal muscle switches from one that promotes autophagy to one that inhibits it.Support or Funding InformationThis work supported by USDA grants 2014‐67015‐21627 and 2011‐67003‐30007.