Heat acclimation increases mitochondrial respiration capacity of C2C12 myotubes and protects against LPS-mediated energy deficit

This work investigated the effect of a 6-day heat acclimation (HA) protocol on myotube metabolic responses at baseline and in response to a subsequent lipopolysaccharide (LPS) challenge. C2C12 myotubes were incubated for 2 h/day at 40 °C for 6 days (HA) or maintained at 37 °C (C). Following 24-h recovery, myotubes were challenged with 500 ng/ml LPS for 2 h, then collected for analysis of protein markers of mitochondrial biogenesis and macronutrient storage. Functional significance of these changes was confirmed with mitochondrial respiration and glycolytic measurements on a Seahorse XF-96 analyzer. HA stimulated mitochondrial biogenesis and increased indicators of mitochondrial content SIRT1 (+?62%); PGC-1α (+?57%); NRF-1 (+?40%); TFAM (+?141%); CS (+?25%); CytC (+?38%); all p?<?0.05]. Altered lipid biosynthesis enzymes p-ACCa:ACC (+?59%; p?=?0.04) and FAS (??86%; p?<?0.01)] suggest fatty acid generation may have been downregulated, whereas increased GLUT4 (+?69%; p?<?0.01) and LDH-B (+?366%; p?<?0.01) suggest aerobic glycolytic capacity may have been improved. Mitochondrial biogenesis signaling in HA myotubes was suppressed by 500 ng/ml LPS (PGC-1α, NRF-1, TFAM; all p?> 0.05) but increased LDH-B (+?30%; p?=?0.02) and CPT-1 (+?55%; p?<?0.01) suggesting improved catabolic function. Basal respiration was increased in HA myotubes (+?8%; p?<?0.01) and HA myotubes maintained elevated basal respiration during LPS challenge (+?8%; p?<?0.01). LPS reduced peak respiration in C myotubes (??6%; p?<?0.01) but did not impair peak respiration in HA myotubes (p?>?0.05). Oxidative reliance was elevated in HA over that in control (+?25%; p?<?0.01) and in HA?+?LPS over C?+?LPS (+?30%; p?<?0.01). In summary, HA stimulated mitochondrial biogenesis in C2C12 myotubes. HA myotubes exhibited (1) elevated basal/peak mitochondrial respiration capacities; (2) greater oxidative reliance; and (3) protection against LPS-mediated respiration impairment. Collectively, these data suggest HA may improve aerobic metabolism in skeletal muscle and protect against LPS-mediated energy deficit.