Targeting inflammation during chronic heat stress improved intramuscular heat shock protein 70, myoblast differentiation, and muscle growth in finishing lambs

Chronic heat stress induces systemic inflammation and impairs growth performance in feedlot livestock. We sought to determine if anti-inflammatory interventions could improve muscle growth and efficiency in heat-stressed finishing lambs. Commercial wethers (40.8 ± 0.7 kg) were fed under heat stress (40 °C) and administered 0.15 mg/kg injectable dexamethasone every 72 h (n = 11), 1200 mg oral fish oil twice daily (n = 11), or no intervention (n = 11). Pair-fed controls were housed under thermoneutral conditions (19 °C, n = 12). Lambs were necropsied after 30 d. Heat stress-induced inflammation was characterized by greater (P < 0.05) circulating (i.e., extracellular) heat shock protein 70 (HSP70) and TNFα that followed a modest, transient increase (P < 0.05) in circulating cortisol. Heat stress also reduced (P < 0.05) circulating eicosapentaenoic acid (EPA) and skeletal muscle (i.e., intracellular) HSP70, which are anti-inflammatory. Daily fish oil partially or fully resolved (P < 0.05) heat stress effects on TNFα, cortisol, EPA, and intramuscular HSP70, but not circulating HSP70. Dexamethasone partially or fully resolved (P < 0.05) heat stress effects on cortisol, EPA, and intramuscular HSP70 but did not resolve TNFα and further worsened (P < 0.05) circulating HSP70. Heat stress-induced insulin resistance was indicated by greater (P < 0.05) circulating insulin and IGF-1 that was resolved (P < 0.05) by fish oil but not dexamethasone. Final bodyweights, average daily gain, and gain-to-feed ratios indicative of growth efficiency were reduced (P < 0.05) by heat stress but recovered (P < 0.05) by fish oil and dexamethasone. Live-animal estimates of ribeye area and fat-free muscle mass were reduced (P < 0.05) by heat stress, as was actual mass for 4 of 6 individual muscles weighed at necropsy. Poor muscle growth in heat-stressed lambs was facilitated in part by reductions (P < 0.05) in total (i.e., pax7+) and tended reductions (P < 0.10) in differentiated (i.e., myogenin+) myoblast populations. Differentiated (but not total) myoblasts tended to be recovered (P < 0.10) by fish oil and dexamethasone interventions, resulting in full or partial recovery of several muscle growth indicators. These findings show that induction of systemic inflammation contributes substantially to impaired muscle growth and efficiency in heat-stressed feedlot lambs. As such, inflammation is a feasible mechanistic target for improving outcomes of heat stress with anti-inflammatory bioactive nutrients.