Abstract Feedlot cattle are exceptionally sensitive to heat stress (HS) with alterations to underlying molecular responses of skeletal muscle attributed to stress response, metabolism, growth, and prolonged adaptability. This study sought to elucidate the molecular response within Longissimus dorsi (LD) in response to acute and chronic HS in finishing steers by measuring exome-wide transcript abundance. Thirty-two crossbred steers (initial BW 364 ± 5 kg) were randomly allotted to control (CON, n = 16) or heat stress (HS, n = 16) treatments. CON steers were maintained at temperature humidity index (THI) 60, while HS steers were subjected to THI of 70-80 for 0900-1700h then returned to control conditions daily for 28-days. At d-1 and d-28, LD samples were collected from a subset of steers (n = 6/treatment) and RNA was extracted for mRNA sequencing performed in 2x150bp format on a NovaSeq6000. Resulting reads were aligned to the Bos taurus reference genome (ARS-UCD2.0), and gene counts obtained using HTSeq. Differentially expressed genes (DEGs) were defined as genes with a Benjamini-Hochberg adjusted p-value 0.10 regardless of log2 Fold Change (log2FC). DEGs underwent gene set enrichment analysis using the g:GOSt function of g:Profiler. Acute (d-1) HS steers exhibited 401 up-regulated and 391 down-regulated DEGs enriched for skeletal muscle growth, metabolism, and structure, satellite cell differentiation, stress response, and repair pathways. Acute HS increased expression of MYH7 (log2FC = 0.63, BH-adj. P = 0.028), attributed to oxidative muscle fiber formation, and IGF1R (log2FC = 0.48, BH-adj. P = 0.015), mTOR (log2FC = 0.27, BH-adj. P = 0.040), and GDFR11 (log2FC = 0.74, BH-adj. P = 0.088) attributed to skeletal muscle growth and regulation. Chronic (d-28) HS altered expression of 82 genes not detected following acute HS, including 53 up-regulated and 29 down-regulated DEGs attributed to mounting stress response, glycolytic muscle fiber formation and metabolism, skeletal muscle wasting and atrophy, and immune response. Chronic HS led to elevated HSP90AA1 (log2 fold change = 0.45, BH-adj. P = 0.081) and MYH1 (log2 fold change = 0.77, BH-adj. P = 0.019) highlighting a sustained heat stress response and a shift in glycolytic fiber formation. Coupled with decreased expression of FOXO1 (log2 fold change = -1.79, BH-adj. P = 0.005) signifying a lessening in muscle wasting and atrophy. These results suggest acute HS trigger rapid changes associated with protection, repair, and growth while chronic HS results in metabolic shift, sustained protein folding chaperone expression, and cellular adaptation.
Eckhardt et al. (Wed,) studied this question.