Comparison of long- and short-rest periods during high-intensity interval exercise on transcriptomic responses in equine skeletal muscle.

The purpose of this study was to elucidate the skeletal muscle transcriptomic response unique to rest duration during high-intensity interval exercise. Thoroughbred horses performed three 1-min bouts of exercise at their maximal oxygen uptake (10.7-12.5 m/s), separated by 15 min (long) or 2 min (short) walking at 1.7 m/s. Gluteus medius muscle was collected before and at 4 h after the exercise and used for RNA sequencing. We identified 1,756 and 1,421 differentially expressed genes in response to the long and short protocols, respectively, using DEseq2 analysis [false discovery rate (FDR) cutoff = 0.05, minimal fold change = 1.5]. The overall transcriptional response was partially aligned, with 43% (n = 949) of genes altered in both protocols, whereas no discordant directional changes were observed. K-means clustering and gene set enrichment analyses based on Gene Ontology biological process terms showed that genes associated with muscle adaptation and development were upregulated regardless of exercise conditions; genes related to immune and cytokine responses were more upregulated following the long protocol, and protein folding and temperature response were highly expressed after the short protocol. We found that 11 genes were upregulated to a greater extent by the short protocol and one was by the long protocol, with GNA13, SPART, PHAF1, and PTX3 identified as potential candidates for skeletal muscle remodeling. Our results suggest that altered metabolic fluctuations dependent on the intermittent pattern of interval exercise modulate skeletal muscle gene expression, and therefore, rest interval length could be an important consideration in optimizing skeletal muscle adaptation.NEW & NOTEWORTHY This is the first study to address the comparison of transcriptional responses to high-intensity interval exercise with two different rest periods in skeletal muscle. The expression of genes related to metabolic adaptations altered in both conditions, while genes associated with immune and cytokine responses and protein folding and temperature response were varied with the length of the rest period. These results provide evidence for rest duration-specific transcriptional response to high-intensity interval training.