Transcriptomic responses of equine skeletal muscle to acute exercise in a hot environment.

Objective Overview

• This study investigated how acute high-intensity exercise affects gene expression in horse skeletal muscle under hot versus cool environmental conditions.
• It aimed to understand how heat stress influences molecular adaptations in equine muscles by examining differences in transcriptional responses after exercise.

Introduction and Background

• Exercise performance typically declines in hot environments due to heat stress, which can negatively affect the body.
• However, heat stress might also trigger physiological and molecular adaptations, particularly in skeletal muscle, that help the organism cope better with heat in the future.
• Understanding these adaptations at the gene expression level is important for optimizing exercise and training regimens, especially for athletic horses that often perform in varied climates.

Methodology

• Subjects: Equine skeletal muscle samples were collected from horses.
• Exercise protocol: Horses performed 3 minutes of high-intensity exercise, specifically at their maximal oxygen uptake speed (VO2 max), to ensure a consistent workload across individuals.
• Environmental conditions: Two different temperature settings were used:
  • Cool: Wet Bulb Globe Temperature (WBGT) of 15°C
  • Hot: WBGT of 30°C
• Sample collection: Muscle biopsies from the middle gluteal muscle were taken 4 hours post-exercise to capture transcriptional changes.
• Gene expression analysis: RNA sequencing (RNA-seq) was used to profile gene expression changes.
  • Differential gene expression was determined using DESeq2 statistical software.
  • Criteria for significance included a false discovery rate (FDR) cutoff of 0.05 and a minimum fold change of 1.5 in gene expression.

Key Findings

• Number of genes differentially expressed after exercise:
  • Hot condition: 176 genes
  • Cool condition: 156 genes
• Overlap and uniqueness:
  • 110 genes changed in both hot and cool conditions, indicating a core response to exercise regardless of temperature.
  • 66 genes responded only in the hot condition, suggesting that heat stress induces additional specific molecular changes.
• Only one gene showed higher expression in hot versus cool environments, indicating that most gene expression changes were related to exercise itself rather than direct temperature effects.
• Pathway analysis revealed:
  • Upregulation of genes involved in the “response to temperature stimulus” pathway exclusively in the hot condition, suggesting heat-specific signaling and adaptation mechanisms.
  • Other pathways linked to general exercise-induced muscle adaptation were similarly regulated in both conditions.

Physiological Implications

• Exercise in hot environments leads to increased pulmonary artery temperature in horses, which could contribute to heat stress at the muscle level.
• The distinct transcriptional activation of temperature response pathways after exercise in the heat may help muscles adapt to cope with thermal stress through mechanisms such as enhanced protein folding, stress response, or cellular protection.
• The findings provide molecular-level insights that could assist in developing training or conditioning programs aimed at improving heat acclimation and performance in horses.

Conclusions

• While exercise induces broadly similar gene expression changes in equine skeletal muscle across environments, heat stress uniquely activates temperature-related genes and pathways.
• The study advances the understanding of how heat exposure during exercise modifies muscle at the molecular level, contributing to adaptations that could protect against thermal damage or improve muscle function under heat stress.
• These insights have potential applications for equine sports science, veterinary care, and management practices in hot climates.