Researchers have taken preliminary steps toward unravelling the roles of two important regulatory proteins produced by muscle cells during exercise in horses.
The Polish research focused on 20 purebred Arabian horses, half of whom were in racehorse training and the remainder being conditioned for Endurance contests.
Sylwester Kowalik and his colleagues from the University of Life Sciences said skeletal muscle is considered the largest endocrine organ determining the maintenance of energy balance.
Adaptive changes in skeletal muscles in response to exercise affect the production, as well as the secretion, of proteins known as myokines, which play a crucial role in energy expenditure.
Kowalik and his colleagues carried out a preliminary study to investigate the impact of two different types of exercise on the circulating level of two myokines, myostatin and irisin, in horses.
Myostatin is a powerful regulator of skeletal muscle mass in mammals.
“Although the origin and metabolism of myostatin are more or less defined, its function appears to be dependent on a network of interactions with other endocrine proteins,” they said. “Among these, irisin has recently been indicated as a factor that may play an essential role in this network.”
However, the precise actions of myokines in equine exercise physiology were still not fully understood, they said.
The horses selected for their study — 10 three-year-old racehorses and 10 Endurance horses — were exposed to quite different training regimes, ultimately competing in Polish national competitions.
The racehorses underwent training sessions that typically culminated in a gallop over 1200m. The Endurance horses, trained over much longer distances, were sampled before and after the completion of 60km in 80km competition rides.
To assess the effect of training sessions on plasma myostatin and irisin concentrations, blood samples were taken at rest and 30 minutes after the end of exercise sessions during their training seasons.
The study team found that single bouts of exercise did not influence plasma irisin levels, regardless of the type of effort, but it did induce an increase in plasma myostatin concentration.
Based on work in humans, it is possible that the measurement of plasma irisin concentrations at least 30 minutes after the end of exercise was performed too early to reveal any changes in its level, they said.
Plasma irisin levels tended to decrease during the race season in racehorses.
“Plasma myostatin was higher in endurance horses than in three-year-old racehorses,” the study team reported in the open-access journal Animals, noting that age may have been a factor in this difference, given that the Endurance horses were older.
The lack of exercise-induced fluctuation in circulating irisin in the studied horses suggests that myostatin released in response to exercise provides a negative feedback signal to irisin release, they said.
Discussing their findings, the researchers said it was worth mentioning one more important function of irisin and myostatin.
“They both play a significant role in bone metabolism as factors influencing bone microstructure and mechanical properties.
“Irisin plays a crucial role in enhancing bone remodelling and increasing cortical bone mass in experimental animals, which ultimately increases bone strength.
“In contrast, myostatin exerts negative effects on bone mass through osteoclast activation, which leads to increased bone remodeling, where bone resorption outpaces bone formation.
“From the exercise physiology point of view, bone and muscle form one functional system and are tightly connected to each other.
“Thus, understanding of these interactions seems to be particularly important in sport horses due to the high training loads to which they are subjected daily.”
Further studies with higher numbers of animals trained on different distances and different ages are needed, they say.
The authors acknowledged there might be some relevant factors that also influenced the concentration of irisin and myostatin in horses, for example, the type of work undertaken by the horses and their routines.
“Therefore, this preliminary study should be considered with caution, and further research is needed to explain the regulation of irisin and myostatin release in exercised horses.”
The study team comprised Sembratowicz, Grzegorz Zięba Ewelina Cholewinska and Anna Czech.
Kowalik, S.; Wiśniewska, A.; Kędzierski, W.; Janczarek, I. Concentrations of Circulating Irisin and Myostatin in Race and Endurace Purebred Arabian Horses—Preliminary Study. Animals 2020, 10, 2268.