Comparison of Shifts in Skeletal Muscle Plasticity Parameters in Horses in Three Different Muscles, in Answer to 8 Weeks of Harness Training

Abstract

Training-induced follow-up of multiple muscle plasticity parameters in postural stability vs. locomotion muscles provides an integrative physiological view on shifts in the muscular metabolic machinery. It can be expected that not all muscle plasticity parameters show the same expression time profile across muscles. This knowledge is important to underpin results of metabolomic studies. Twelve non-competing Standardbred mares were subjected to standardized harness training. Muscle biopsies were taken on a non-training day before and after 8 weeks. Shifts in muscle fiber type composition and muscle fiber cross-sectional area (CSA) were compared in the m. pectoralis, the m. vastus lateralis, and the m. semitendinosus. In the m. vastus lateralis, which showed most pronounced training-induced plasticity, two additional muscle plasticity parameters (capillarization and mitochondrial density) were assessed. In the m. semitendinosus, additionally the mean minimum Feret's diameter was assessed. There was a significant difference in baseline profiles. The m. semitendinosus contained less type I and more type IIX fibers compatible with the most pronounced anaerobic profile. Though no baseline fiber type-specific and overall mean CSA differences could be detected, there was a clear post-training decrease in fiber type specific CSA, most pronounced for the m. vastus lateralis, and this was accompanied by a clear increase in capillary supply. No shifts in mitochondrial density were detected. The m. semitendinosus showed a decrease in fiber type specific CSA of type IIAX fibers and a decrease of type I fiber Feret's diameter as well as mean minimum Feret's diameter. The training-induced increased capillary supply in conjunction with a significant decrease in muscle fiber CSA suggests that the muscular machinery models itself toward an optimal smaller individual muscle fiber structure to receive and process fuels that can be swiftly delivered by the circulatory system. These results are interesting in view of the recently identified important fuel candidates such as branched-chain amino acids, aromatic amino acids, and gut microbiome-related xenobiotics, which need a rapid gut–muscle gateway to reach these fibers and are less challenging for the mitochondrial system. More research is needed with that respect. Results also show important differences between muscle groups with respect to baseline and training-specific modulation.

Baselining physiological parameters in three muscles across three equine breeds. What can we learn from the horse?

“…As mentioned previously, also parameters such as fCSA and mfCSA are of importance for metabolic and performance capacity typology ( Dechick et al, 2020 ; de Meeûs d’Argenteuil et al, 2021b ). In horses, breed differences have been reported with that respect.…”

“…Other energy cycles furnish themselves mainly via fuels that are being supplied on demand. The latter option obviously requires thorough adaptation of capillarization and reshaping towards a smaller muscle fiber size in order to allow for swift diffusion of candidate fuels ( Giacomello et al, 2020 ; Betz et al, 2021 ; de Meeûs d’Argenteuil et al, 2021b ).…”

“…Several different morphophysiological features, that are specific to the muscular compartment, and a longitudinal follow-up of the evolution of those features can be used to obtain a view on not only basal metabolic activity, but also adaptation of the muscular machinery in response to certain challenges, such as training. Some of these features can be visualized histochemically, such as muscle fiber type composition, the fiber specific and mean cross sectional area (fCSA and mfCSA), and the degree of capillarization and mitochondrial density present within a certain muscle ( Moro et al, 2019 ; de Meeûs d’Argenteuil et al, 2021b ; van Boom et al, 2022 ). Other factors can be monitored biochemically, for instance the activity of certain enzyme systems that catalyze different energy cycles ( Warren et al, 2014 ; Srisawat et al, 2017 ; Moro et al, 2019 ).…”

“…When an individual is subjected to a certain training load, muscle plasticity will occur, leading to shifts in one or more of the above-mentioned parameters ( Tyler et al, 1998 ; Larson et al, 2019 ; Moro et al, 2019 ; de Meeûs d’Argenteuil et al, 2021a ; de Meeûs d’Argenteuil et al, 2021b ). From a comparative viewpoint, it is key to keep in mind that important baseline differences exist between these parameters when comparing animal species and within the same animal species, when comparing breeds ( Snow and Guy, 1980 ; Kohn et al, 2011 ; Schiaffino and Reggiani, 2011 ; Rivero and Hill, 2016 ).…”

“…We have previously shown that shifts in metabolic fingerprint and shifts in fCSA and capillarization precede shifts in fiber type composition and mitochondrial density in answer to training, meaning that metabolic fingerprint, muscle fCSA and muscle fiber capillarization evolution need to be viewed as early biomarkers for muscular metabolic adaptation ( de Meeûs d’Argenteuil et al, 2021b ). We have also shown that, in horses, in answer to aerobic training, the muscular machinery models itself towards an optimal smaller individual muscle fCSA to receive and process fuels that can be swiftly delivered by the circulatory system ( de Meeûs d’Argenteuil et al, 2021b ). With that respect, gut microbiome-derived metabolites showed important upregulation after training in Standardbred horses.…”