Regulation of satellite cells by extrinsic factors during recovery from exercise in horses
The vast majority of horses engage in some form of exercise, whether it be for leisure or competition. Despite almost half of the approximately 7.2 million horses engaging in structured athletic work, very little is known about one of the most critical facets of recovery: satellite cells (SCs). Satellite cells lie adjacent to the myofiber of skeletal muscle, poised to enter the myogenic program and fuse to the nearby muscle after a damaging event. Hepatocyte growth factor (HGF) and insulin-like growth factor-1 (IGF-1) transcript abundance increased after an exhaustive bout of endurance exercise in concert with myogenic regulator factors and preceding increased SC abundance in a previous study. This suggests that SCs may participate in repair of exercise-induced muscle damage. To assess the role of HGF in this process, equine SCs (eqSCs) were isolated from the gluteus medius of mature thoroughbred geldings for activation, proliferation and differentiation assays. Activation was not accelerated by 1, 5, 10, or 25 ng/mL HGF. Instead, 25 ng/mL HGF increased the proliferation rate of eqSC via protein kinase C δ and decreased differentiation. The influence of dietary L-citrulline, an amino acid that has the potential to influence SC activity and nutrient availability by its metabolism to L-arginine, was assessed during recovery from exercise in unfit adult horses. To model submaximal exercise, horses were exercised for 1 h at an average heart rate of 116 bpm, suggested to be typical of a heavy exercise session by the National Research Council. L-citrulline decreased myogenin mRNA abundance compared to controls while exercise increased peroxisome proliferator-activated receptor gamma coactivator 1- α (PGC1α) mRNA abundance, a master regulator of energy metabolism, at 1 d post-exercise. Although SCs were not activated in response to a single bout of submaximal exercise, metabolic regulators increased in the early period of recovery. Through these studies eqSC dynamics during exercise are better defined.