Effect of Branched-Chain Amino Acid and N-acetylcysteine Supplementation Post-Exercise on Muscle mTOR Signaling in Exercising Horses

Recovery from intense exercise is crucial in the equine athlete and numerous nutritional interventions have been proposed to benefit exercise recovery. Branched-chain amino acids (leucine, isoleucine, valine; BCAA) have been associated with improved muscle protein synthesis post-exercise. N-acetylcysteine (NAC) is a precursor to glutathione, a powerful antioxidant involved in many biological pathways, and supplementation has been reported to improve exercise performance and oxidative stress. However, there is some evidence that NAC may disrupt the mTOR signaling pathways associated with protein synthesis. The objective of this study was to determine if there is an undesirable effect on the mTOR pathway with NAC supplementation or if concurrent supplementation with BCAAs might counteract this effect. Eight conditioned Thoroughbred geldings (9.5 ± 2.7 years, 581 ± 65kg, BW) were fed a grain meal 30 m post-exercise supplemented with nothing (control), BCAA (20 g/d; 2:1:1 Leu:Ile:Val), NAC (10 g/d), or both for 21 d in a replicated Latin square design. On d 20 of supplementation, horses performed a standardized exercise test on a highspeed treadmill including a 1.5 mi gallop at a target heart rate of 180–190 bpm. Relative to exercise, blood samples were collected at –24 h, every 30 m for 1 h before and 4 h after exercise, and at 24 h. Plasma was subsequently analyzed for amino acid concentrations using HPLC. Muscle samples were collected from the gluteus medius by percutaneous muscle biopsy at –24, 1, 4, and 24 h relative to exercise for evaluation of major proteins in the mTOR pathway using Western immunoblot analysis. Data were analyzed using PROC MIXED with repeated measures (SAS v9.4) using treatment, time, and their interaction as fixed effects. Exercise increased plasma levels of alanine, leucine, and isoleucine (P < 0.001), but not valine (P > 0.05). BCAA alone and in combination with NAC elevated plasma BCAA concentrations post-feeding for at least 4 h post-exercise (P < 0.001). Regardless of treatment, exercise resulted in the activation of Akt and rpS6 (P < 0.001) at 1 h and 4 h post-exercise, respectively. Importantly, NAC did not adversely affect the activation of any mTOR signaling proteins relative to control (P > 0.05). The greatest effect on mTOR signaling was due to exercise. At the dosage supplemented, NAC did not have detrimental effects on the activation of the mTOR pathway. These results provide novel preliminary data on the safety of NAC supplementation in exercising horses in regards to the mTOR pathway and protein synthesis.

Hauss, A., C. Loos, A. Gerritsen, K. Urschel, and J. Pagan. 2021. Effect of branched-chain amino acid and N-acetylcysteine supplementation post-exercise on muscle mTOR signaling in exercising horses. Journal of Equine Veterinary Science 100:103524.