Effects of Graded Levels of Protein Intake on mTOR Signaling and Muscle Protein Fractional Synthesis Rates in Horses

Muscle mass is determined by the balance between rates of muscle protein synthesis and degradation, with muscle protein synthesis being more responsive to anabolic stimuli such as feeding. The amount of protein consumed in a single concentrate meal may be highly variable, depending on the feed being used, and it is presently unknown how much protein is needed to maximize muscle protein synthesis. Dietary protein not only provides the amino acids to be used in protein synthesis, but amino acids have also activate the mammalian target of rapamycin (mTOR) pathway, the main signaling pathway that regulates muscle protein synthesis. We have previously measured the activation of proteins in the mTOR signaling pathway; however, the activation of mTOR signaling only measures the potential for protein synthesis to occur and not actual rates of muscle protein synthesis. The infusion of a stable isotope, such as [ring-2H5]phenylalanine, can be used to directly measure the fractional synthesis rate (FSR), but muscle protein FSR has not been previously measured in horses. The goal of this research is to study the activation of mTOR signaling and muscle FSR rates in mature horses following the consumption of a meal containing different amounts of crude protein. Eight mature (~8 – 16 years old) horses will be studied under each of 4 conditions, administered in a randomly determined order: fasting (0 g protein/kg BW), low protein intake (0.15 g protein/kg BW), moderate protein intake (0.30 g protein/kg BW) and high protein intake (0.60 g protein/kg BW). Horses will be studied using a 4 hour primed, constant infusion of [ring-2H5]phenylalanine. Following 2 hours of isotope equilibration, horses will be fed their treatment feed. Gluteus medius muscle samples will be collected at 60 and 120 min post-feeding to measure the activation of mTOR signaling factors and the isotopic enrichment of the muscle protein, which will be used to calculate muscle FSR. An improved understanding of the regulation of muscle protein synthesis could allow for the development of targeted nutritional strategies to ensure the development and maintenance of muscle mass across the lifespan.