Effect of feeding a high-protein diet following an 18-hour period of feed withholding on mammalian target of rapamycin-dependent signaling in skeletal muscle of mature horses

Abstract

In mature horses, feeding resulted in increased mammalian target of rapamycin signaling and the mechanism appeared to be independent of an increase in Akt phosphorylation at Ser⁴⁷³. Results indicate that adult horses may be able to increase rates of muscle protein synthesis in response to feeding and that dietary amino acids appear to be the main mediators of this effect.

Effect of gluteus medius muscle sample collection depth on postprandial mammalian target of rapamycin signaling in mature Thoroughbred mares

“…Generally, studies 20,36–38 in which a beneficial effect on muscle mTOR signaling activation was identified for dietary supplementation with leucine or branched chain amino acids involved a rate of supplementation that resulted in at least a doubling of plasma leucine concentration. Although plasma amino acid concentrations were not measured in our study, previous studies 4,5 involving similar horse feeding protocols showed that plasma branched chain amino acid concentrations were only 24% to 45% higher than preprandial concentrations 60 to 80 minutes after feeding. The muscle branched chain amino acid concentrations at the 8-cm biopsy depth were significantly greater than those at the other depths in the present study, but the 25% to 33% increase in their concentrations did not appear large enough to elicit an increase in the mTOR signaling response.…”

“…The amino acid composition of the high-protein pellet has been reported elsewhere. 5 A feeding protocol similar to the one used in the present study resulted in a 4-fold increase in plasma insulin concentration, an approximately 25% to 125% increase in plasma concentrations of indispensable (essential) amino acids, and an increase in the degree of upstream and downstream mTOR signaling in the gluteus medius muscle in the postprandial versus postabsorptive state. 4,5 …”

“…4,5 However, to the authors’ knowledge, ours is the first study to examine the effects of gluteus medius muscle biopsy depth on the degree of mTOR signaling activation in fed horses.…”

“…Although little research has been conducted regarding the regulation of mTOR signaling in equine skeletal muscle, research performed by our group has shown that mTOR signaling in that muscle type increases after feeding in growing 4 and mature 4,5 horses and that the magnitude of the signaling response to feeding is greater in yearlings than in 2-year-old horses. 4 In 2 previous studies of mTOR signaling in horses, biopsy specimens of the gluteus medius muscle were obtained from a standardized depth of 6 cm 5 or at 50% of the muscle’s depth, 4 but muscle fiber types at the site of biopsy were not characterized in either study.…”

“…4 In 2 previous studies of mTOR signaling in horses, biopsy specimens of the gluteus medius muscle were obtained from a standardized depth of 6 cm 5 or at 50% of the muscle’s depth, 4 but muscle fiber types at the site of biopsy were not characterized in either study. Research evidence suggests that variation in muscle fiber type can alter the degree of mTOR signaling activation in response to anabolic stimuli (exercise in rodents 22,23 and humans 24,25 and feeding in piglets 26 ), with type II fibers being more responsive to exercise and feeding stimuli.…”