Ingestion of 20 g intact protein is sufficient to maximally stimulate MPS and APS after resistance exercise. Phosphorylation of candidate signaling proteins was not enhanced with any dose of protein ingested, which suggested that the stimulation of MPS after resistance exercise may be related to amino acid availability. Finally, dietary protein consumed after exercise in excess of the rate at which it can be incorporated into tissue protein stimulates irreversible oxidation.

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We examined the impact of chronic stress on rat growth rate and intestinal epithelial physiology and the role of mast cells in these responses. Mast cell-deficient (Ws/Ws) rats and +/+ littermate controls were submitted to water avoidance stress or sham stress, 1 h/day, for 5 days. Seven hours after the last sham or stress session, jejunal segments were mounted in Ussing chambers, in which secretion and permeability were measured. Body weight (as a growth index) and food intake were determined daily. Stress increased baseline jejunal epithelial ion secretion (indicated by short-circuit current), ionic permeability (conductance), and macromolecular permeability (horseradish peroxidase flux) in +/+ rats, but not in Ws/Ws rats, compared with nonstressed controls. Stress induced weight loss and reduced food intake similarly in the groups. In +/+ rats, these parameters remained altered 24-72 h after the cessation of stress. Modulation of stress-induced mucosal mast cell activation may help in the management of certain intestinal conditions involving epithelial pathophysiology.

BackgroundMinimizing the number of muscle biopsies has important methodological implications and minimizes subject discomfort during a stable isotope amino acid infusion. We aimed to determine the reliability of obtaining a single muscle biopsy for the calculation of muscle protein fractional synthetic rate (FSR) as well as the amount of incorporation time necessary to obtain that biopsy after initiating a stable isotope infusion (Study 1). The calculation of muscle protein FSR requires tracer steady-state during the stable isotope infusion. Therefore, a second aim was to examine if steady-state conditions are compromised in the precursor pools (plasma free or muscle intracellular [IC]) after ingestion of a tracer enriched protein drink and after resistance exercise (Study 2).MethodsSixteen men (23 ± 3 years; BMI = 23.8 ± 2.2 kg/m2, means ± SD) were randomized to perform Study 1 or Study 2 (n = 8, per study). Subjects received a primed, constant infusion of L-[ring-13C6]phenylalanine coupled with muscle biopsies of the vastus lateralis to measure rates of myofibrillar protein synthesis (MPS). Subjects in Study 2 were fed 25 g of whey protein immediately after an acute bout of unilateral resistance exercise.ResultsThere was no difference (P = 0.3) in rates of MPS determined using the steady-state precursor-product equation and determination of tracer incorporation between sequential biopsies 150 min apart or using plasma protein as the baseline enrichment, provided the infusion length was sufficient (230 ± 0.3 min). We also found that adding a modest amount of tracer (4% enriched), calculated based on the measured phenylalanine content of the protein (3.5%) in the drink, did not compromise steady-state conditions (slope of the enrichment curve not different from zero) in the plasma free or, more importantly, the IC pool (both P > 0.05).ConclusionsThese data demonstrate that the single biopsy approach yields comparable rates of muscle protein synthesis, provided a longer incorporation time is utilized, to that seen with a traditional two biopsy approach. In addition, we demonstrate that enriching protein-containing drinks with tracer does not disturb isotopic steady-state and thus both are reliable techniques to determine rates of MPS in humans.

Churchward-Venne TA, Cotie LM, MacDonald MJ, Mitchell CJ, Prior T, Baker SK, Phillips SM. Citrulline does not enhance blood flow, microvascular circulation, or myofibrillar protein synthesis in elderly men at rest or following exercise. Am J Physiol Endocrinol Metab 307: E71-E83, 2014. First published May 13, 2014; doi:10.1152/ajpendo.00096.2014.-Aging is associated with anabolic resistance, a reduced sensitivity of myofibrillar protein synthesis (MPS) to postprandial hyperaminoacidemia, particularly with low protein doses. Impairments in postprandial skeletal muscle blood flow and/or microvascular perfusion with hyperaminoacidemia and hyperinsulinemia may contribute to anabolic resistance. We examined whether providing citrulline, a precursor for arginine and nitric oxide synthesis, would increase arterial blood flow, skeletal muscle microvascular perfusion, MPS, and signaling through mTORC1. Twentyone elderly males (65-80 yr) completed acute unilateral resistance exercise prior to being assigned to ingest a high dose (45 g) of whey protein (WHEY) or a low dose (15 g) of whey protein with 10 g of citrulline (WHEY ϩ CIT) or with 10 g of nonessential amino acids (WHEY ϩ NEAA). A primed, continuous infusion of L-[ring-13 C6] phenylalanine with serial muscle biopsies was used to measure MPS and protein phosphorylation, whereas ultrasound was used to measure microvascular circulation under basal and postprandial conditions in both a rested (FED) and exercised (EX-FED) leg. Argininemia was greater in WHEY ϩ CIT vs. WHEY and WHEY ϩ NEAA from 30 to 300 min postexercise (P Ͻ 0.001), but there were no treatment differences in blood flow or microvascular perfusion (all P Ͼ 0.05). Phosphorylation of p70S6K-Thr 389 was greater in WHEY vs. WHEY ϩ NEAA (P ϭ 0.02). Postprandial MPS was greater in WHEY vs. WHEY ϩ CIT and WHEY ϩ NEAA under both FED (WHEY: ϳ128%; WHEY ϩ CIT: ϳ56%; WHEY ϩ NEAA: ϳ38%) and EX-FED (WHEY: ϳ251%; WHEY ϩ CIT: ϳ124%; WHEY ϩ NEAA: ϳ108%) conditions (P ϭ 0.003). Citrulline coingestion with a low quantity of protein was ineffective in augmenting the anabolic properties of protein compared with nonessential amino acids. citrulline; myofibrillar protein synthesis; elderly DECLINES IN MUSCLE MASS WITH AGING are due to rates of myofibrillar protein synthesis (MPS) being less than the net rate of muscle protein breakdown (37). Aminoacidemia and physical exercise are potent anabolic stimuli that elicit the synthesis of new muscle proteins; however, aging is associated with a reduced MPS response following both hyperaminoacidemia (54) and resistance exercise (28), a phenomenon termed "anabolic resistance" (for review, see Ref. 10). Although the mechanisms underlying anabolic resistance are not fully understood, age-related impairments in blood flow may be involved (31,48,50).Aminoacidemia and insulinemia increase MPS (54), arterial blood flow (54), and muscle microvascular perfusion (52) in the young; however, these responses are blunted in the elderly (48, 54). Age-related impairments in vascular function may be...