Abstract
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. Twenty-one 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 + CIT) or with 10 g of nonessential amino acids (WHEY + 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 + CIT vs. WHEY and WHEY + NEAA from 30 to 300 min postexercise (P < 0.001), but there were no treatment differences in blood flow or microvascular perfusion (all P > 0.05). Phosphorylation of p70S6K-Thr(389) was greater in WHEY vs. WHEY + NEAA (P = 0.02). Postprandial MPS was greater in WHEY vs. WHEY + CIT and WHEY + NEAA under both FED (WHEY: ~128%; WHEY + CIT: ~56%; WHEY + NEAA: ~38%) and EX-FED (WHEY: ~251%; WHEY + CIT: ~124%; WHEY + NEAA: ~108%) conditions (P = 0.003). Citrulline coingestion with a low quantity of protein was ineffective in augmenting the anabolic properties of protein compared with nonessential amino acids.