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-13C6] 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-Thr389was 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.
Effects of Estrogen Replacement and Lower Androgen Status on Skeletal Muscle Collagen and Myofibrillar Protein Synthesis in Postmenopausal Women
