Time‐dependent changes in muscle protein synthesis during amino acids infusion

Abstract

It was reported that an anabolic effect of intravenous infusion of leucine on muscle protein is transient in human and neonatal pig. And it is hypothesized that availability of all essential amino acids (EAA) is necessary to sustain protein synthesis increased by leucine infusion. In order to test this hypothesis, we determined fractional synthesis rate (FSR) of skeletal muscle protein during primed and constant infusion of leucine alone (100 mg·kg−1·h−1, prime: 66.7 mg/kg) or same amount of leucine with other essential amino acids (600 mg·kg−1·h−1 in total, prime: 400 mg/kg) for 4 hours in ketamine-xylazine anesthetized rats. FSR was determined by flooding dose method using L-[ring-2H5]-phenylalanine. During the infusion of leucine alone, plasma leucine concentration was increased 3–4 fold and plasma concentrations of all other EAA were decreased. During the EAA infusion, plasma concentrations of each EAA were increased 2–5 fold. FSR showed transient increase at 15–30 min of leucine infusion and then declined, whereas FSR stayed elevated throughout EAA infusion. Leucine stimulation of muscle protein synthesis is transient and EAA are needed to be infused concomitantly in order to maintain increased muscle protein synthesis rate as hypothesized.