Glutamate ingestion: the plasma and muscle free amino acid pools of resting humans

Monosodium glutamate (MSG) ingestion is known to increase plasma glutamate concentration, and MSG infusion stimulates insulin secretion. We investigat ed the impact of MSG ingestion on both the plasma and intramuscular amino a cid pools. Nine postprandial adults ingested MSG (150 mg/kg) and rested for 105 min. Venous blood was sampled preingestion and then every 15 min; vast us lateralis muscle biopsies were taken preingestion and at 45, 75, and 105 min postingestion. Venous plasma glutamate and aspartate concentrations in creased (P less than or equal to 0.05) similar to 700-800 and 300-400%, res pectively, after 30-45 min. Although several other plasma amino acids incre ased modestly, the rise in glutamate accounted for similar to 80% of the in crease in total plasma amino acids. In addition, plasma insulin increased t hreefold after 15 min; this occurred before a significant increase in plasm a glutamate, indicating a feed-forward stimulation from the gastrointestina l tract. The intramuscular amino acid pool was remarkably constant, with on ly glutamate increasing (P less than or equal to 0.05) by 3.56 mmol/kg dry wt. By 105 min, the plasma and muscle amino acids had returned to resting c oncentrations. This increase in muscle glutamate concentration could accoun t for similar to 40% of the MSG ingested; we propose that resting skeletal muscle is a major sink for the glutamate and metabolizes it to aspartate.