Human erythrocyte and plasma amino acid concentrations during exercise

Purpose: This investigation examined the effects of exercise and maltodextrin (Md) or placebo (Pl) ingestion on plasma and erythrocyte concentrations of amino acids. Methods: The erythrocyte and plasma concentrations of 17 amino acids, as well as plasma glucose and insulin, were analyzed in eight healthy trained male subjects before, during, and 25 min after 90-min cycle ergometer exercise at 65% peak oxygen uptake. The two treatments involved ingestion of orange-flavored water (Pl) or orange-flavored 10% maltodextrin solution (Md). Results: Two-way ANOVA revealed 1) that plasma concentrations of alanine and tyrosine changed significantly during the treatments, 2) that the plasma concentrations were significantly different between treatments for glycine and threonine, 3) that all erythrocyte concentrations increased significantly throughout the treatments except for arginine and tyrosine, and 4) that there were no significant differences in erythrocyte concentrations between the treatments. Three-way ANOVA highlighted the significant differences in the time responses between plasma and erythrocyte concentrations; the changes in erythrocyte levels from rest being significantly different from plasma for all amino acids except aspartic acid, glycine, and ornithine. Plasma glucose concentrations became elevated and remained above rest values in Md but fell below rest values in Pl; the differences in concentration between treatments were significant. Correspondingly, plasma insulin was significantly higher in Md during exercise. Conclusion: These results highlight that far from being slow in the uptake of amino acids, the erythrocyte in fact sequesters amino acids at an appreciable rate during exercise without a corresponding elevation in the plasma amino acids. For a greater understanding of amino acid changes during exercise, the analysis of both plasma and erythrocytes is recommended. In vitro studies have consistently shown that amino acid uptake by human erythrocytes is slow (27). It is generally believed that plasma rather than erythrocytes is the vehicle of amino acid transport between tissues (20) with the notion that the erythrocytes are of little if any significance in the interorgan transfer of amino acids (14). Consequently, most experimental measures of circulating amino acids have been restricted to the quantification of plasma concentrations (2,4,5,13,21). Relatively few analytical data are available on the actual amino acid content of the erythrocyte, although the red blood cell is well established as a cellular model of amino acid “transporter” systems that could exist in a cell (28). Some researchers have proposed that erythrocytes are significantly involved in the transport of amino acids. In vivo studies have shown that amino acid uptake or delivery by/to the erythrocytes is faster in vivo than expected from in vitro, indicating that the erythrocyte may have a specific role in the interorgan transport of amino acids in dogs (10,11), sheep (16), and humans (3,7,14). Differences in plasma arterial-venous concentrations are measured to establish fluxes in amino acids, yet the role and significance of the erythrocyte in these processes has not been seriously considered. The function of the erythrocyte as an amino acid transporter may be accentuated under exercise conditions as the concentration of the free amino acid pool becomes increased, particularly under conditions of glycogen depletion (6). Carbohydrates reduce but do not eliminate the exercise induced elevation in amino acid catabolism (8,18,19). Thus, the ingestion of carbohydrate during exercise is expected to decrease the fuel required from amino acids, including an attenuation of the glucose-alanine cycle, which is normally maintained during exercise (13). Carbohydrate ingestion also influences any insulin dependent erythrocyte amino acid fluxes as observed by Aoki et al. (3). Because no studies have so far been undertaken concerning erythrocyte amino acids and exercise, nor the influence of carbohydrate ingestion on erythrocyte amino acids, this investigation was undertaken to provide the followinginformation. First, to provide data on the erythrocyte amino acids concentrations during exercise. Second, to determine whether the changes in plasma and erythrocyte amino acids during exercise and recovery follow a similar pattern, and finally, to assess the influence of ingesting carbohydrate upon both plasma and erythrocyte concentrations of amino acids during exercise and into recovery.