Interdependent Fluxes of Amino Acids and Sodium Ion in the Pigeon Red Blood Cell

The Na + -dependent uptake of each of eight neutral amino acids by the pigeon red blood cell in vitro is associated with an increase in Na + influx, whereas three neutral amino acids that have uptakes little affected by Na + cause no significant stimulation of Na + movements. No net uptake of Na + is detectable during incubation with any of these amino acids, and stimulation of Na + efflux, as well as influx, is seen directly during the uptake of glycine, alanine, and β-alanine. Neither the uptake of alanine nor the associated Na + movements are inhibited by ouabain at a concentration sufficient to block the normal active extrusion of Na + from the cell. The stimulation of Na + influx by alanine and glycine occurs additively when both amino acids are present together. Analysis of the cellular concentrations of several neutral amino acids indicates that all the apparent uptake of glycine and alanine caused by Na + can be accounted for in terms of homogeneous and heterogeneous exchange with analogous endogenous amino acids. The Na + -dependent increments of influx (Δ v s ) of glycine, alanine, and β-alanine parallel quantitatively the associated increases of Na + influx (Δ v n a ) over the concentration ranges tested, although the kinetics and the stoichiometry are different for each amino acid, the mean values of the ratio Δ v n a :Δ v s being 1.53, 2.52, and 0.96, respectively. Possible interpretations of these findings are discussed.