The k+-driven amino-acid cotransporter of the larval midgut of lepidoptera - is na+ an alternative substrate

Amino acid accumulation within brush-border membrane vesicles (BBMV) from the larval midgut of Lepidoptera is driven by a K+ gradient. However, it can also be driven by a Na+ gradient, although with reduced efficiency. To examine the possibility that sodium and potassium ions are handled by the same amino acid transporter, glycine uptake into BBMV from Philosamia cynthia Drury was measured in the presence of a pH gradient and of a transmembrane electrical potential difference, i.e. in simulated 'physiological' conditions. The kinetics of glycine uptake at extravesicular saturating Na+ or K+ concentrations discloses a higher affinity of the cotransporter for the amino acid in the presence of Na+ but a maximum transport rate with K+. Glycine uptake at a fixed concentration as a function of external Na+ or K+ concentration yields curves that show saturation but do not fit a rectangular hyperbola, with Hill coefficients less than 1 with Na+ and greater than 1 with K+. These coefficients vary according to glycine concentration. Increasing the concentration of extravesicular Na+ at a saturating external K+ concentration reduced glycine uptake to 70% of the control value. This inhibition curve is compatible with competition between the two cations for the same cotransporter and with the presence of different kinetic constants with Na+ or K+. The data are consistent with a steady-state random two-substrate mechanism for glycine transport, with Na+ and K+ as alternative substrates.