Kinetic properties of sodium transport pathways in the river lamprey Lampetra fluviatilis erythrocytes

To activate Na super(+)/H super(+) exchange, intracellular pH (pH sub(i)) of erythrocytes of the river lamprey Lampetra fluviatilis were changed from 6 and 8 using nigericin. The Na super(+)/H super(+) exchanger activity was estimated from the values of amiloride-sensitive components of Na super(+) ( super(22)Na) inflow or of H super(+) outflow from erythrocytes. Kinetic parameters of the carrier functioning were determined by using Hill equation. Dependence of Na super(+) and H super(+) transport on pH sub(i) value is described by hyperbolic function with the Hill coefficient value (n) close to 1. Maximal rate of ion transport was within the limits of 9-10 mmol/l cells/min, and the H super(+) concentration producing the exchanger 50% activation amounted to 0.6-1.0 kM. Stimulation of H super(+) outcome from acidified erythrocytes (pH sub(i) 5.9) with increase of H super(+) concentration in the incubation medium is described by Hill equation with n value of 1.6. Concentration Na super(+) for the semimaximal stimulation of H super(+) outcome amounted to 10 mM. The obtained results indicate the presence in lamprey erythrocytes of only binding site for H super(+) from the cytoplasm side and the presence of positive cooperativity in Na super(+)-binding from the extracellular side of the Na super(+)/H super(+) exchanger. Na super(+) efflux from cells in the Na super(+)-free medium did not change at a 10-fold increase of H super(+) concentration in the incubation medium. The presented data indicate differences of kinetic properties of the lamprey erythrocyte Na super(+)/H super(+) exchanger and of this carrier isoforms in mammalian cells. In intact erythrocytes the dependence of the amiloride-sensitive Na super(+) inflow on its concentration in the medium is described by Hill equitation with n 1.6. The Na super(+) concentration producing the 50% transport activation amounted to 39 mM and was essentially higher as compared with that in acidified erythrocytes. These data confirm conception of the presence of two amiloride-sensitive pathways of Na+ transport in lamprey erythrocytes.