Extracellular Na+ inhibits Na+/H+ exchange: cell shrinkage reduces the inhibition

Department of Biology, Syracuse University, Syracuse, New York 13244-1220 Submitted 30 December 2003 ; accepted in final form 18 March 2004 Na + /H + exchangers (NHE) are ubiquitous transporters participating in regulation of cell volume and pH. Cell shrinkage, acidification, and growth factors activate NHE by increasing its sensitivity to intracellular H + concentration. In this study, the kinetics were studied in dog red blood cells of Na + influx through NHE as a function of external Na + concentration ([Na + ] o ). In cells in isotonic media, [Na + ] o inhibited Na + influx >40 mM. Osmotic shrinkage activated NHE by reducing this inhibition. In cells in isotonic media + 120 mM sucrose, there was no inhibition, and influx was a hyperbolic function of [Na + ] o . The kinetics of Na + -inhibited Na + influx were analyzed at various extents of osmotic shrinkage. The curves for inhibited Na + fluxes were sigmoid, indicating more than one Na + inhibitory site associated with each transporter. Shrinkage significantly increased the Na + concentration at half-maximal velocity of Na + -inhibited Na + influx, the mechanism by which shrinkage activates NHE. erythrocytes; cell volume regulation; amiloride; kinetics of sodium ion influx Address for reprint requests and other correspondence: P. B. Dunham, BRL, Syracuse Univ., 130 College Pl., Syracuse, NY 13244–1220 (E-mail: pbdunham{at}syr.edu ).