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

Abstract

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+]oinhibited 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.