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

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