22Na+ fluxes in thymic lymphocytes: I. Na+/Na+ and Na+/H+ exchange through an amiloride-insensitive pathway

Abstract

The Na+ transport pathways of normal rat thymocytes were investigated. Na+ conductance was found to be lower than K+ conductance, which is consistent with reported values of membrane potential. In contrast, the isotopically measured Na+ permeability was > 10-fold higher than that of K+, which indicates that most of the flux is electroneutral. Cotransport with Cl- (or K+ and Cl−) and countertransport with Ca2+ were ruled out by ion substitution experiments and use of inhibitors. Countertransport for Na+ or H+ through the amiloride-sensitive antiport accounts for only 15-20% of the resting influx. In the presence of amiloride, 22Na+ uptake was increased in Na+- loaded cells, which suggests the existence of Na+/Na+ countertransport. Cytoplasmic pH determinations using fluorescent probes indicated that under certain conditions this amiloride-resistant system will also exchange Na+ for H+, as evidenced by an internal Na+'-dependent acidification of cells suspended in Na+- free media. The rate of cytoplasmic acidification is proportional to internal [Na+] but inversely related to extracellular [Na+]. Moreover, 22Na+ uptake is inhibited by increasing external [H+]. The results support the existence of a substantial amiloride-insensitive, electroneutral cation exchange system capable of transporting Na+ and H+. © 1984, Rockefeller University Press., All rights reserved.