title: Activation of Na+/H+ exchange in lymphocytes by osmotically induced volume changes and by cytoplasmic acidification.
abstract: After swelling in hypotonic solutions, peripheral blood mononuclear cells (PBM) shrink toward their original volumes. Upon restoration of isotonicity, the cells initially shrink but then regain near-normal size again. This regulatory volume increase (RVI) is abolished by removal of Na+o or Cl-o or by addition of amiloride. RVI is unaffected by removal of K+o or by ouabain and is only partially inhibited by 1 mM furosemide. As a result of increased influx, the cells gain both Na+ and K+ during reswelling. In contrast, only Na+ content increases in the presence of ouabain. Amiloride largely eliminates the changes in the content of both cations. Using diS-C3-(5), no significant membrane potential changes were detected during RVI, which suggests that the fluxes are electroneutral. The cytoplasmic pH of volume-static cells was measured with 5,6-dicarboxyfluorescein. After acid loading, the addition of extracellular Na+ induced an amiloride-inhibitable alkalinization, which is consistent with Na+/H+ exchange. Cytoplasmic pH was not affected by cell shrinkage itself, but an internal alkalinization, which was also amiloride sensitive and Na+ dependent, developed during reswelling. In isotonic lightly buffered solutions without HCO-3, an amiloride-sensitive acidification of the medium was measurable when Na+ was added to shrunken PBM. K+ was unable to mimic this effect. The observations are compatible with the model proposed by Cala (J. Gen. Physiol. 1980. 76:683-708), whereby an electroneutral Na+o/H+i exchange is activated by osmotic shrinking. Cellular volume gain occurs as Cl-o simultaneously exchanges for either HCO-3i or OH-i. Na+i is secondarily replaced by K+ through the pump, but this step is not essential for RVI.
mesh_terms: D000136:Acid-Base Equilibrium; D000584:Amiloride / Q000494:pharmacology; D001692:Biological Transport; D002478:Cells, Cultured; D003470:Culture Media; D006801:Humans; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials; D008954:Models, Biological; D009000:Monocytes / Q000166:cytology / Q000378:metabolism*; D009997:Osmotic Pressure; D010042:Ouabain / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*
chemical_list: D003470:Culture Media; D010042:Ouabain; D000584:Amiloride; D006859:Hydrogen; D012964:Sodium