title: Na+-H+ exchange and Na+ entry across the apical membrane of Necturus gallbladder.
abstract: The role of Na+-H+ exchange in Na+ transport across the apical membrane was evaluated in Necturus gallbladder epithelium by means of intracellular Na+ activity (aNai) and 22Na+ uptake measurements. Under control conditions, complete replacement of Na+ in the mucosal solution with tetramethylammonium reduced aNai from 14.0 to 6.9 mM in 2 min (P less than 0.001). Mucosal addition of the Na+-H+ exchange inhibitor amiloride (10(-3) M) reduced aNai from 15.0 to 13.3 mM (P less than 0.001), whereas bumetanide (10(-5) and 10(-4) M) had no effect. Na+ influx across the apical membrane was studied by treating the tissues with ouabain, bathing them in Na-free solutions, and suddenly replacing the mucosal solution with an Na-containing solution. When the mucosal solution was replaced with Na-Ringer's, aNai increased at approximately 11 mM/min. This increase was inhibited by 54% by amiloride (10(-3) M, P less than 0.001) and was unaffected by bumetanide (10(-5) M). Amiloride-inhibitable Na+ fluxes across the apical membrane were also induced by the imposition of pH gradients. Na+ influx was also examined in tissues that had not been treated with ouabain. Under control conditions, 22Na+ influx from the mucosal solution into the epithelium was linear over the first 60 s and was inhibited by 40% by amiloride (10(-3) M, P less than 0.001) and by 19% by bumetanide (10(-5) M, P less than 0.025). We conclude that Na+-H+ exchange is a major pathway for Na+ entry in Necturus gallbladder, which accounts for at least half of apical Na+ influx both under transporting conditions and during exposure to ouabain. Bumetanide-inhibitable Na+ entry mechanisms may account for only a smaller fraction of Na+ influx under transporting conditions, and cannot explain influx in ouabain-treated tissues. These results support the hypothesis that NaCl entry results primarily from the operation of parallel Na+-H+ and Cl--HCO-3 exchangers, and not from a bumetanide-inhibitable NaCl cotransporter.
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D002034:Bumetanide / Q000494:pharmacology; D002463:Cell Membrane Permeability / Q000187:drug effects; D002712:Chlorides / Q000378:metabolism; D004848:Epithelium / Q000378:metabolism; D005704:Gallbladder / Q000378:metabolism*; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D007474:Ion Exchange; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D009337:Necturus; D010042:Ouabain / Q000494:pharmacology; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*
chemical_list: D002712:Chlorides; D007473:Ion Channels; D000644:Quaternary Ammonium Compounds; D002034:Bumetanide; D010042:Ouabain; D000584:Amiloride; D006859:Hydrogen; D012964:Sodium; C027917:tetramethylammonium