title: Na/H exchange in cultured chick heart cells. pHi regulation.
abstract: The purpose of this study was to establish the existence of Na/H exchange in cardiac muscle and to evaluate the contribution of Na/H exchange to pHi regulation. The kinetics of pHi changes in cultured chick heart cells were monitored microfluorometrically with 6-carboxyfluorescein and correlated with Nai content changes analyzed by atomic absorption spectrophotometry; transmembrane H+ movements were evaluated under pH stat conditions. After induction of an intracellular acid load by pretreatment with NH4Cl, a regulatory cytoplasmic alkalinization occurred with a t1/2 of 2.9 min. pHi regulation required external Na+ and was concomitant with transmembrane H+ extrusion as well as a rapid rise in Nai content in an Na/H ratio of 1:1. Microelectrode recordings of membrane potential demonstrated directly the electroneutral character of pHi regulation. Acid-induced net Na+ uptake could be either stimulated by further decreasing pHi or inhibited by decreasing pHo; Na+ uptake was unaffected by tetrodotoxin (10 micrograms/ml), quinidine (10(-3) M), DIDS (10(-4) M), Clo-free solution, or HCO3-free solution. Amiloride (10(-3) M) maximally inhibited both pHi regulation and Na+ uptake; the ID50 for amiloride inhibition of Na+ uptake was 3 microM. Nao-dependent H+ extrusion showed half-maximal activation at 15 mM Nao; Li+, but not K+ or choline+, could substitute for Na+ to support H+ extrusion. Cao-free solution also stimulated acid-induced Na+ uptake. We conclude that pHi regulation following an acid load in cardiac muscle cells is by an amiloride-sensitive, electroneutral Na/H exchange. Stimulation of Na/H exchange up to 54 pmol/cm2 X s indicates the rapidity of this exchange across cardiac cell membranes. Na/H exchange may also participate in steady state maintenance of pHi.
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D002642:Chick Embryo; D002712:Chlorides / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007474:Ion Exchange; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*
chemical_list: D001639:Bicarbonates; D002712:Chlorides; D000584:Amiloride; D006859:Hydrogen; D012964:Sodium; D002118:Calcium