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 NH4C1, 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 pH* regulation. Acid-induced net Na+ uptake could be either stimulated by further decreasing pHi or inhibited by decreasing pH0; Na+ uptake was unaffected by tetrodotoxin (10-4 g/ml), quinidine (10-3 M), DIDS (10-4 M), Clo-free solution, or HCOs-free solution. Amiloride (10-s M) maximally inhibited both pHi regulation and Na+ uptake; the IDSo for amiloride inhibition of Na+ uptake was 3 µM. Nao-dependent H+ extrusion showed half-maximal activation at 15 µM Na+; Li+, but not K+ or choline+, could substitute for Na+ to support H+ extrusion. Ca2+-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·s indicates the rapidity of this exchange across cardiac cell membranes. Na/H exchange may also participate in steady state maintenance of pHi. © 1985, Rockefeller University Press., All rights reserved.