Na/H exchange in cultured chick heart cells: pH_i regulation

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 pH_i regulation. The kinetics of pH_i changes in cultured chick heart cells were monitored microfluorometrically with 6-carboxyfluorescein and correlated with Na_i 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 Na_i 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 pH_i 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 HCO_s-free solution. Amiloride (10^-s M) maximally inhibited both pHi regulation and Na⁺ uptake; the ID_So 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 pH_i 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 pH_i.