Na(+)-H+ exchange and Na(+)-dependent HCO3- influx both contribute to recovery of intracellular pH (pHi) after an acidosis induced by using the NH4Cl prepulse technique in mammalian and avian cardiac tissue. We have investigated the relative contributions of these mechanisms to pHi recovery during respiratory acidosis in the Langendorff-perfused ferret heart with and without correction of extracellular pH (pHo). pHi was measured from the chemical shift of the exogenous 31P nuclear magnetic resonance pH indicator 2-deoxy-D-glucose 6-phosphate. Intrinsic intracellular buffering capacity, calculated from the change in intracellular HCO3- concentration after a change in CO2, was reduced from approximately 33 (no inhibitors of acid extrusion present) to 19 +/- 5 mM when H+ extrusion during the acid loading phase was inhibited. During respiratory acidosis (pHo approximately 6.95), the proton efflux rate (JH) calculated at pHi 6.85 was 0.30 +/- 0.04 mmol.l-1.min-1 (n = 9). When pHo was corrected by increasing external HCO3- concentration to 60 mM during respiratory acidosis (pHo approximately 7.33), JH was 1.11 +/- 0.11 mmol.l-1.min-1 (n = 7), and when pHo was partially corrected by the addition of 50 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid to the perfusion solution (pHo approximately 7.1), JH was 0.64 +/- 0.08 mmol.l-1.min-1 (n = 6). In all three groups Na(+)-H+ exchange and HCO3- influx each contributed approximately 50% to acid-equivalent efflux.(ABSTRACT TRUNCATED AT 250 WORDS)