INTRACELLULAR PH RECOVERY DURING RESPIRATORY-ACIDOSIS IN PERFUSED HEARTS

Na+-H+ exchange and Na+-dependent HCO, influx both contribute to recovery of intracellular pH (pH(i)) 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 pH(i) recovery during respiratory acidosis in the Langendorff-perfused ferret heart with and without correction of extracellular pH (pH(o)). pH(i) was measured from the chemical shift of the exogenous P-31 nuclear magnetic resonance pH indicator 2-deoxy-D-glucose B-phosphate. Intrinsic intracellular buffering capacity, calculated from the change in intracellular HCO3- concentration after a change in CO2, was reduced from similar to 33 (no inhibitors of acid extrusion present) to 19 +/- 5 mM when H+ extrusion during the acid loading phase was inhibited. During respiratory acidosis (pH(o) approximate to 6.95), the proton efflux rate (J(H)) calculated at pH(i) 6.85 was 0.30 +/- 0.04 mmol.l(-1).min(-1) (n = 9). When pH(o) was corrected by increasing external HCO3- concentration to 60 mM during respiratory acidosis (pH(o) approximate to 7.33), J(H) was 1.11 +/- 0.11 mmol.l(-1).min(-1) (n = 7), and when pH(o) was partially corrected by the addition of 50 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid to the perfusion solution (pH(o) approximate to 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 similar to 50% to acid-equivalent efflux. pH(i) changes associated with fluxes of CO2 across the cardiac sarcolemma were about six times more rapid than pH(i) changes associated with Na+-H+ exchange or HCO3- influx. This suggests that under conditions where there is a gradient of CO2 across the cardiac sarcolemma, for example, after reperfusion of the ischemic myocardium, CO2 efflux will be a significant contributor to pH(i) recovery.