REGULATION OF INTRACELLULAR PH IN THE PERFUSED HEART BY EXTERNAL HCO3- AND NA+-H+ EXCHANGE

Both Na-dependent HCO3- influx and the Na+-H+ antiport have been shown to contribute to recovery from intracellular acidosis in avian and mammalian cardiac tissue. We have investigated the participation of these mechanisms in the recovery of intracellular pH (pH(i)) after an acid load in the Langendorff-perfused ferret heart. pH(i) was measured from the phosphorus-31 nuclear magnetic resonance chemical shift of 2-deoxy-D-glucose 6-phosphate. Basal pH(i) was higher in HCO3--buffered solution (7.05 +/- 0.01; n = 8) than in nominally HCO3--free N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) solution (6.98 +/- 0.02; n = 9). Addition of 5-(N-ethyl-N-isopropyl)amiloride (EIPA) caused a significant fall in pH(i) in HEPES solution (6.91 +/- 0.02; n = 5) but not in HCO3- solution (7.02 +/- 0.02; n = 5). Intrinsic intracellular buffering capacity in 0 Na+-HEPES solution was 37 +/- 2 mmol/I (n = 4), and additional buffering due to HCO3--CO2 was approximately 13 mmol/l in HCO3- solution. After an intracellular acidosis induced by an NH4Cl prepulse, the proton efflux rate (J(H)) at pH(i) 6.90 was 0.5 +/- 0.2 mmol-l-1.min-1 (n = 14) in HEPES solution and 1.2 +/- 0.4 mmol-l-1.min-1 (n = 13) in HCO- solution. The addition of 1 muM EIPA effectively blocked proton efflux in HEPES solution (J(H) < 0.1 mmol-l-1.min-1; n = 8), whereas it slowed pH(i) recovery in HCO3- solution (J(H) = 0.6 +/- 0.2 mmol.l-1.min-1; n = 9). There was no recovery of pH(i) in Na+-free HCO3- solution (J(H) < 0.1 mmol.l-1.min-1; n = 3). The Na+-H+ antiport and a mechanism requiring both external Na+ and HCO3- each contribute approximately 50% to proton efflux at pH(i) 6.90 during the recovery from intracellular acidosis in the isolated perfused mammalian heart.