1. Using microfluorimetry, intracellular pH, buffering capacity and intracellular pH recovery from intracellular acidosis were determined in isolated adult rat ventricular myocytes, in buffers with and without HCO-3.

2. In nominally HCO-3-free media, the intracellular pH was higher than in HCO-3-containing media. Buffering capacity at resting intracellular pH and at a pH of about 6.3 was also lower in HCO-3-free media.

3. In HCO-3-free media, recovery from an acid load after an NH4C1 prepulse was almost completely inhibited by the Na+/H+ antiport activity specific inhibitor ethylisopropyl amiloride. However, in the presence of HCO-3, H+ efflux rate was enhanced, and ethylisopropyl amiloride led to only partial inhibition of H+ efflux. Complete inhibition was achieved only with further addition of the anion-transport inhibitor 4,4′-di-isothiocyanatostilbene-2,2′-disulphonate.

4. Thus, in adult rat ventricular myocytes, recovery from intracellular acidosis in the absence of HCO-3 was almost wholly due to Na+/H+ antiport activity. In the more physiological situation with HCO-3 present, a third of the recovery from an intracellular acid load was attributed to an additional external Na+-dependent di-isothiocyanatostilbene-disulphonate-sensitive H+ efflux.

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