Cytosolic pH regulation in perfused rat liver: role of intracellular bicarbonate production

The contribution of metabolic bicarbonate to cytosolic pH (pH(cyto)) regulation was studied on isolated perfused rat liver using phosphorus-31 NMR spectroscopy. Removal of external HCO3- decreased proton efflux from 18.6 +/- 5.0 to 1.64 +/- 0.29 mu mol/min per g liver wet weight (w.w.) and pH(cyto) from 7.17 +/- 0.06 to 6.87 +/- 0.06. In the nominal absence of bicarbonate, inhibition of carbonic anhydrase by acetazolamide induced a further decrease of proton efflux of 0.69 +/- 0.26 mu mol/min per g liver w.w, reflecting a reduction in metabolic CO2 hydration, and hence a decrease of H+ and HCO3- supplies. Even though 27% of the proton efflux was amiloride-sensitive under bicarbonate-free conditions, amiloride did not change pH(cyto), revealing the contribution of additional regulatory processes. Indeed, pH regulation was affected by the combined use of 4-acetanlido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) and amiloride since pH(cyto) decreased by 0.16 +/- 0.05 and proton efflux by 0.60 +/- 0.14 mu mol/min per g liver w.w. The data suggest that amiloride-sensitive or SITS-sensitive transport activities could achieve, by themselves, pH(cyto) regulation. The involvement of two mechanisms, most likely Na+/H+ antiport and Na+:HCO3- symport, was confirmed in the whole organ under intracellular and extracellular acidosis, The evidence of Na-dependent transport of HCO3- in the absence of exogenous bicarbonate implies that the amount of metabolic bicarbonate is sufficient to effectively participate to pH(cyto) regulation. (C) 1998 Elsevier Science B.V. All rights reserved.