pH regulation in perfused rat liver: respective role of Na(+)-H+ exchanger and Na(+)-HCO3- cotransport

Na(+)-H+ antiport and Na(+)-HCO3- symport are involved in intracellular pH (pHi) homeostasis in cultured hepatocytes. We have studied the occurrence of these transport systems in the intact rat liver by 31P nuclear magnetic resonance. Livers perfused with a Krebs medium (25 mM HCO3-, pH 7.4, 37 degrees C) displayed a cytosolic pH 7.18 +/- 0.05 (n = 32). In response to an acid load (35 mM isobutyric acid), pHi remained constant. The same result was obtained in the presence of 1 mM amiloride (with or without acid load), indicating that the amiloride-sensitive Na(+)-H+ exchanger is inactive at external physiological pH (pHe). Under systemic acidosis (6.5-7.0 pHe range), during the acid load, pHi decreased with increased external proton concentrations and became amiloride sensitive. The pHi set point for the activation of the Na(+)-H+ exchange is 7.0. In the absence of HCO3-, livers showed a constant acidic shift of pHi (0.2 pH unit) in the 6.5-7.5 pHe range. Perfusion with 1 mM stilbene derivative (4-acetamido-4'-isothiocyanostilbene-2-2' disulfonic acid) in the presence of HCO3- and at pHe 7.4 induced a dramatic pHi fall (delta pH 0.15), further accentuated during an acid load (delta pH 0.25). Our results suggest that 1) the symport is always involved in pH homeostasis over a large range of pH variation (6.5-7.5) and 2) the Na(+)-H+ exchanger is activated under systemic acidosis as soon as pHi reaches the set point value.