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 (pH(i)) homeostasis in cultured hepatocytes. We have studied the occurrence of these transport systems in the intact rat liver by P-31 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), pH(i) 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 (pH(e)). Under systemic acidosis (6.5-7.0 pH(e) range), during the acid load, pH(i) decreased with increased external proton concentrations and became amiloride sensitive. The pH(i) 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 pH(i) (0.2 pH unit) in the 6.5-7.5 pH(e) range. Perfusion with 1 mM stilbene derivative (4-acetamido-4'-isothiocyanostilbene-2-2' disulfonic acid) in the presence of HCO3- and at pH(e) 7.4 induced a dramatic pH(i) fall (DELTApH 0.15), further accentuated during an acid load (DELTApH 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 pH(i) reaches the set point value.