Pumping ions: Regulation of intracellular pH in hepatocytes

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Abstract

In rat hepatocytes, basolateral Na+-H+ exchange and Na+-HCO3 cotransport function as acid extruders. To assess mechanisms of acid loading, intracellular pH (pHi) recovery from an alkaline load was analyzed in short-term cultured rat hepatocyte monolayers using the pH-sensitive dye BCECF. Electrophysiological techniques were also used to assess the role of the membrane potential (Vm). Cells were alkaline loaded by suddenly reducing external CO2 and HCO3 (from 10% and 50 mM, respectively, to 5% and 25 mM) at constant pH. After this maneuver, pHi rapidly rose by 0.13 ± 0.03 pH units (pHu) and recovered to baseline at an initial rate of 0.026 ± 0.009 pHu/min. Intracellular buffering power was estimated from the dependence of pHi on [NH4]o and varied between 70 and 10.5 mM/pHu in a pHi range of 6.5-7.6. Initial pHi recovery corresponded to a rate of OH efflux (JOH) of 1.76 ± 0.71 mM/min and was blocked by 0.5 mM DIDS (0.003 ± 0.002; JOH = 0.18 ± 0.06) or by 1 mM H2DIDS (0.001 ± 0.002; JOH = 0.26 ± 0.08) and by removal of [Cl]o (0.003 ± 0.007; JOH = 0.28 ± 0.07). The dependence of JOH on [Cl]o exhibited saturation kinetics with an apparent Km for [Cl]o of 5.1 mM. pHi recovery was Na+ independent and was not inhibited by substitution of Na+ with NMDG (0.045 ± 0.09; JOH = 2.94 ± 0.59). During an alkaline load, cell Vm hyperpolarized from –33.4 ± 1.8 to –43.4 ± 2.8 mV, mainly due to an increase in K+ conductance by a factor of 2.8 ± 0.3. Ba2+ blocked these changes and depolarized Vm by 12.1 ± 1.2 mV but had no effect on pHi recovery (0.025 ± 0.007; JOH = 1.46 ± 0.60), excluding that Vm functions as a major force for HCO3 extrusion after an alkaline load. The rate of JOH was directly proportional to the pHi reached after the alkaline load and varied fourfold over the pHi range of 7.25-7.46. These data indicate that pHi recovery from an alkaline load in rat hepatocytes is mediated by an electroneutral Na+-independent Cl-HCO3 exchanger.

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