Pumping ions: Regulation of intracellular pH in hepatocytes
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.
