CL- CHANNELS OF THE GASTRIC PARIETAL-CELL THAT ARE ACTIVE AT LOW PH

HCl secretion across mammalian gastric parietal cell apical membrane m ay involve Cl- channels. H+-K+-ATPase-containing membranes isolated fr om gastric mucosa of histamine-stimulated rabbits were fused to planar lipid bilayers. Channels were recorded with symmetric 800 mM CsCl sol utions, pH 7.4. A linear current-voltage (I-V) relationship was obtain ed, and conductance was 28 +/- 1 pS at 800 mM CsCl. Conductance was 6. 9 +/- 2 pS at 150 mM CsCl. Reversal potential was +22 mV with a fivefo ld cis-trans CsCl concentration gradient, indicating that the channel was anion selective with a discrimination ratio of 6:1 for Cl- over Cs +. Anion selectivity of the channel was I- > Cl greater-than-or-equal- to Br > NO3-, and gluconate was impermeant. Channels obtained at pH 7. 4 persisted when pH of medium bathing the trans side of the bilayer (p H(trans)) was reduced to pH 3, without a change in conductance, linear ity of I-V relationship, or ion selectivity. In contrast, asymmetric r eduction of pH of medium bathing the cis side of the bilayer from 7.4 to 3 always resulted in loss of channel activity. At pH 7.4, open prob ability (P(o)) of the channel was voltage dependent, i.e., predominant ly open at +80 mV but mainly closed at -80 mV. In contrast, with low p H(trans), channel P(o) at -80 mV was increased 3.5-fold. The Cl- chann el was Ca2+ indifferent. In absence of ionophores, ion selectivity for support of H+-K+-ATPase activity and H+ transport was consistent with that exhibited by the channel and could be limited by substitution wi th NO3-, whereas maximal H+-K+-ATPase activity was in-different to ani on present, demonstrating that anion transport can be rate limiting. C l- channels with similar characteristics (conductance, linear I-V rela tionship, and ion selectivity) were also present in H+-K+-ATPase-conta ining vesicles isolated from resting (cimetidine-treated) gastric muco sa, exhibiting at -80 mV a pH-independent approximately 3.5-fold lower P(o) than stimulated vesicle channels. At -80 mV, reduction of pH(tra ns) increased P(o), of both resting and stimulated Cl- channels by fiv e- to sixfold. Changing membrane potential from 0 to -80 mV across sti mulated vesicles increased Cl- channel activity an additional 10-fold. These findings suggest a model wherein stimulation of gastric HCl sec retion involves modification of preexisting but inactive Cl- channels that are present in tubulovesicles together with H+-K+-ATPase, and, wh en these channels move to the apical membrane, their activity is incre ased by negative membrane potential and by low extracytosolic pH acros s the secretory membrane of the parietal cell.