PH GRADIENT EFFECTS ON CHLORIDE TRANSPORT ACROSS BASOLATERAL MEMBRANE-VESICLES FROM GUINEA-PIG JEJUNUM

1. The effects of alkaline-inside pH gradients on Cl-36(-) uptake were quantified by using brush-border membrane (BBM) and basolateral membr ane (BLM) vesicles from guinea-pig jejunum. 2. With BBM vesicles, a pH (o)/pH(i) gradient of 5.0/7.5 yielded fast overshoots involving a rand om, non-obligatory Cl--H+ symport, strongly inhibited by CCCP. In cont rast, BLM vesicles responded to similar pH gradients with much smaller , delayed overshoots, unaffected by CCCP. 3. The initial Cl- entry rat es into BLM vesicles were a function of each pH(o), pH(i) and Delta pH value. They were stimulated by valinomycin in the presence of inward- directed K+ gradients. Short-circuiting the membrane potential with eq uilibrated K+ and valinomycin inhibited pH gradient-dependent Cl- upta ke, but only partially. 4. Taken together, these results indicate that guinea-pig jejunal BLM vesicles possess both Cl- conductance and Cl-- H+ symport activities. 5. Even when different, the BBM and the BLM sym porters are mechanistically similar. Neither of them involves a Cl--OH - antiport, nor a simultaneous Cl--anion exchange mechanism. Rather, f or each membrane, all of these activities (symport, anion exchange) ca n be explained in terms of a single mobile carrier acting as a random, non-obligatory Cl--H+ symporter where exchange occurs simply by count erflow. Net Cl- translocation via either the ternary (Cl--C-H+) or the binary (Cl--C) complexes accounts, respectively for the existence of two, operationally distinct, electroneutral and rheogenic components. 6. The BBM symporter appears to involve an AE2 protein, but the molecu lar identity of the BLM one remains to be established.