Cl(-)-HCO3- antiport in rat lacrimal gland

With the use of analytical subcellular fractionation and tracer uptake methods we have demonstrated the presence of a Cl(-)-HCO3- antiport mechanism in the rat exorbital lacrimal gland. We find that outwardly directed gradients of HCO3- and of 35Cl- accelerated the flux of 36Cl- into isolated membrane vesicles. Because vesicle membrane potentials were clamped to 0 mV with K+-valinomycin, the observed anion gradient-dependent acceleration of Cl- influx could not be attributed to conductive fluxes. The antiporter had an apparent K0.5 for Cl- between 6 and 10 mM. It was sensitive to the stilbene derivatives 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). It was also sensitive to the loop diuretic furosemide, which has frequently been used in tests for NaKCl2 symporter activity. Other anions inhibited anion gradient-driven Cl- influx in the sequence SCN- greater than NO3- greater than Cl- greater than HCO3- greater than SO2-4. The density distribution of Cl(-)-HCO3- antiport activity indicated that approximately 80% of the transporter was associated with intracellular membranes, suggesting the presence of cytoplasmic pools of functional antiporters. Because several studies have already shown the presence of Na+-H+ antiporter activity in lacrimal acinar cell basolateral membranes, a cellular model for lacrimal acinar electrolyte secretion is proposed in which a parallel array of Cl- -HCO3- and Na+-H+ antiporters mediates the Na+-dependent accumulation of Cl- against its electrochemical potential gradient.