Chloride transport across the membrane of parotid secretory granules

Abstract

The Cl- transport pathways in secretory granules isolated from the parotid glands of rats were characterized by the technique of ionophore-induced lysis in defined salt solutions. The granules were shown to possess a Cl- conductance that exhibited a distinct anion selectivity with a sequence I- greater than Br- greater than Cl- greater than F- greater than SO4(2-) much greater than gluconate-. This conductance could be reduced approximately 40% by the stilbene 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) from the cytoplasmic side; the half-maximal concentration for inhibition was 50 microM. Furthermore, the apparent Cl- conductance was reduced by outwardly directed granule H+ gradients and stimulated by inwardly directed gradients. An outwardly directed H+ gradient mimics the in vivo environment and may serve in a regulatory capacity, providing for a tonic inhibition of transport until the granule fuses with the luminal membrane. The granules also possessed a Cl(-)-HCO3- exchange based on electroneutrality of Cl- uptake and stimulation of this uptake by HCO3-. This pathway displayed a different anion selectivity, I- greater than Br- greater than F- greater than Cl- much greater than SO4(2-) much greater than gluconate-, and was not inhibited by SITS on the cytoplasmic side. The presence of these electrolyte transport pathways in the granule membrane is consistent with the production of primary fluid by parotid acinar cells after fusion of granules with the luminal plasma membrane.