Membrane-specific regulation of Cl- channels by purinergic receptors in rat submandibular gland acinar and duct cells

Abstract

Measurement of [Cl-](i) and the Cl- current in the rat salivary submandibular gland (SMG) acinar and duct cells was used to evaluate the role of Cl- channels in the regulation of [Cl-](i) during purinergic stimulation. Under resting conditions [Cl-](i) averaged 56 ± 8 and 26 ± 7 mM in acinar and duct cells, respectively. In both cells, stimulation with 1 mM ATP resulted in Cl- efflux and subsequent influx. Inhibition of NaKCl2 cotransport had no effect on [Cl-](i) changes in duct cells and inhibited only about 50% of Cl- uptake in acinar cells. Accordingly, low levels of expression of NaKCl2 cotransporter protein were found in duct cells. Acinar cells expressed high levels of the cotransporter. Measurement of Cl- current under selective conditions revealed that acinar and duct cells express at least five distinct Cl- channels; a ClCO-like, volume-sensitive, inward rectifying, Ca2+-activated and CFTR-like Cl- currents. ATP acting on both cell types activated at least two channels, the Ca2+-activated Cl- channel and a Ca2+-independent glibenclamide-sensitive Cl- current, possibly cystic fibrosis transmembrane regulator (CFTR). Of the many nucleotides tested only 2'-3'-benzoylbenzoyl (Bz)-ATP and UTP activated Cl- channels in SMG cells. Despite their relative potency in increasing [Ca2+](i), BzATP in both SMG cell types largely activated the Ca2+-independent, glibenclamide- sensitive Cl- current, whereas UTP activated only the Ca2+-dependent Cl- current. We interpret this to suggest that BzATP and UTP largely activate Cl- channels residing in the membrane expressing the receptor for the active nucleotide. The present studies reveal a potentially new mechanism for transcellular Cl- transport in a CFTR-expressing tissue, the SMG. Coordinated action of the P2z (luminal) and P2u (basolateral) receptors can mediate part of the transcellular Cl- transport by acinar and duct cells to determine the final electrolyte composition of salivary fluid.