Inhibition of Ca2+-dependent Cl- secretion in T84 cells:: membrane target(s) of inhibition is agonist specific

Previous studies have indicated that Ca2+- dependent Cl- secretion across monolayers of T84 epithelial cells is subject to a variety of negative influences that serve to limit the overall extent of secretion. However, the downstream membrane target(s) of these inhibitory influences had not been elucidated. In this study, nuclide efflux techniques were used to determine whether inhibition of Ca2+-dependent Cl- secretion induced by carbachol, inositol 3,4,5,6-tetrakisphosphate, epidermal growth factor, or insulin reflected actions at an apical Cl- conductance, a basolateral K+ conductance, or both. Pretreatment of T84 cell monolayers with carbachol or a cell-permeant analog of inositol 3,4,5,6-tetrakisphosphate reduced the ability of subsequently added thapsigargin to stimulate apical I-125(-), but not basolateral Rb-86(+), efflux. These data suggested an effect on an apical Cl- channel. Conversely, epidermal growth factor reduced Ca2+- stimulated Rb-86(+) but not I-125(-) efflux, suggesting an effect of the growth factor on a K+ channel. Finally, insulin inhibited I-125(-) and Rb-86(+) effluxes. Thus effects of agents that inhibit transepithelial Cl- secretion are also manifest at the level of transmembrane transport pathways. However, the precise nature of the membrane conductances targeted are agonist specific.