Regulation of conductive Cl- transport in human fibroblasts

Under normal growth conditions, approximately 20% of the efflux of Cl- from human fibroblasts occurs via an electrically conductive pathway or Cl- channel. This basal Cl- conductance is insensitive to the Cl- -anion exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and to the Cl- -cation cotransport inhibitor bumetanide. Exposure of the cells to dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP) for 15 min increases the electrically conductive component of Cl- efflux by approximately 20%. Unlike the basal Cl- conductance, the cAMP-activated channel is DIDS sensitive, indicating that cAMP activates a different Cl- pathway from the one responsible for the basal Cl- conductance. Elevation of intracellular Ca2+ by addition of the ionophore A23187 also stimulates Cl- efflux via a DIDS inhibitable, electrically conductive Cl- pathway. That the cAMP- and Ca2+-stimulated pathways are different is suggested by the observation that simultaneous exposure of cells to optimal levels of dibutyryl cAMP and A23187 results in an increased Cl- efflux equal to the sum of the two factors acting independently. Prostaglandin E1, a known activator of adenylate cyclase, also elevates the levels of intracellular free Ca2+ in these cells and concomitantly activates both the cAMP- and the Ca2+-stimulated Cl- channels. Although regulated, Cl- channels are known to function in the modulation of nerve and muscle excitability, their role in fibroblast function is not clear.