ABSTRACT
Whole-cell patch-clamp recordings of membrane currents were performed in combination with measurements of mediator secretion from mucosal-type mast cells (rat basophilic leukemia cells, subline 2H3), to determine the involvement of membrane conductances induced upon depletion of intracellular Ca2+ stores. In patch-clamp experiments, ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid-induced depletion of internal Ca2+ stores led to activation of two distinct membrane conductances, a Ca2+ current and a Cl- current. The Ca2+ current was blocked by 100 microM La3+, which did not affect the Cl- current. In contrast, 500 microM 4,4'-diisothiocyanato-2,2'-disulfonic acid produced selective blocked of the Cl- current. Remarkably, the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), niflumic acid, and N-phenylanthranilic acid (NPAA) inhibited not only the Cl- current but also the Ca2+ current. IC50 values for the blockade of the Ca2+ inward current by NPPB, niflumic acid, and NPAA were determined to be 23, 150, and 190 microM, respectively. In secretion experiments, thapsigargin-induced depletion of internal Ca2+ stores stimulated serotonin release, which was found to be strictly dependent on extracellular Ca2+. In the presence of 100 microM La3+ secretion was almost completely inhibited. In contrast, only 50% of secretion was suppressed by 500 microM 4,4'-diisothiocyanato-2,2'-disulfonic acid, which fully blocked the Cl- current without affecting Ca2+ influx, as monitored by electrophysiological experiments. The other Cl- channel blockers produced a very different pattern for the inhibitory dose dependence of secretion, with IC50 values for NPPB, niflumic acid, and NPAA of 23, 60, and 180 microM, respectively. Taken together, these findings suggest that Ca2+ store depletion leads to concomitant activation of Cl- and Ca2+ currents. Blockade of the latter is apparently an additional mode of action for diarylaminocarboxylate-type Cl- channel blockers inhibiting mast cell secretory responses.
