Stilbenes stimulate T84 Cl- secretion by elevating Ca2+

Abstract

Basolateral but not apical application of 10-200 microM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) or 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) to T84 monolayers produced a transient increase in short-circuit current (Isc), followed by a sustained inhibition. 4,4'-Dinitrostilbene-2,2'-disulfonic acid (DNDS) had no effect. The increase in Isc produced by DIDS represents Cl- secretion and appears to result from Ca2+ elevation, because in all respects except time course the response to DIDS mimicked the response to the Ca(2+)-elevating agent thapsigargin. Fura-2 measurements established that thapsigargin elevates Ca2+ in T84 cells, but Ca2+ responses to DIDS could not be established directly because DIDS absorbs strongly at the critical wavelengths. Responses to DIDS and thapsigargin were 1) blocked by bumetanide; 2) not blocked by basolateral Ba2+; 3) completely nonadditive; 4) strongly synergistic with basal levels of Isc or with Isc increases produced by elevating adenosine 3',5'-cyclic monophosphate (cAMP; with forskolin) or guanosine 3',5'-cyclic monophosphate (with heat-stable enterotoxin); and 5) reversibly abolished by removal of basolateral Ca2+. Interactions between Ca2+ and cAMP-elevating agents strongly support a model of Cl- secretion in which apical Cl- conductance is activated by cyclic nucleotides but not by Ca2+ while basolateral K+ channels are activated by Ca2+. In contrast with this mechanism, whole cell patch-clamp recordings of nonconfluent T84 cells indicated that DIDS and other Ca(2+)-elevating agents stimulated an increase in Cl- conductance. Thus increases in cytosolic free Ca2+ in nonconfluent T84 cells activate conductances that differ from those in confluent monolayers.