The present study demonstrates the activation of Cl- channels in HT29 cells by agonist (ATP, neurotensin, carbachol) increasing cytosolic Ca2+, by hypotonic cell swelling and by cGMP. Cell-attached nystatin patch-clamp (CAN) as well as slow and fast whole-cell recordings were used. The cell membrane potential was depolarized in a dose-dependent manner with half-maximal effects at 0.4 mumol/l for ATP, 60 pmol/l for neurotensin and 0.8 mumol/l for carbachol. The depolarization, which was caused by Cl- conductances increases, occurred within 1 s and was accompanied by a simultaneous and reversible increase of the input conductance of the cell-attached membrane from 295 +/- 32 pS to 1180 +/- 271 pS (ATP; 10 mumol/l, n = 21) and 192 +/- 37 pS to 443 +/- 128 pS (neurotensin; 1 nmol/l, n = 8). The effects of the agonists could be mimicked by ionomycin (0.2 mumol/l), suggesting that an increase in intracellular Ca2+ was responsible for the activation of Cl- channels. The depolarization was followed by a secondary hyperpolarization. Hypotonic cell swelling also depolarized the cells and induced an increase in the membrane conductance. With 120 mmol/l NaCl the depolarization was 10 +/- 0.8 mV and the cell-attached conductance increased from 228 +/- 29 pS to 410 +/- 65 (n = 26) pS. NaCl at 90 mmol/l and 72.5 mmol/l had even stronger effects. Comparable conductance increases were also obtained when the different agonists or hypotonic cell swelling were examined in whole cell experiments. 5-Nitro-2-(3-phenylpropylamino)-benzoate (1 mumol/l) did not prevent the effects of Ca(2+)-increasing hormones and of hypotonic solutions.(ABSTRACT TRUNCATED AT 250 WORDS)