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Department of Physiology, Queen's University, Kingston, Canada.

1. In epithelial cells, hyposmotic stress induces visible cell swelling and large Cl- currents, which deactivate on return to isotonic solutions and are abolished by 0.1-0.5 mM DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid). During depolarizing voltage clamp pulses, the currents activate rapidly and show time-dependent relaxation with associated tail currents on return to negative potentials. 2. We used whole-cell and outside-out patch recording to study volume activation of Cl- currents in the epithelial cancer cell lines H69AR and HeLa S5. In a 210 or 160 mosmol l-1 hyposmotic bathing solution containing 90 mM NaCl, 1 mM Ca2+ and 1 mM Mg2+, current relaxation was rapid, occurred positive to the Cl- reversal potential and reduced current to < 30% of its peak level at +100 mV. 3. Replacement of most bath inorganic cations by N-methyl-D-glucamine (NMDG) at constant Cl- concentration and osmolarity eliminated most of the current relaxation and caused an increase in steady-state current levels. Steady-state current was 85 +/- 6% of peak current at +100 mV in NMDG-Cl bath solution. This ratio fell to 55 +/- 2% (n = 5) when 1 mM Mg2+ was re-added to the bath. 4. Re-addition of Mg2+ or other Group II metals (Ca2+, Sr2+, Ba2+) induced immediate changes in current relaxation in a dose- and species-dependent manner. Concentrations of Mg2+ as low as 0.1 mM were effective in causing Cl- current relaxation. The IC50 for steady-state current block by external Mg2+ was 1.75 mM.(ABSTRACT TRUNCATED AT 250 WORDS)