Volume-sensitive basolateral K+ channels were studied in apically amphotericin B-permeabilized HT-29/B6 monolayers in Ussing chambers with current fluctuation analysis. The basolateral K+ conductance and Lorentzian K+ channel noise were osmotically activated in presence of Cl- concentrations greater than or equal to 74 mM. Under isotonic conditions with 148 mM Cl-, a large transepithelial K+ current of 500 +/- 16.8 microA/cm2 and a spontaneous Lorentzian K+ channel noise with a corner frequency of 29.8 +/- 1.6 Hz (n = 31) were observed. Increasing extracellular osmolalities by addition of sucrose sensitively decreased the K+ current across the basolateral membrane. Half-maximal sucrose concentration was 20 +/- 6 mM for this shrinkage maneuver. The osmotically sensitive K+ pathway was similarly activated with the halide Br- and selective for K+ over Rb+ (4:1). The established K+ channel blockers lidocaine [50% inhibitory concentration (IC50) = 49.0 +/- 3.7 microM], quinidine (IC50 = 10.1 +/- 1.3 microM), and also the chloride channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (IC50 = 114 +/- 2.1 microM) completely inhibited basolateral K+ currents, whereas 46% of K+ current was blocked by barium (IC50 = 95.3 +/- 23.2 microM). Osmotic sensitivity of this K+ conductance made a correction for hypertonic effects of added blockers necessary, and considerable osmotic effects of blockers at commonly used doses were shown. All blockers induced dose dependently additional Lorentzian noise, indicating a direct inhibitory action on basolateral K+ channels. In this human Cl- secretory cell line, volume-sensitive K+ channels are localized only in the basolateral membrane and may modulate osmotic regulation when HT-29 cells swell.