A, gramicidin-treated cells expressing Δ(241–680) were equilibrated in 20/120 Na/K-PSS containing 0.3 mm EGTA; at the arrow labelled ‘Isotonic’, 5 ml of 20/120 Na/K-PSS + 0.3 mm EGTA was applied. The fura-2 fluorescence intensities with 334 and 380 nm excitation, corrected for background, are depicted. The trace labelled ‘Ratio * 35’ depicts the 334/380 ratio multiplied by a factor of 35 in order to scale to the figure. B, gramicidin-treated Δ(241–680) cells were equilibrated with 70/70 Na/K-PSS containing 0.3 mm EGTA. Two minutes prior to beginning the recordings, the medium was changed to 70/140 Na/mannitol-PSS. The labelled arrows designate the application of 5 ml of 70 Na-PSS (Hypotonic) or 70/500 Na/mannitol-PSS (Hypertonic). These experiments have been repeated more than 3 times with similar results.