Membrane Biology 9 Springer-Verlag New York Inc. 1992 Biphasic Increase of Apical CI- Conductance by Muscarinic Stimulation of HT-29cI.19A Human Colon Carcinoma Cell Line: Evidence for Activation of Different CI-

Summary. The modulation of ion transport pathways in filter-grown monolayers of the CI-secreting subclone (19A) of the human colon carcinoma cell line HT-29 by muscarinic stimulation was studied by combined Ussing chamber and microimpalement experiments. Basolateral ddition of 10 4 M carbachol induced acomplex poly-phasic change of the cell potential consisting of (i) a fast and short (30-sec) depolarization of 15 + 1 mV from a resting value of-52-+ l mV and an increase of the fractional resistance ofthe apical membrane (first phase), (ii) a repolarization of22 + I mV leading to a hyperpolarization f the cell (second phase), (iii) a depolarization of l l-+ 1 mV and a decrease of the fractional resistance of the apical membrane (the third phase), (iv) and sometimes, a hyperpolarization f 6 +- 1 mV and an increase of the fractional resistance of the apical membrane (fourth phase)