I ntracellular alkalinization is known to be associated with tumorigenic transformation. Besides phenotypical alterations alkali-transformed Madin-Darby canine kidney (MDCK) cells exhibit a spontaneously oscillating cell membrane potential (PD). Using single-channel patch clamp techniques, it was the aim of this study to identify the ion channel underlying the rhythmic hyperpolarizations of the PD. In the cell-attached patch configuration, we found that channel activity was oscillating. The frequency of channel oscillations is 1.1 +/- 0.1 min-1. At the peak of oscillatory channel activity, single-channel current was -2.7 +/- 0.05 pA, and in the resting state it was -1.95 +/- 0.05 pA. Given the single-channel conductance of 53 +/- 3 pS for inward (and of 27 +/- 5 pS for outward) current the difference of single-channel current amplitude corresponded to a hyperpolarization of approximately 14 mV. The channel is selective for K+ over Na+. Channel kinetics are characterized by one open and by three closed time constants. The channel is Ca2+ sensitive. Half maximal activation in the inside-out patch mode is achieved at a Ca2+ concentration of 10 mumol/liter. In addition, we also found a 13-pS K+ channel that shows no oscillatory activity in the cell-attached patch configuration and that was not Ca2+ sensitive. We conclude that the Ca(2+)-sensitive 53-pS K+ channel is underlying spontaneous oscillations of the PD. It has virtually identical biophysical properties as a Ca(2+)-sensitive K+ channel in nontransformed parent MDCK cells. Hence, alkali-induced transformation of MDCK cells did not affect the channel protein itself but its regulators thereby causing spontaneous fluctuations of the PD.