Extracellular detection of K+ release during migration of transformed Madin-Darby canine kidney cells

Madin Darby canine kidney cells transformed by alkaline stress (MDCK-F cells) constitutively migrate at a rate of about 1 mu um . min(-1). Migration depends on the intermittent activity of a Ca2+-stimulated, 53-pS K+ channel (K-Ca channel) that is inhibitable by charybdotoxin. In the present study we examined whether this intermittent K-Ca channel activity results in a significant K+ loss across the plasma membrane. K+ efflux from MDCK-F cells should result in a transient increase of extracellular K+ ([K+](e)) in the close vicinity of a migrating cell. However, due to the rapid diffusion of K+ ions into the virtually infinite extracellular space, such a transient increase in [K+](e) was too small to be detected by conventional K+-Selective electrodes. Therefore, we developed a ''shielded ion-sensitive microelectrode'' (SIM) that limited diffusion to a small compartment, formed by a shielding pipette which surrounded the tip of the K+-sensitive microelectrode. The SIM improved the signal to noise ratio by a factor of at least three, thus transient increases of [K+](e) in the vicinity of MDCK-F cells became detectable. They occurred at a rate of 1.3 min(-1). The cell releases 40 fmol K+ during each burst of intermittent K-Ca channel activity, which corresponds to about 15% of the total cellular K+ content. Since transmembrane K+ loss must be accompanied by anion loss and therefore leads to a decrease of cell volume, these findings support the hypothesis that intermittent Volume changes are a prerequisite for the migration of MDCK-F cells.