Ca²⁺-activated K⁺ channels in cultured medullary thick ascending limb cells

The conductive properties of a clone of medullary thick ascending limb (MTAL) cells (GRB-MAL1) were assessed using conventional microelectrodes and the patch clamp technique. The apical cell membrane potential (V(a)) of MTAL cells was -46 ± 3 mV. Addition of Ba²⁺ (1 mM) to the apical solution induced a 22 ± 2 mV depolarization of V(a), whereas furosemide hyperpolarized V(a) by -5 ± 1 mV. In the cell-attached patch configuration, the most frequently occurring channel had a single channel conductance of 121 ± 5 pS and carried outward current. In excised patches, current movement was down to the electrochemical K⁺ gradient. Fluctuations were activated by depolarization of V(a) and by increasing Ca²⁺ concentration on the intracellular face. Micromolar amounts of Ba²⁺ on the intracellular face of the membrane inhibited channel activity. We conclude that cultures of MTAL cells GRB-MAL1 retain at least two of the properties of the mature phenotype, namely, an apical K⁺ conductance and a sensitivity to loop diuretics; the most frequently occurring channel in the apical cell membrane is a Ca²⁺-activated, maxi-K⁺ channel; and, finally Ca²⁺-activated K⁺ channels may play a role in generating the apical K⁺ conductance in cultured MTAL cells.