ATP-SENSITIVE K+-SELECTIVE CHANNELS OF INNER MEDULLARY COLLECTING DUCT CELLS

The inner medullary collecting duct (IMCD) in vivo has the capacity to either secrete or reabsorb K+. How ever, a selective K+ conductance has not been described previously in the IMCD. In the present study, the patch-clamp method was used to determine the presence and properties of K+-selective channels in the apical membrane of the inner medullary collecting duct cell line, mIMCD-3. Two types of K+-selective channels were observed in both cell-attached and excised patches. The most predominant K+ channel, a smaller conductance K channel (SK), was present in cell-attached patches with 140 mM KCl (high bath K+) but not with 135 mM NaCl plus 5 mM KCl (low bath K+) in the bathing solution. The single-channel conductance of SK was 36 pS with inward currents and 29 pS with outward currents in symmetrical 140 mM KCl. SK was insensitive to both voltage and Ca2+ However, SK was inhibited significantly by millimolar concentrations of ATP in excised patches. A second K+-selective channel [a larger K+ channel (BK)] displayed a single-channel conductance equal to 132 pS with inward currents and 90 pS with outward currents in symmetrical 140 mM KCl solutions. BK was intermittently activated in excised inside-out patches by Mg2+-ATP in concentrations from 1 to 5 mM. With complete removal of Mg2+, BK was insensitive to ATP. BK was also insensitive to potential and Ca2+ and was observed in cell-attached patches with 140 mM KCl in the bath solution. Both channels were blocked reversibly by 1 mM Ba2+ from the intracellular surface but not by external Ba2+. In addition, a reduction in internal pH from 7.4 to 6.8 had no effect on the activity of either channel. It is concluded that the apical membrane of mIMCD-3 cells contains both a small K+-selective channel that is inhibited by physiological concentrations of ATP and a large K+-selective channel that is activated by Mg2+-ATP.