ATP-SENSITIVE K-SELECTIVE CHANNELS OF INNER MEDULLARY COLLECTING DUCTCELLS()

The inner medullary collecting duct (IMCD) in vivo has the capacity to either secrete or reabsorb K+. How ever, a selective K+ conductance h as not been described previously in the IMCD. In the present study, th e patch-clamp method was used to determine the presence and properties of K+-selective channels in the apical membrane of the inner medullar y collecting duct cell line, mIMCD-3. Two types of K+-selective channe ls were observed in both cell-attached and excised patches. The most p redominant K+ channel, a smaller conductance K channel (SK), was prese nt in cell-attached patches with 140 mM KCl (high bath K+) but not wit h 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 2 9 pS with outward currents in symmetrical 140 mM KCl. SK was insensiti ve to both voltage and Ca2+ However, SK was inhibited significantly by millimolar concentrations of ATP in excised patches. A second K+-sele ctive channel [a larger K+ channel (BK)] displayed a single-channel co nductance equal to 132 pS with inward currents and 90 pS with outward currents in symmetrical 140 mM KCl solutions. BK was intermittently ac tivated in excised inside-out patches by Mg2+-ATP in concentrations fr om 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 ce ll-attached patches with 140 mM KCl in the bath solution. Both channel s 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 con cluded that the apical membrane of mIMCD-3 cells contains both a small K+-selective channel that is inhibited by physiological concentration s of ATP and a large K+-selective channel that is activated by Mg2+-AT P.