CYCLIC NUCLEOTIDE-GATED CATION CHANNELS MEDIATE SODIUM-ABSORPTION BY IMCD (MIMCD-K2) CELLS

The inner medullary collecting duct cell line, mIMCD-K2, absorbs Na+ b y an amiloride-sensitive, electrogenic mechanism. The goal of the pres ent study was to characterize the amiloride-sensitive, Na+-conducting channels responsible for electrogenic Na+ absorption. To this end, we measured Na+ currents in single cells with the patch-clamp technique a nd Na+ currents across monolayers mounted in Ussing-type chambers. In whole cell patch-clamp experiments, amiloride-sensitive, inward Na+ cu rrents were mediated by nonselective cation channels. In single-channe l patch-clamp experiments, amiloride- and guanosine 3',5'-cyclic monop hosphate (cGMP)-sensitive, 20-pS nonselective cation channels (i.e., C NG channels) were identified in the apical membrane. CNG channels were inhibited by amiloride, diltiazem, ethylisopropylamiloride (EIPA), an d 8-bromo-cGMP and were permeable to Ca2+ and Mg2+. Epithelial Na+ cha nnels were never observed in whole cell or single-channel recordings. Na+ absorption across confluent monolayers was inhibited with a rank o rder potency of benzamil > amiloride > phenamil much greater than EIPA > diltiazem. Our data are most consistent with the view that CNG chan nels mediate electrogenic Na+ absorption across mIMCD-K2 cells.