CELL-SPECIFIC EXPRESSION OF AMILORIDE-SENSITIVE, NA-CONDUCTING ION CHANNELS IN THE KIDNEY()

Amiloride-sensitive, electrogenic Na+ absorption across the distal nep hron plays a vital role in regulating extracellular fluid volume and b lood pressure. Recently, two amiloride-sensitive, Na+-conducting ion c hannel cDNAs were cloned. One, an epithelial Nai-selective channel (EN aC), is responsible for Na+ absorption throughout the distal nephron. The second, a guanosine 3',5'-cyclic monophosphate (cGMP)-inhibitable cation channel, is conductive to Na+ and Ca2+ and contributes to Na+ a bsorption across the inner medullary collecting duct (IMCD). As a firs t step toward understanding the segment-specific contributions(s) of c GMP-gated cation channels and ENaC to Na+ and Ca2+ uptake along the ne phron, we used in situ reverse transcription-polymerase chain reaction (RT-PCR) hybridization, solution-phase RT-PCR, and Western blot analy sis to examine the nephron and cell-specific expression of these chann els in mouse kidney cell lines and/or dissected nephron segments. cGMP -gated cation channel mRNA was detected in proximal tubule, medullary thick ascending limb (mTAL), distal convoluted tubule (DCT), cortical collecting duct (CCD), outer medullary collecting duct (OMCD), and IMC D. cGMP-gated cation channel protein was detected in DCT CCD, and IMCD cell lines. These observations suggest that hormones that modulate in tracellular cGMP levels may regulate Na+, and perhaps Ca2+, uptake thr oughout the nephron. mRNA for alpha-mENaC, a subunit of the mouse ENaC , was detected in mTAL, DCT, CCD, OMCD, and IMCD. Coexpression of alph a-mENaC and cGMP-gated cation channel mRNAs in mTAL, DCT, CCD, OMCD, a nd IMCD suggests that both channels may contribute to Na+ absorption i n these nephron segments.