title: A bumetanide-sensitive, apically localized Na+2Cl-K+ cotransport in the rat inner medullary collecting duct.
abstract: To better characterize loop diuretic-sensitive ion fluxes in the inner medullary collecting duct (IMCD) we examined them in IMCD cells grown as a primary culture on permeable supports. A polarization of the cells with their basolateral side to the support was confirmed morphologically by electron microscopy and functionally by flux studies with ouabain. Within 7 days cells developed a transepithelial resistance of 974+/-52 omega per cm2 and a low transepithelial potential difference (-0.7+/-0.8 mV). Measurements of intracellular ion content by electron probe microanalysis in IMCD depleted of intracellular ions by preincubation in a Na(+)-K(+)-Cl(-)-free medium revealed, compared to the control receiving solvent, significant reductions in intracellular Na+ content (-17.6% within 10 min) and intracellular Cl- content (-43.8% within 30 min) by the addition of bumetanide (10(-4) mol/l) to the apical but not basolateral incubation medium. In 22Na+ and 86Rb+ isotope uptake studies, fluxes from the apical side were significantly inhibited at bumetanide concentrations of 100 micromol/l by 0.27+/-0.10 and 0.21+/-0.04 nmol/cm2 in 10 min, respectively, whereas basolateral fluxes of 86Rb+ but not 22Na+ were significantly reduced by this substance. Removal of Cl- had a similar but not additional effect. mRNA encoding the apical isoform of the Na+2Cl(-)K+ cotransporter could be specifically amplified by reverse transcriptase polymerase chain reaction from the inner medulla and highly purified IMCD cells. Northern blot of mRNA isolated from the inner medulla with a riboprobe of the apical isoform revealed a transcript of approximately 4.9 kb. This probe localized under "low-stringency" conditions to the IMCD in in situ hybridization studies. These results suggest the presence of an apically localized isoform of bumetanide-sensitive Na+2Cl(-)K+ cotransport in at least a subfraction of IMCD cells. This transport may be involved in the ultimate adjustment of urinary electrolyte concentration by this final segment of the tubular system.
authors.affiliation: Abteilung Nephrologie und Rheumatologie, Georg August-Universität Göttingen, Germany. cgrupp@gwdg.de
authors.forename: C
authors.initials: C
authors.lastname: Grupp
authors.forename: R
authors.initials: R
authors.lastname: Stelmach
authors.forename: I
authors.initials: I
authors.lastname: Troche
authors.forename: G A
authors.initials: GA
authors.lastname: Müller
mesh_terms: D000818:Animals; D015152:Blotting, Northern; D002034:Bumetanide / Q000494:pharmacology*; D002352:Carrier Proteins / Q000096:biosynthesis / Q000378:metabolism*; D002478:Cells, Cultured; D002712:Chlorides / Q000378:metabolism; D004232:Diuretics / Q000494:pharmacology*; D004573:Electrolytes / Q000652:urine; D004577:Electron Probe Microanalysis; D017403:In Situ Hybridization; D007679:Kidney Medulla / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D007685:Kidney Tubules, Collecting / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D008297:Male; D008854:Microscopy, Electron; D011188:Potassium / Q000378:metabolism; D012333:RNA, Messenger / Q000096:biosynthesis; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism; D028021:Sodium-Potassium-Chloride Symporters
chemical_list: D002352:Carrier Proteins; D002712:Chlorides; D004232:Diuretics; D004573:Electrolytes; D012333:RNA, Messenger; D028021:Sodium-Potassium-Chloride Symporters; D002034:Bumetanide; D012964:Sodium; D011188:Potassium