Contribution of the Na+-K+-2Cl(-) cotransporter NKCC1 to Cl- secretion in rat OMCD

In rat kidney the "secretory" isoform of the Na+-K+-2Cl(-) cotransporter (N KCC1) localizes to the basolateral membrane of the alpha -intercalated cell . The purpose of this study was to determine whether rat outer medullary co llecting duct (OMCD) secretes Cl- and whether transepithelial Cl- transport occurs, in part, through Cl- uptake across the basolateral membrane mediat ed by NKCC1 in series with Cl- efflux across the apical membrane. OMCD tubu les from rats treated with deoxycorticosterone pivalate were perfused in vi tro in symmetrical HCO3-/CO2-buffered solutions. Cl- secretion was observed in this segment, accompanied by a lumen positive transepithelial potential . Bumetanide (100 muM), when added to the bath, reduced Cl- secretion by 78 %, although the lumen positive transepithelial potential and fluid flux wer e unchanged. Bumetanide-sensitive Cl- secretion was dependent on extracellu lar Na+ and either K+ or NH4+, consistent with the ion dependency of NKCC1- mediated Cl- transport. In conclusion, OMCD tubules from deoxycorticosteron e pivalate-treated rats secrete Cl- into the luminal fluid through NKCC1-me diated Cl- uptake across the basolateral membrane in series with Cl- efflux across the apical membrane. The physiological role of NKCC1-mediated Cl- u ptake remains to be determined. However, the role of NKCC1 in the process o f fluid secretion could not be demonstrated.