Cellular localization of the potassium channel Kir7.1 in guinea pig and human kidney

Background. K+ channels have important functions in the kidney, such as mai ntenance of the membrane potential, volume regulation, recirculation, and s ecretion of potassium ions. The aim of this study was to obtain more inform ation on the localization and possible functional role of the inwardly rect ifying K+ channel, Kir7.1. Methods. Kir7.1 cDNA (1114 bp) was isolated from guinea pig kidney (gpKir7. 1), and its tissue distribution was analyzed by reverse transcriptase-polym erase chain reaction (:RT-PCR). In addition, a genomic DNA fragment (6153 b p) was isolated from a genomic library. cRNA was expressed in Xenopus laevi s oocytes for functional studies. Immunohistochemistry and RT-PCR were used to localize Kir7.1 in guinea pig and human kidney. Results. The expression of gpKir7.1 in Xenopus laevis oocytes revealed inwa rdly rectifying K+ currents. The reversal potential was strongly dependent on the extracellular K+ concentration, shifting from -14 mV at 96 mmol/L K to -90 mV at 1 mmol/L K+. gpKir7.1 showed a low affinity for Ba2+ Signific ant expression of gpKir7.1 was found in brain, kidney, and lung, but not in heart, skeletal muscle, liver, or spleen. Immunocytochemical detection in guinea pig identified the gpKir7.1 protein in the basolateral membrane of e pithelial cells of the proximal tubule. RT-PCR analysis identified strong g pKir7.1 expression in the proximal tubule and weak expression in glomeruli and thick ascending limb. In isolated human tubule fragments, RT-PCR showed expression in proximal tubule and thick ascending limb. Conclusion. Our results suggest that Kir7.1 may contribute to basolateral K + recycling in the proximal tubule and in the thick ascending limb.