Sodium transport in rat renal papillary collecting tubule cells in culture

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Abstract
Sodium transport in the papillary collecting duct (PCD) is poorly understood because of the inaccessibility of the distal nephron to micropuncture. Cultured rat renal papillary collecting tubule (RPCT) cells were investigated as a model for the PCD. RPCT cells have the morphologic appearance and hormonal responsiveness of the papillary collecting tubule. Sodium transport was studied using 22Na+ uptake measurements. Sodium uptake, measured at 23°C in the absence of K+ and in the presence of 0.5 mM ouabain, was saturable at 100 mM extracellular NaCI, and half‐maximal uptake occurred at 40 mM NaCI. The accumulation of 22Na+ appeared to be intracellular and was regulated by (Na+, K+)‐ATPase activity, since activation of the Na+/K+ pump with K+ reduced 22Na+ accumulation by 90%. The time course for uptake was linear, showed only a single component, and followed first order kinetics with a t½ of 16 min. Amiloride and lithium inhibited 22Na+ influx, and a Dixon plot was linear, with a Ki of 16 μM amiloride. Chloride replacement or 1 mM furosemide, with or without K+, reduced uptake by only 20%. Sodium efflux from RPCT cells in the presence of ouabain showed a similar time course (t½, 15 min) and was also inhibited by amiloride (IC50 =20 μM). Increased extracellular pH stimulated 22Na+ uptake and inhibited 22Na+ efflux. Addition of permeable organic acids, acetate, and bicarbonate, enhanced 22Na+ uptake. These results are consistent with Na+/H+ and Na+/Na+ exchange as mechanisms of 22Na+ uptake in the RPCT cell. This exchanger may be important in regulation of transepithelial sodium flux, maintenance of intracellular pH and cell volume, and hormonal stimulation of the papillary collecting duct.