Electrophysiological Localization of Transporters in the Distal Nephron Segments

Abstract

The distal convoluted tubule (DCT) and the connecting tubule (CNT) from the rabbit kidney were investigated by electrophysiological techniques, isotopic flux measurement, and microfluorometry, in order to characterize the ion transport properties of cell membranes. When the DCT and the CNT were per-fused in vitro, the transepithelial voltage (VT) displayed lumen negative, and the transepithelial resistances (RT) were relatively low, as in the category of leaky epithelia. Random cellular impalement revealed that the basolateral membrane voltage (VB) of the DCT showed Gaussian distribution, whereas the CNT consisted of two cell populations, having different VB and different fractional resistance of the apical membrane (fRA). The CNT cell had a high VB. and lower fRA and the intercalated (IC) cell in the CNT had a low VB and higher fRA Using ion substitution and channel inhibitors, the conductive properties of DCT cells and CNT cells revealed that the luminal membrane had both Na+ and K+ conductances and that the basolateral membrane had both K+ and Cl-conductances. Intercalated cells of CNT had only a Cl-conductance in the basolateral membrane. Besides Na+ conductance in the luminal membrane of CNT cells, two other modes of Na+ entry process were observed. One pathway was the Na/Cl cotransporter, revealed by isotopic ion flux studies, which was inhibitable with thiazide diuretics, and the other was a nonselective cation conductance, which was not sensitive to amiloride and was opened by parathyroid hormone (PTH). From intracellular calcium measurement and calcium flux studies, it was found that the latter mode serves as a route for calcium entry pathway in CNT cells.