Na channels in the rat connecting tubule

Epithelial Na channels were investigated using patch-clamp techniques in connecting tubule (CNT) segments isolated from rat kidney. Cell-attached patches with Li+ in the patch pipette contained channels with conductances for inward currents of 13 - 16 pS and slow opening and closing kinetics, similar to properties of Na channels in the cortical collecting tubule (CCT). Macroscopic amiloride-sensitive currents (I-Na) were also observed under whole cell clamp conditions. These currents were undetectable in cells from control rats but were large when the animals were infused with aldosterone ( 1,380 +/- 340 pA/cell at a holding potential of - 100 mV) or fed a high-K diet ( 670 +/- 260 pA/cell) for 1 wk. Under both of these conditions, currents in cells of the CNT were two- to fourfold larger than currents in cells of the CCT of the same animals. In aldosterone-treated animals, currents in cells of the initial collecting tubule (iCT) were intermediate, such that the relative magnitude of I-Na was as follows: CNT > iCT > CCT. Quantitative analysis of the results suggests that the maximal capacity of the aggregate population of CNTs to reabsorb Na could be as high as 18 mumol/min, or similar to10% of the filtered load of Na. This capacity is similar to10 times higher than that of the CCT.