In a recent micropuncture study electrodes with relatively large tips (3 to 5 μ O.D.) and hence, low tip resistances were used to measure the transepithelial potential difference (PD) across the proximal tubule of the rat kidney. The present study reexamines the PD of the distal tubule of the rat kidney using such electrodes. In contrast to previous studies where a negative PD has been uniformly found in the tubule, the transtubular PD was found to be previously oriented (+3.7 mv) when particular efforts were made to puncture the earliest accessible segments. In accord with previous observations, the PD of the late segment was consistently negative (mean, -19.6 mv). Morphologic examination of the epithelium at the site of puncture suggests that in the very early distal tubule where positive potentials are recorded, the epithelium is characteristic of the distal convoluted tubule. By contrast, in the latter part of the distal tubule, where negative potentials are recorded, the epithelium displays the morphologic characteristics of the cortical collecting duct. The results of these studies suggest that the net transport properties of the distal tubule, that is the region of the nephron beginning just beyond the macula densa and extending to the first junction with another renal tubule, are a composite of activities of at least two types of epithelium.
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