Determination of electrical resistance of the isolated cortical collecting tubule and its possible anatomical location.

The electrical properties of renal epithelia have been determined principally by micropuncture studies of neplhron segments located at the surface of the kidney. Deeper segments, owing to their inaccessibility to micropuncture cannot be studied directly, and consequently, their contribution to the ultimate formation of urine has been assessed by indirect methods. The introduction of the isolated tubule perfusion technique(l) provided the opportunity for studying individual nephron segments not only under conditions which allowed greater experimental manipulation, but wlhiclh also permitted direct study of nephron segments such as the cortical collecting tubule. One of the early goals was to develop techniques to determine the potential difference and the transepitlhelial resistance in this nephron segment. Studies by Burg, Isaacson and Grantham(2) showed that the cortical collecting tubule was capable of maintaining a negative luminal potential near -25 mV and that the transepithelial resistance was much larger than that of the proximal tubule. These studies, however, were subject to some uncertainty owing to tlle possibility that significant leaks could have occurred not only at the perfusing end of the tubule, but more importantly at the distal end of the isolated tubule where the tubule was held in a single glass micropipette. In order to overcome this difficulty which could lead to a significant source of error in the measurements of both potential difference and resistance, Sylgard 184 was used to electrically seal the clistal end of the tubule into the collecting pipette(3-5). This modification of the original system proved useful not only in studies of the electrical properties of the isolated tubules, but also in studies using isotopic tracers to determine the permeability to nonelectrolytes(6). In order to establish the validity of the techniques, studies were undertaken to show that the isolated tubule behaved like