Factors governing sodium and chloride transport across the inner medullary collecting duct

Based on topographical, morphological and physiological characteristics, the three portions of the collecting duct system are the cortical collecting tubule (CCT), the outer medullary collecting duct (OMCD) and the inner medullary collecting duct (IMCD). The IMCD is a direct continuation of the OMCD and extends from the junction between the outer and inner medulla to the tip of the papilla. Also, due to the morphological differences observed along the IMCD, Madsen and Tisher [1] recently have proposed a subdivision of the IMCD into three portions as the outer third (IMCD1), middle third (IMCD2) and inner third (IMCD3), despite there being no abrupt divisions between the three segments. The cells of IMCD1 are similar to the intercalated cells and principal cells of the OMCD. In the rat the intercalated cells constitute approximately 10% of the cells in the IMCD1. However, in most of the IMCD2 and the entire IMCD3 there is only one cell type which appears to be different from both principal and intercalated cells of the OMCD [1]. The functional differences between IMCD1 and the terminal two segment portions of IMCD were initially demonstrated by Sands and Knepper [2]. They showed that IMCD2 and IMCD3 exhibit similar permeabilities to urea, whereas significant differences were demonstrated between IMCD1 and the terminal IMCD (IMCD2 and IMCD3). Differences related to the trans-epithelial potential have been also described between these segments. Stanton [3] has recently demonstrated the presence of small lumen-negative PD (-2.2 ± 0.3 mV) in the initial part of IMCD obtained from Brattleboro rats. Previous results from our laboratory [4] which did not take into account the three portions of IMCD, showed that the PD was not significantly different from zero (0.7 ± 0.2 mV) throughout the length of the IMCD. These observations led us to compare the transepithelial PD observed in the IMCD with the middle (IMCD2) and distal parts (IMCD3) of Wistar rats. Our results confirmed the presence of a slightly lumen-negative PD in the initial segments which differed significantly from the slightly lumen-positive voltage observed in the middle third and distal portion of IMCD (Fig. 1). Sands, Nonoguchi and Knepper [5] showed under basal conditions that the ratio between apparent Na+ and Cl- permeabilities (PNa, PCl) was significantly higher in the initial IMCD than in the terminal IMCD. Thus, the IMCD can be subdivided into two distinct subsegments, the initial IMCD or IMCD1