WATER PERMEABILITY OF APICAL AND BASOLATERAL CELL-MEMBRANES OF RAT INNER MEDULLARY COLLECTING DUCT

To quantify the pathways for water permeation through the kidney medulla, knowledge of the water permeability (P(osmol)) of individual cell membranes in inner medullary collecting duct (IMCD) is required. Therefore IMCD segments from the inner two thirds of inner medulla of Sprague-Dawley rats were perfused in vitro using a setup devised for rapid bath and luminal fluid exchanges (half time, t1/2 of 55 and 41 ms). Differential interference contrast microscopy, coupled to video recording, was used to measure volume and approximate surface areas of single cells. Volume and volume-to-surface area ratio of IMCD cells were strongly correlated with their position along the inner medullary axis. Transmembrane water flow (J(v)) was measured in response to a variety of osmotic gradient (DELTA-II) presented on either basolateral or luminal side of the cells. The linear relation between J(v) and DELTA-II yielded the cell membrane P(osmol), which was then corrected for membrane in foldings. Basolateral membrane P(osmol) was 126 +/- 3-mu-m/s. Apical membrane P(osmol) rose from a basal value of 26 +/- 3-mu-m/s to 99 +/- 5-mu-m/s in presence of antidiuretic hormone (ADH). Because of amplification of basolateral membrane, the ADH-stimulated apical membrane remained rate-limiting for transcellular osmotic water flow, and the IMCD cell did not swell significantly. Calculated transcellular P(osmol), expressed in terms of smooth luminal surface, was 64-mu-m/s without ADH and 207-mu-m/s with ADH. IMCD cells in anisosmotic media displayed almost complete volume regulatory decrease but only partial volume regulatory increase.