Video measurement of basolateral membrane hydraulic conductivity in the proximal tubule.

Isolated, lumen-collapsed S1, S2, and S3 proximal tubule segments from the rabbit were exposed acutely to media made hypotonic or hypertonic by adjusting the concentration of the impermeant solute raffinose. The result was a water flux into or out of the cells across their basolateral cell membranes and a consequent swelling or shrinking of the cells. From tubule volume changes measured at 1/60-s intervals during the first 0.03-0.2 s of video recordings, the earliest water fluxes were found to be 0.76 +/- 0.04 nl X min-1 X mm-1 X mosM-1 in S1, 0.53 +/- 0.03 in S2, and 0.35 +/- 0.04 in S3. When normalized to outer tubule surface areas, these fluxes yield statistically different hydraulic conductivities of about 5,500, 4,000, and 3,000 microns X s-1 in the three segments. However, when normalized to the basolateral membrane surface areas, the basolateral membrane hydraulic conductivities are all approximately 300 microns X s-1 and not statistically different. If one assumes that the hydraulic conductivities of the basolateral and apical cell membranes are equal, the latter value agrees with reported transtubular measurements and is sufficient to allow nearly isotonic transcellular absorption to occur with driving forces of 2-3 mosM.