Video measurement of basolateral NaCl reflection coefficient in proximal tubule.

Isolated, lumen-collapsed, proximal and distally occluded segments of rabbit S1 and S2 proximal tubule were equilibrated in isotonic NaCl or isosmotic raffinose medium and then exposed acutely to hypotonic or hypertonic raffinose or NaCl solution. The result was a water flux per millimeter tubule length, JVo, across the basolateral cell membranes and a consequent cell swelling or shrinkage that could be measured by a video technique in the initial 0.1 s or less after a change from steady state. The cell volume change was proportional to the applied osmolality difference, delta pi, and differed consistently with the solute employed. From the equation JVo/delta pi = sigma LpA, where sigma is the basolateral membrane reflection coefficient for the osmotic solute used and LpA is the membrane hydraulic conductivity per millimeter tubule length, and from the assumption that sigma raffinose = 1, sigma NaCl was obtained by dividing the JVo/delta pi values from the NaCl studies by those from the raffinose studies. For both S1 and S2 segments, sigma NaCl was found to be approximately 0.5. A similar value was obtained from the rate of cell shrinkage immediately after isosmolar exchange of raffinose for NaCl medium.

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.