Relative osmotic effects of raffinose, KCl, and NaCl across basolateral cell membrane

Lumen-collapsed segments of rabbit S2 proximal tubule were bathed in isotonic medium and then exposed acutely to a medium made hypertonic by the addition of raffinose, NaCl, KCl, Na gluconate, K gluconate, or choline Cl. The result was a rapid efflux of water and a shrinking of the tubule, which could be measured by video techniques within the first 0.1 s. After reequilibration in isotonic medium, each tubule was then exposed to a second hypertonic medium to provide a direct comparison between two different solutes, either NaCl vs. KCl or raffinose vs. any one of the other solutes. Because raffinose is impermeant across the basolateral cell membrane, the ratio of its effect to that of another solute is a measure of the reflection coefficient (sigma) of that other solute. The following results were obtained: sigma KCl = 0.70 +/- 0.02, sigma K gluconate = 0.97 +/- 0.07, sigma Na gluconate = 0.84 +/- 0.06, and sigma choline Cl = 0.75 +/- 0.06. We previously have reported sigma NaCl = 0.56 +/- 0.07. If sigma of each salt is considered to be the arithmetic average of its component parts, and if gluconate and choline are considered to be impermeant, we also obtain sigma Na+ = 0.68, sigma K+ = 0.94, and sigma Cl- = 0.50.