COUPLED TRANSPORT OF SOLUTE AND WATER ACROSS RABBIT GALLBLADDER EPITHELIUM.

Recent studies of the isolated fish gallbladder by Diamond (2-4) showed that active, electrically neutral transport of sodium chloride provides the driving force for the absorption of water from the gallbladder lumen. The extent to which water absorption is coupled to active solute transport was indicated by the demonstration of net water absorption against significant osmotic gradients across the gallbladder wall. Similar findings were reported in studies of the isolated rabbit gallbladder (5) in which it was also demonstrated that bicarbonate participates in active solute transport. Transport of water against osmotic gradients was demonstrated in the canine gallbladder inl vivo by Grim (6), who suggested that the findings were due to pinocytosis. "Uphill" water transport was also demonstrated in isolated intestinal segments by Parsons and Wingate (7). In an effort to find a physical explanation for the remarkable coupling between solute and water movement observed in certain tissues, Curran (8) proposed and with McIntosh and Ogilvie successfully tested (9, 10) a model consisting of two membranes arranged in series, which could account for apparent "uphill" transport of water. Patlak, Goldstein, and Hoffman (11) recently published a detailed mathematical treatment of the Curran model from which they were able to predict a number of unexpected characteristics of such a system.