Sucrose fluxes and junctional water flow across Necturus gall bladder epithelium

When sucrose is present in a luminal solution bathing Necturus gall bladder epithelium, the secreted fluid is also found to contain sucrose at more than 0.9 of the luminal concentration. A first-order convection–diffusion equation has been used to calculate the emergent sucrose con­centration, by using values for the dimensions of the paracellular system and assuming that the sucrose flow is extracellular. The results indicate that most of the sucrose flow across the junctions must be convective in nature, and the size of the junctional water flow required amounts to most of the overall epithelial water flow, i. e. during secretion the fluid is generated by junctional flow and not by standing-gradient osmosis over the membrane lining the interspaces. The junctional complexes at the luminal side of the interspaces are not zonulae occludentes but parallel alignments of the cell membranes with a constant (peak to peak) spacing of 14.2 nm. As the reflexion coefficient of these junctions for salt is unknown, but could be low, it is by no means certain that the junctional water flow is driven by osmosis.