OSMOTIC WATER-FLOW PATHWAYS ACROSS NECTURUS GALLBLADDER - ROLE OF THETIGHT JUNCTION

To explore the quantitative significance of passive water flow through tight junctions of leaky epithelia, transepithelial water flow rates were measured in Necturus gallbladder mounted in chambers. Osmotic flo ws generated by raffinose gradients were asymmetrical with the greater flow in the mucosal-to-serosal direction. In tissue fixed in situ, in tercellular spaces were dilated during mucosal-to-serosal flow and clo sed during serosal-to-mucosal flow. Tight junctions were focally separ ated (blistered), which correlated with the magnitude of mucosal-to-se rosal flow. Blisters were not observed during serosal-to-mucosal flow or in nontransporting gallbladders. In freeze-fracture replicas, blist ers appeared as pockets between intramembranous strands. Protamine, wh ich decreases electrical conductance and increases depth and complexit y of the tight junction, reduced osmotic water flow by similar to 30% in the mucosal-to-serosal direction (100 mosmol/kg gradient) without a ltering serosal-to-mucosal flow. We suggest that in the steady state, at least 30% of osmotically driven water passes transjunctionally in t he mucosal-to-serosal direction, but flow is transcellular in the sero sal-to-mucosal direction. Directionally divergent pathways may account for flow asymmetry.