AJP - Gastrointestinal and Liver Physiology, Vol 266, Issue 4 722-G730, Copyright © 1994 by American Physiological Society
Osmotic water flow pathways across Necturus gallbladder: role of the tight junction
K. Loeschke and C. J. Bentzel
Department of Medicine, East Carolina University, School of Medicine, Greenville, North Carolina 27858-4354.
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 flows
generated by raffinose gradients were asymmetrical with the greater flow in
the mucosal-to-serosal direction. In tissue fixed in situ, intercellular
spaces were dilated during mucosal-to-serosal flow and closed during
serosal-to-mucosal flow. Tight junctions were focally separated
(blistered), which correlated with the magnitude of mucosal-to-serosal
flow. Blisters were not observed during serosal-to-mucosal flow or in
nontransporting gallbladders. In freeze-fracture replicas, blisters
appeared as pockets between intramembranous strands. Protamine, which
decreases electrical conductance and increases depth and complexity of the
tight junction, reduced osmotic water flow by approximately 30% in the
mucosal-to-serosal direction (100 mosmol/kg gradient) without altering
serosal-to-mucosal flow. We suggest that in the steady state, at least 30%
of osmotically driven water passes transjunctionally in the
mucosal-to-serosal direction, but flow is transcellular in the
serosal-to-mucosal direction. Directionally divergent pathways may account
for flow asymmetry.
