In vertebrate salt-secreting epithelia, Na + moves passively down an electrochemical gradient via a paracellular pathway. We assessed how this pathway is modified to allow Na + secretion in hypersaline environments. Mummichogs (Fundulus heteroclitus) acclimated to hypersaline [2× seawater (2SW), 64‰] for 30 days developed invasive projections of accessory cells with an increased area of tight junctions, detected by punctate distribution of CFTR (cystic fibrosis transmembrane conductance

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... r) immunofluorescence and transmission electron miscroscopy of the opercular epithelia, which form a gill-like tissue rich in ionocytes. Distribution of CFTR was not explained by membrane raft organization, because chlorpromazine (50 μmol l −1 ) and filipin (1.5 μmol l −1 ) did not affect opercular epithelia electrophysiology. Isolated opercular epithelia bathed in SW on the mucosal side had a transepithelial potential (V t ) of +40.1±0.9 mV (N=24), sufficient for passive Na + secretion (Nernst equilibrium voltage≡E Na =+24.11 mV). Opercular epithelia from fish acclimated to 2SW and bathed in 2SW had higher V t of +45.1±1.2 mV (N=24), sufficient for passive Na + secretion (E Na =+40.74 mV), but with diminished net driving force. Bumetanide block of Cl − secretion reduced V t by 45% and 29% in SW and 2SW, respectively, a decrease in the driving force for Na + extrusion. Estimates of shunt conductance from epithelial conductance (G t ) versus short-circuit current (I sc ) plots (extrapolation to zero I sc ) suggested a reduction in total epithelial shunt conductance in 2SW-acclimated fish. In contrast, the morphological elaboration of tight junctions, leading to an increase in accessory-cell-ionocyte contact points, suggests an increase in local paracellular conductance, compensating for the diminished net driving force for Na + and allowing salt secretion, even in extreme salinities.