Tight Junction Elaboration Aids Hypersaline Secretion in Euryhaline Teleost Fish

Abstract

We examined how the Na+ conductive paracellular pathway between ionocytes and accessory cells is modified in hypersaline conditions to allow Na+ secretion in extreme environments. Mummichogs (Fundulus heteroclitus) were acclimated to seawater (SW) or hypersaline conditions (2SW, 64 ‰) for 30 days. In 2SW fish, invasive cellular projections of accessory cells with increased area of simple tight junctions were detected by the punctate distribution of CFTR immunofluorescence and by TEM ultrastructure of the opercular epithelium (OE), a gill‐like tissue rich in mitochondrion‐rich ionocytes. Punctate CFTR distribution was not explained by membrane raft organization, as chlorpromazine (50 μM) and filipin (1.5 μM) were ineffective in changing electrophysiology of OE. Isolated OE from SW fish with SW on the mucosal side had a transepithelial potential (Vt) of +40.1 + 0.9 mV (n = 24), sufficient to allow passive Na+ secretion (Nernst equilibrium potential ≡ ENa = +24.11 mV). OE from fish acclimated to 2SW and bathed in 2SW (mucosal side) had significantly higher Vt of +45.1 + 1.2 mV (n = 24; P < 0.001) and plasma Na+ was slightly elevated, sufficient for passive Na+ secretion (ENa = +40.74 mV), but with a severely diminished net driving force. Paradoxically, estimates of shunt conductance from epithelial conductance (Gt ) vs. short‐circuit current (Isc) plots (extrapolation to zero Isc) revealed significant reduction in total epithelial shunt conductance in 2SW acclimated fish. We conclude that whereas most epithelial tight junctions become less conductive in hypersaline conditions, those localized to the paracellular Na+ exit pathway become more conductive, compensating for lower driving force. Funded by NSERC and CFI.