Paracellular pathway remodeling enhances sodium secretion by teleost fish in hypersaline environments

Abstract

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 (2SW, 64 ‰) for 30 days developed invasive projections of accessory cells with increased area of tight junction, detected by punctate distribution of CFTR immunofluorescence and TEM of opercular epithelia (OE), a gill-like tissue rich in ionocytes. CFTR distribution was not explained by membrane raft organization, as chlorpromazine (50 μM) and filipin (1.5 μM) did not affect OE electrophysiology. Isolated OE bathed in SW on the mucosal side had a transepithelial potential (Vt) of +40.1+0.9 mV (n=24), sufficient for passive Na+ secretion (Nernst equilibrium voltage≡ENa=+24.11 mV). OE from fish acclimated to 2SW bathed in 2SW had higher Vt of +45.1+1.2 mV (n=24), sufficient for passive Na+ secretion (ENa=+40.74 mV), but with diminished net driving force. Bumetanide block of Cl− secretion reduced Vt 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 (Gt ) vs. short-circuit current (Isc) plots (extrapolation to zero Isc) suggested a reduction in total epithelial shunt conductance in 2SW acclimated fish. In contrast, the morphological elaboration of tight junctions, an increase in accessory cell-ionocyte contact points, suggests an increase local paracellular conductance, compensating for the diminished net driving force for Na+ and allowing salt secretion even in extreme salinities.