Esophageal desalination is mediated by Na⁺, H⁺ exchanger-2 in the gulf toadfish (Opsanus beta)

Esophageal desalination is a crucial step in the gastrointestinal water absorption pathway, as this pre-intestinal processing establishes the osmotic conditions necessary for water absorption. Previous work has shown that esophageal Na⁺ absorption is amiloride sensitive; however, it is as yet unclear if Na⁺, H⁺ exchangers (NHE) or Na⁺ channels (ENaC) are responsible. The purpose of the current study was therefore to investigate the roles that NHE isoforms may play in this process in a marine teleost, the gulf toadfish (Opsanus beta), as well as what role NHE isoforms may play in the downstream intestinal Na⁺ transport. A combination of symmetrical current clamp and asymmetrical voltage clamp experiments showed the esophagus to contain both an ion absorptive current (I_sc=0.83±0.68) and serosal side negative transepithelial potential (TEP=-4.9±0.6). ²²Na uptake (J^Na_m→s) was inhibited by 0.5mM EIPA, with no effect of 0.1mM amiloride, 1mM furosemide or 1mM thiazide. A Cl^- free saline reduced J^Na_m→s by 40% while also reducing conductance and reversing TEP. These results suggest that both transcellular and paracellular components contribute to esophageal Na⁺ transport, with transcellular transport mediated by NHE. The NHE1, NHE2 and NHE3 genes were amplified and tissue distribution analysis by real-time PCR showed high NHE2 expression levels in the esophagus and stomach. Little NHE3 expression was observed throughout the gastrointestinal tract, and NHE2 expression was absent from the intestine. Hypersalinity (60ppt) had no effect on the expression profile of NHE2, slc4a2, scl26a6, CAc or V-type ATPase (β-subunit), suggesting that esophageal desalination is less flexible in response to osmotic stress than the intestine.