Esophageal desalination of seawater in flounder: role of active sodium transport.

The esophagus of the flounder, Pseudopleuronectes americanus, was studied to determine how salinity of ingested seawater (SW) is decreased before fluid absorption in the intestine. Drinking rate was 2.5 ml X h-1 X kg-1. Stomach fluid osmolality was 45% that of seawater, and intestinal fluid was isosmotic to plasma. Esophagus and stomach were nearly impermeable to 28Mg; thus Mg concentrations were accurate indicators of fluid addition and NaCl removal between pharynx and stomach. Measurements of water and ion fluxes across isolated esophageal epithelium mounted in Ussing chambers and bathed by Ringer solution showed that the tritiated water flux was lower in esophagus than in intestine and that 22Na flux ratio was 1.4 (Jm leads to s/Js leads to m) regardless of acclimation medium (100 or 10% SW). Potential difference was zero, and electrical resistance averaged 90 omega X cm2. Mucosal-to-serosal Na transport was inhibited by 0.1 mM amiloride, 0.1 mM ouabain, and Cl-free medium, whereas 1.0 mM furosemide had no effect. Net esophageal Na absorption (mucosal-to-serosal) averaged 10.0 mumol X h-1 X cm-2 with mucosa exposed to SW and was inhibited 46% by 0.1 mM ouabain. Taken together the above observations suggest a role for both passive and active esophageal Na transport in SW desalination.