GROSELL, M.; TAYLOR, J.; University of Miami: Intestinal bicarbonate secretion is a novel aspect of marine teleost osmoregulation

Drinking and intestinal water absorption forms a critical component of marine teleost fish osmoregulation. Ingested seawater is desalinized in the esophagus prior to entering the intestine where active absorption of Na⁺ and Cl^– drives water absorption. In addition to salt absorption via co-transport systems apical anion exchange is important for osmoregulation accounting for as much as 70% of the intestinal Cl^– absorption. The apical anion exchange process perform active HCO₃^– secretion and active Cl^– absorption which in the gulf toadfish is driven by extrusion of H⁺ across the basolateral membrane. The cellular substrate for the Cl^–/HCO₃^– exchange is endogenous CO₂ arising from epithelial respiration with CO₂ hydration facilitated in part by carbonic anhydrase. The HCO₃^– and H⁺ resulting from CO₂ hydration is exchanged for Cl^– across the apical membrane and Na⁺ across the basolateral membrane, respectively. Basolateral H⁺ extrusion by Na⁺/H⁺ exchange is fueled by the electrochemical Na⁺ gradient maintained by the basolateral Na⁺/K⁺-ATPase. The energy required for the active transport of Cl^– and HCO₃^– is thus provided by the activity of the Na⁺/K⁺-ATPase enzyme. Hydration of CO₂ provides a net gain of an osmolyte (Cl^–) providing osmotic drive for water absorption which is critical for successful marine osmoregulation. The anion exchange process results in luminal HCO₃^– concentrations of ~ 100 mM with pH approaching 9.0 and voided rectal fluids therefore accounts for substantial base excretion. Although rectal base secretion is not involved in dynamic acid-base balance regulation, variations in intestinal HCO₃^– secretion is matched by net acid excretion across the gills. (Supported by NSF-IBN 0416440 and a University of Miami graduate student fellowship).