GROSELL, M.*; TAYLOR, J.R.; GENTZ, J.; University of Miami; University of Miami; University of Miami: Marine teleost osmoregulation involves highly acidic and hyperosmotic fluid absorption by the intestine

In marine teleost fish, continuous diffusive water loss is compensated by seawater ingestion. Imbibed seawater is desalinized by the esophagus resulting in roughly iso-osmotic intestinal fluids allowing for water absorption across the intestinal epithelium. In addition to apical Na⁺:K⁺:2Cl^– co-transport (NKCC), apical anion exchange (AE) contributes by as much as 70 % to Cl^– uptake and thereby water absorption. In the gulf toadfish, Opsanus beta, hydration of endogenous epithelial CO₂ provides most of the HCO₃^– for exchange with Cl^– across the apical membrane. The H⁺ resulting from the CO₂ hydration reaction is excreted across the basolateral membrane via Na⁺/H⁺ exchange (NHE) which rely on the Na⁺ gradient established by the lateral Na⁺/K⁺-ATPase (NKA). The high intestinal HCO₃^– secretion rates (0.3-0.4 �mol cm²/h) resulting from AE suggest high metabolic rate of the intestinal epithelium which is consistent with high levels of NKA and a high density of mitochondria. Both secondary active HCO₃^– secretion and Cl^– uptake via AE as well as Na⁺ and Cl^– absorption via NKCC is ultimately fueled by the activity of NKA. The metabolic waste product, CO₂, arising in part from the oxidative phosphorylation required to sustain NKA activity is used (after being hydrated) as a counter ion for Cl^– uptake via AE resulting in highly energy efficient Cl^– absorption. Furthermore, the exchange of cellular CO₂, which is osmotically benign, for Cl^– uptake provides an osmotic driving force for water absorption. The basolateral H⁺ extrusion results in a highly acidic absorbate with a calculated osmotic pressure of 500-600 mOsm. (Supported by NSF IAB 0416440).