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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

Intestinal carbonic anhydrase, bicarbonate, and proton carriers play a role in the acclimation of rainbow trout to seawater

¹Rosensteil School of Marine Atmospheric Sciences, University of Miami, Florida; and ²University of Ottawa, Ottawa, Ontario, Canada

Submitted 2 March 2007 ; accepted in final form 28 August 2007

Abrupt transfer of rainbow trout from freshwater to 65% seawater caused transient disturbances in extracellular fluid ionic composition, but homeostasis was reestablished 48 h posttransfer. Intestinal fluid chemistry revealed early onset of drinking and slightly delayed intestinal water absorption that coincided with initiation of NaCl absorption and HCO₃^– secretion. Suggestive of involvement in osmoregulation, relative mRNA levels for vacuolar H⁺-ATPase (V-ATPase), Na⁺-K⁺-ATPase, Na⁺/H⁺ exchanger 3 (NHE3), Na⁺-HCO₃^– cotransporter 1, and two carbonic anhydrase (CA) isoforms [a general cytosolic isoform trout cytoplasmic CA (tCAc) and an extracellular isoform trout membrane-bound CA type IV (tCAIV)], were increased transiently in the intestine following exposure to 65% seawater. Both tCAc and tCAIV proteins were localized to apical regions of the intestinal epithelium and exhibited elevated enzymatic activity after acclimation to 65% seawater. The V-ATPase was localized to both basolateral and apical regions and exhibited a 10-fold increase in enzymatic activity in fish acclimated to 65% seawater, suggesting a role in marine osmoregulation. The intestinal epithelium of rainbow trout acclimated to 65% seawater appears to be capable of both basolateral and apical H⁺ extrusion, likely depending on osmoregulatory status and intestinal fluid chemistry.

water absorption; HCO₃^– secretion; intestinal H⁺ transport; tCAIV; tCAc

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