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

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.