Regulation of apical h ⁺-atpase activity and intestinal HCO ³ _- secretion in marine fish osmoregulation

The absorption of Cl ^- and water from ingested seawater in the marine fish intestine is accomplished partly through Cl ^-/HCO ₃ ^- exchange. Recently, a H ^- pump (vacuolar-type H ⁺-ATPase) was found to secrete acid into the intestinal lumen, and it may serve to titrate luminal HCO ₃ ^- and facilitate further Cl ^-/HCO ₃ ^- exchange, especially in the posterior intestine, where adverse concentration gradients could limit Cl ^-/HCO ₃ ^- exchange. The H ⁺ pump is expressed in all intestinal segments and in gill tissue of gulf toadfish (Opsanus beta) maintained in natural seawater. After acute transfer of toadfish to 60 ppt salinity, H ⁺ pump expression increased 20-fold in the posterior intestine. In agreement with these observations was a fourfold-increased H ⁺- ATPase activity in the posterior intestine of animals acclimated to 60 ppt salinity. Interestingly, Na ⁺-K ⁺-ATPase activity was elevated in the anterior intestine and gill, but not in the posterior intestine. Apical acid secretion by isolated intestinal tissue mounted in Ussing chambers fitted with pH-stat titration systems increased after acclimation to hypersalinity in the anterior and posterior intestine, titrating >20% of secreted bicarbonate. In addition, net base secretion increased in hypersalinity-acclimated fish and was ~70% dependent on serosal HCO ₃ ^-. Protein localization by immunohistochemistry confirmed the presence of the vacuolar-type H ⁺-ATPase in the apical region of intestinal enterocytes. These results show that the H ⁺ pump, especially in the posterior intestine, plays an important role in hypersaline osmoregulation and that it likely has significant effects on HCO ₃ ^- accumulation in the intestinal lumen and, therefore, the continued absorption of Cl ^- and water.