Regulation of apical H+-ATPase activity and intestinal HCO3- secretion in marine fish osmoregulation

Guffey S, Esbaugh A, Grosell M. Regulation of apical H+-ATPase activity and intestinal HCO3- secretion in marine fish osmoregulation. Am J Physiol Regul Integr Comp Physiol 301: R1682-R1691, 2011. First published August 24, 2011; doi:10.1152/ajpregu.00059.2011.-The absorption of Cl- and water from ingested seawater in the marine fish intestine is accomplished partly through Cl-/HCO3- 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 HCO3- and facilitate further Cl-/HCO3- exchange, especially in the posterior intestine, where adverse concentration gradients could limit Cl-/HCO3- 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 similar to 70% dependent on serosal HCO3-. 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 HCO3- accumulation in the intestinal lumen and, therefore, the continued absorption of Cl- and water.