Molecular and functional regionalization of bicarbonate secretion cascade in the intestine of the European sea bass (Dicentrarchus labrax)

In marine fish the intestinal HCO3– secretion is the key mechanism to enable luminal aggregate formation and water absorption. Using the sea bass (Dicentrarchus labrax), the present study aimed at establishing the functional and molecular organization of different sections of the intestine concerning bicarbonate secretion and Cl− movements. The proximal intestinal regions presented similar HCO3– secretion rates, while differences were detected in the molecular expression of the transporters involved and on regional HCO3– concentrations. The anterior region presented significantly higher Na+/K+-ATPase activity, Cl− transepithelial transport and basolateral slc4a4, apical slc26a6 and slc26a3 expression levels. In the mid intestine, the total HCO3– content was significantly increased in the fluid as in the carbonate aggregates. In the rectum no HCO3– secretion was observed and was characterized by the diminished HCO3– total content, residual molecular expression of slc4a4, slc26a6 and slc26a3, higher H+-ATPase activity and expression, suggesting the existence of a different bicarbonate handling mechanism. The possible regulation of HCO3– secretion by extracellular HCO3– and increased intracellular cAMP levels were also investigated. cAMP did not affect HCO3– secretion, although Cl− secretion was enhanced by cftr. HCO3– secretion rise due to the HCO3– basolateral increment showed that at resting levels slc4a4 was not a limiting step for secretion. The transcellular/intracellular dependence of apical HCO3– secretion differed between the proximal regions. In conclusion, intestinal HCO3– secretion has a functional region-dependent organization that was not reflected by the anterior-posterior regionalization on HCO3– secretion and expression profiles of chloride/water absorption related genes.