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

In marine fish the intestinal HCO 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 HCO secretion rates, while differences were detected in the molecular expression of the transporters involved and on regional HCO 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 HCO content was significantly increased in the fluid as in the carbonate aggregates. In the rectum no HCO secretion was observed and was characterized by the diminished HCO 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 HCO secretion by extracellular HCO and increased intracellular cAMP levels were also investigated. cAMP did not affect HCO secretion, although Cl secretion was enhanced by cftr. HCO secretion rise due to the HCO basolateral increment showed that at resting levels slc4a4 was not a limiting step for secretion. The transcellular/intracellular dependence of apical HCO secretion differed between the proximal regions. In conclusion, intestinal HCO secretion has a functional region-dependent organization that was not reflected by the anterior-posterior regionalization on HCO secretion and expression profiles of chloride/water absorption related genes.