Postprandial acid-base balance and ion regulation in freshwater and seawater-acclimated European flounder, Platichthys flesus

The effects of feeding on both acid-base and ion exchange with the environment, and internal acid-base and ion balance, in freshwater and seawater-acclimated flounder were investigated. Following voluntary feeding on a meal of 2.5-5% body mass and subsequent gastric acid secretion, no systemic alkaline tide or respiratory compensation was observed in either group. Ammonia efflux rates more than doubled from 489 +/- 35 and 555 +/- 64 mu mol kg(-1) h(-1) under control conditions to 1,228 +/- 127 and 1,300 +/- 154 mu mol kg(-1) h(-1) post-feeding in freshwater and seawater-acclimated fish, respectively. Based on predictions of gastric acid secreted during digestion, we calculated net postprandial internal base gains (i.e., HCO3- secreted from gastric parietal cells into the blood) of 3.4 mmol kg(-1) in seawater and 9.1 mmol kg(-1) supercript stopin freshwater-acclimated flounder. However, net fluxes of ammonia, titratable alkalinity, Na+ and Cl- indicated that branchial Cl-/HCO3- and Na+/H+ exchange played minimal roles in counteracting these predicted base gains and cannot explain the absence of alkaline tide. Instead, intestinal Cl-/HCO3- exchange appears to be enhanced after feeding in both freshwater and seawater flounder. This implicates the intestine rather than the gills as a potential route of postprandial base excretion in fish, to compensate for gastric acid secretion.