Basolateral NBCe1 plays a rate-limiting role in transepithelial intestinal HCO₃^- secretion, contributing to marine fish osmoregulation

Although endogenous CO₂ hydration and serosal HCO ₃^- are both known to contribute to the high rates of intestinal HCO₃^- secretion important to marine fish osmoregulation, the basolateral step by which transepithelial HCO ₃^- secretion is accomplished has received little attention. Isolated intestine HCO₃^- secretion rates, transepithelial potential (TEP) and conductance were found to be dependent on serosal HCO₃^- concentration and sensitive to serosal DIDS. Elevated mucosal Cl^- concentration had the unexpected effect of reducing HCO₃^- secretion rates, but did not affect electrophysiology. These characteristics indicate basolateral limitation of intestinal HCO₃^- secretion in seawater gulf toadfish, Opsanus beta. The isolated intestine has a high affinity for serosal HCO ₃^- in the physiological range (K_m=10. 2mmoll¹). indicating a potential to efficiently fine-tune systemic acid-base balance. We have confirmed high levels of intestinal tract expression of a basolateral Na⁺/HCO₃- cotransporter of the electrogenic NBCeI isoform in toadfish (tfNBCel), which shows elevated expression following salinity challenge, indicating its importance in marine fish osmoregulation. When expressed in Xenopus oocytes, isolated tfNBCel has transport characteristics similar to those in the isolated tissue, including a similar affinity for HCO₃^- (K_m=8.5mmoll- ¹. Reported affinity constants of NBC1 for Na⁺ are generally much lower than physiological Na⁺ concentrations, suggesting that cotransporter activity is more likely to be modulated by HCO₃- rather than Na⁺ availability in vivo. These similar functional characteristics of isolated tfNBCel and the intact tissue suggest a role of this cotransporter in the high HCO₃^- secretion rates of the marine fish intestine.