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

SUMMARY Although endogenous CO2 hydration and serosal HCO3– are both known to contribute to the high rates of intestinal HCO3– secretion important to marine fish osmoregulation, the basolateral step by which transepithelial HCO3– 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 HCO3– in the physiological range (Km=10.2 mmol l–1), 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 NBCe1 isoform in toadfish (tfNBCe1), which shows elevated expression following salinity challenge, indicating its importance in marine fish osmoregulation. When expressed in Xenopus oocytes, isolated tfNBCe1 has transport characteristics similar to those in the isolated tissue, including a similar affinity for HCO3– (Km=8.5 mmol l–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 tfNBCe1 and the intact tissue suggest a role of this cotransporter in the high HCO3– secretion rates of the marine fish intestine.