Fig. 8

A suite of transporters and enzymes form a metabolon at the apical membrane of enterocytes for H⁺ and HCO₃⁻ secretion and recycling to form carbonate precipitates in SW eels. VHA may also be involved in H⁺ secretion into the lumen (see Fig. 7). This idea of metabolon formation was first proposed in the mammalian intestine and may be applicable to the teleost intestine (see 6.3). H⁺ and HCO₃⁻ secreted into the lumen by apical transporters are catalyzed by membrane-bound CAIV to produce CO₂ and H₂O, which are recycled into the cell via AQP1. Cytosolic CAII then produces H⁺ and HCO₃⁻ again for continuous supply of the ions into the lumen. NHE3, SLC26a3/6 and CFTR are bound on NHERF for mutual regulation. CAIV also facilitates Mg/CaCO₃ production in the microenvironment under the mucus layer, which is maintained by continuous secretion of mucus from goblet cells (see 4.5). CAII is known to bind to the NHE/AE complex and to AQP to regulate their activity. CFTR and SLC26a3/6 bind through the R domain and the STAS domain of each transporter. For details, see 6.3. For abbreviation definitions, see the list