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Fig. 8 | Zoological Letters

Fig. 8

From: The digestive tract as an essential organ for water acquisition in marine teleosts: lessons from euryhaline eels

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

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