Why do hagfish have gill "chloride cells" when they need not regulate plasma NaCl concentration?

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Abstract
Two enzymes implicated in branchial ion transport, Na+-K+-ATPase and carbonic anhydrase, were localized in gill iononcytes ("chloride cells") of the Pacific hagfish, Eptatretus stouti, by light microscopic histochemical techniques. In hagfish, ouabin-sensitive Na+-K+-ATPase activity was confined to apical halves of ionocytes, where most of the cytoplasmic tubular potassium cyanate sensitive carbonic anhydrase activity occurred throughout the cytoplasm and nucleus of hagfish ionocytes. Biochemical assay of hagfish gill homogenates for Na+-K+-ATPase yielded a specific activity of 3.1 .mu.mol Pi .cntdot. .mu.mol Pi .cntdot. mg protein-1 .cntdot. h-1 at 37.degree. C. This resembles values we obtained for freshwater fish (Carassius auratus: 3.3 .mu.mol Pi .cntdot. mg protein-1; Tilapia shirana: 3.7 .mu.mol Pi .cntdot. mg protein-1 .cntdot. h-1), and is less than values we obtained for marine teleosts (Pomacentrus spp.: 13 .mu.mol Pi .cntdot. mg protein-1 .cntdot. h-1; Gillichthys mirabilis: 6.7 .mu.mol Pi .cntdot. mg protein-1 .cntdot. h-1). Hagfish resemble freshwater teleosts in many other gill features related to ion transport. The presence of carbonic anhydrase in gill ionocytes of hagfish supports the proposal that these cells function in acid-base regulation, i.e., that they exchange H+ for Na+ and HCO3- for Cl-.