The roles of different cell types involved in ion transport in the fish gill have been widely debated but it is commonly accepted that mitochondria-rich chloride cells and pavement cells play key roles in ion and acid—base transport in freshwater fish. Mitochondria-rich cells function in acid—base regulation by altering chloride and carbonate exchange, while pavement cells are currently believed to be the site of sodium uptake at the gill.

Galvez and colleagues focus on the mitochondria-rich chloride cells in the freshwater trout gill. They use a novel magnetic bead separation technique to isolate different mitochondria-rich cell subtypes. The existence of these cell subtypes in fish gill had been proposed but evidence to support their existence was lacking.

Different sub-types of mitochondria-rich cells in other animals have been identified by looking at the differential binding of peanut lectin agglutinin (PNA) to the apical surfaces of mitochondria-rich cell types. Galvez and colleagues adopted this technique to distinguish between subtypes of mitochondria-rich cells in the fish gill.

Research on the fish gill from their lab indicates that PNA binds to mitochondria-rich chloride cells on the apical surface of the gill epithelium. Here, they use PNA binding to separate mitochondria-rich gill cells into PNA⁺ and PNA^- populations. They identify the existence of at least two mitochondria-rich cell subtypes. The ultrastructure of the PNA⁺ cells was characteristic of mitochondria-rich chloride cells, whereas the PNA^- cell morphology was reminiscent of pavement cells.

The functional properties of these subtypes in acid—base regulation were examined by the expression of two proteins important in epithelial transport. The Na⁺-K⁺-ATPase is involved in energising both sodium and chloride uptake across the gill and the H⁺-ATPase provides an electrochemical gradient that drives sodium movement. The expression of these proteins under normal, acidosis and alkalosis conditions was examined.

Both ATPases were expressed in the PNA⁺ and PNA^- cell types. However, both of these proteins were expressed differently in the two subtypes during acid—base disturbances. Most notably, only the PNA^- cell types responded during acidosis, by increasing expression of the H⁺-ATPase.

The authors suggest that the site of proton excretion in gill tissue is the PNA^- cells and that the PNA⁺ cells are analogous in function to the β-mitochondria-rich chloride cells of the mammalian kidney collecting ducts, which are responsible for base-secretion. In the mammalian kidney, there is a functional separation of mitochondria-rich cell types into acid-secreting and base-secreting cells. Separating and identifying different subtypes in the fish gill is both important and interesting to determine whether the functional separation also occurs in gill tissue.

Galvez and colleagues are now hoping to clone the apical anion exchanger in the PNA⁺ mitochondria-rich chloride cells of the fish gill. The use of magnetic bead separation to enrich for PNA⁺ mitochondria-rich chloride cells should help improve the ability of cloning rare transport proteins on this cell type.