Histochemical localization of Na+–K+ ATPase and carbonic anhydrase activity in gills of 17 fish species

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Abstract
Activities of two enzymes considered to be involved in NaCl regulation, Na+–K+ ATPase and carbonic anhydrase, were localized in gill epithelia of 14 teleost, 2 agnathan, and 1 shark species through light microscopic histochemistry. Findings were confirmed by use of appropriate inhibitors (ouabain, acetazolamide). Na+–K+ ATPase activity was detected in chloride cells of most marine teleost species (six of eight) and of marine leopard shark and hagfish, but never in freshwater fish gills. In general, this finding agrees with past biochemical assays showing gill Na+–K+ ATPase activity to be highest in marine teleosts. Staining for carbonic anhydrase took one of three patterns among species: gill pavement cells or chloride cells, or both, were stained. Interspecific distribution of these patterns bore little relation to taxonomy or to habitat salinity, although chloride cells of euryhaline teleosts seemed more likely to stain than chloride cells of stenohaline teleosts, freshwater or marine. Given the lack of a definite correlation with salinity, it is concluded that fish gill carbonic anhydrase may not function in NaCl regulation as much as in acid–base regulation; the enzyme's role in preventing systemic pH imbalance is discussed.

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