Carbonic anhydrase cytochemistry in mitochondria‐rich cells of salamander larvae gill epithelium as related to age and H+ and Na+ concentrations

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Abstract
Carbonic anhydrase (CAH) was localized in the mitochondria‐rich cells (MRC) of 1‐week‐old salamander larvae gill epithelium, in both MRC and pavement cells of 6‐week‐old larvae, and in regenerated stems of previously amputated gills. CAH activity of the MRC was measured quantitatively using a microscope densitometric technique. Changes in CAH activity per cell and changes in the numbers of CAH‐positive MRC were followed under different H+ and Na+ concentrations at the two age groups. CAH activity per cell increased with age, whereas the numbers of CAH‐positive MRC dropped. CAH activity per cell in the 1‐week‐old age group reached maximal values at pH 7.4 and stayed relatively high in the more alkaline media. Moderate increases of Na+ concentrations had small but significant effects on increasing CAH activity of gill MRC. When taking into consideration not only the changes in cellular activity but also the changes in the number of CAH‐positive cells under the different acclimation media, an activity index (ICAH) was calculated. Thus, the ICAH in the 1‐week‐old was found to be dependent on the decline of ambient H+ concentrations (expressed as increasing pH), reaching maximal effect at pH 8.0. On the other hand, raising the Na+ concentrations of the acclimation media to 110 and 220 mOsm/liter caused a maximal inhibition of tissue CAH activity as expressed by ICAH. In conclusion, it is suggested that salamander larvae gill MRC take part in the adaptation of the larvae to changing H+ concentrations of their milieu rather than in their adaptation to changes in its osmolality.