Renal carbonic anhydrase in the quail Coturnix coturnix japonica: I. Activity and distribution in male and female metanephros.

Carbonic anhydrase activity was studied in the quail metanephros by means of histochemical, histophotometrical and biochemical methods. Male and female samples were examined separately in order to show sex-related differences in enzyme activity and localization. The staining patterns revealed differential distribution of reaction product in the different tubular segments. The initial portion of proximal tubules showed positivity on the brush border in female kidneys only. Extra situ investigations provided further evidence of sexual dimorphism resulting in higher values of enzyme activity for female than for male kidneys. In both sexes, marked staining was detected at the distal tubule level where histophotometric analysis confirmed the highest amount of reaction product. Moreover, the intracellular staining distribution at this site proved to be similar to that observed for mammalian proximal convoluted tubules. In the collecting ducts, a mosaic-like pattern was found with respect to both carbonic anhydrase staining and metachromatic properties. The functional significance of the presence of enzyme in the different renal tubules is discussed by comparison with the mammalian kidney. A model is proposed whereby the distal tubules represent the main sites of urinary acidification and bicarbonate reabsorption.