Localization of carbonic anhydrase in swimbladder of European eel (Anguilla anguilla) and perch (Perca fluviatilis).

The distribution of carbonic anhydrase in swimbladder tissue and especially in gas gland cells of the European eel (Anguilla anguilla) and the perch (Perca fluviatilis) was analysed using histochemical staining according to Hansson (1967), with modifications proposed by Riddersträle (1991). While in the European eel, gas gland cells are distributed as a single layered epithelium over the whole secretory part of the swimbladder, the gas gland of the perch consists of a compact, richly vascularized 'multilayered' epithelium, in which gas gland cells have contact with the swimbladder lumen via small channels. In spite of these differences in organization, membranes of gas gland cells near blood vessels are richly folded in both species. A strong histochemical staining for carbonic anhydrase was observed in these membrane foldings. With prolonged incubation times a positive reaction was also observed in the cytoplasm of gas gland cells. In addition, the vascular endothelium and the erythrocytes showed a positive histochemical reaction. No staining, however, was visible in apical membranes towards the lumen of the swimbladder. In the perch, swimbladder epithelial cells outside the gas gland showed no positive staining of carbonic anhydrase. The results thus indicate that carbonic anhydrase activity is especially concentrated in membranes facing blood vessels. This suggests that a rapid equilibrium of the CO2/HCO3- reaction in the intracellular as well as in the extracellular space is essential for swimbladder function.