Aquaporin expression and cholesterol content in eel swimbladder tissue

Leakiness of the swimbladder wall of teleost fishes must be prevented to avoid diffusional loss of gases out of the swimbladder. Guanine incrustation as well as high concentrations of cholesterol in swimbladder membranes in midwater and deep-sea fish have been connected to a reduced gas permeability of the swimbladder wall. On the other hand, the swimbladder is filled by diffusion of gases, mainly oxygen and CO₂ , from the blood and the gas gland cells into the swimbladder lumen. In swimbladder tissue of the zebrafish and the Japanese eel aquaporin mRNA has been detected, and the aquaporin protein has been considered important for the diffusion of water, which may accidentally be gulped by physostome fish when taking an air breath. In the present study expression of two aquaporin 1 genes (Aqp1aa and Aqp1ab) in swimbladder tissue of the European eel, a functional physoclist fish, was assessed by immunohistochemistry, and expression of both genes was detected in endothelial cells of swimbladder capillaries as well as in basolateral membranes of gas gland cells. In addition, Aqp1ab was present in apical membranes of swimbladder gas gland cells. We also found high concentrations of cholesterol in these membranes, which were several-fold higher than in muscle tissue membranes. In yellow eels the cholesterol concentration exceeded the concentration detected in silver eel swimbladder membranes. We suggest that aquaporin 1 in swimbladder gas gland cells and endothelial cells facilitates CO₂ diffusion into the blood, enhancing the switch-on of the Root effect, which is essential for the secretion of oxygen into the swimbladder. It may also facilitate CO₂ diffusion into the swimbladder lumen along the partial gradient established by CO₂ production in gas gland cells. Cholesterol has been shown to reduce the gas permeability of membranes and thus could contribute to the gas tightness of swimbladder membranes, which is essential to avoid diffusional loss of gas out of the swimbladder. This article is protected by copyright. All rights reserved.