Influence of temperature and membrane lipid composition on the osmotic water permeability of teleost gills.

Osmotic water uptake was measured gravimetrically in isolated, ligated gill arches from trout (acclimated to and incubated at 5 degrees and 20 degrees C) and tilapia (21.5 degrees and 33 degrees C). For both species, incubation of arches at the higher temperature led to 1.5- to 3-fold greater measures of water weight gain. However, gills from warmer-acclimated trout and tilapia had 1- to >3-fold lower the initial rate and 1.5- to >2.5-fold lower the extent of water uptake seen in colder-acclimated conspecifics. Both the incubation temperature sensitivity and the acclimation effects are consistent with transmembrane water permeation. Calcium-free incubations (permitting paracellular water movement) also indicated that interfacial cell membranes contribute to gill permeability characteristics; without calcium, trout gill osmotic water uptake values increased 1.5- to 2-fold, and the temperature dependence of water uptake decreased (initial rate) or was eliminated (extent). The specific contribution of cholesterol to restricting barrier membrane water permeability was indicated by concentration-dependent increases in water uptake in the presence of either nystatin (a cholesterol-complexing, pore-forming agent) or methyl-beta-cyclodextrin (which selectively depletes membrane cholesterol). In addition, a cholesterol-specific cytochemical probe (filipin) intensely labeled the apical surface membranes of trout and tilapia gill epithelium. In summary, these studies implicate membrane cholesterol in determining water permeability in fish gills.