Passive and active transport properties of a gill model, the cultured branchial epithelium of the freshwater rainbow trout (Oncorhynchus mykiss).

Branchial epithelia of freshwater rainbow trout were cultured on permeable supports, polyethylene terephthalate membranes ("filter inserts"), starting from dispersed gill epithelial cells in primary culture. Leibowitz L-15 media plus foetal bovine serum and glutamine, with an ionic composition similar to trout extracellular fluid, was used. After 6 days of growth on the filter insert with L-15 present on both apical and basolateral surfaces, the cultured preparations exhibited stable transepithelial resistances (generally 1000-5000 ohms cm2) typical of an electrically tight epithelium. Under these symmetrical conditions, transepithelial potential was zero, and unidirectional fluxes of Na+ and Cl- across the epithelium and permeability to the paracellular marker polyethylene glycol-4000 (PEG) were equal in both directions. Na+ and Cl- fluxes were similar to one another and linearly related to conductance (inversely related to resistance) in a manner indicative of fully conductive passive transport. Upon exposure to apical fresh water, transepithelial resistance increased greatly and a basolateral-negative transepithelial potential developed. At the same time, however, PEG permeability and unidirectional effluxes of Na+ and Cl- increased. Thus, total conductance fell, and ionic fluxes and paracellular permeability per unit conductance all increased greatly, consistent with a scenario whereby transcellular conductance decreases but paracellular permeability increases upon dilution of the apical medium. In apical fresh water, there was a net loss of ions from the basolateral to apical surfaces as effluxes greatly exceeded influxes. However, application of the Ussing flux ratio criterion, in two separate series involving different methods for measuring unidirectional fluxes, revealed active influx of Cl- against the electrochemical gradient but passive movement of Na+. The finding is surprising because the cultured epithelium appears to consist entirely of pavement-type cells.