PERMEABILITY AND MORPHOLOGY OF A CULTURED BRANCHIAL EPITHELIUM FROM THE RAINBOW-TROUT DURING PROLONGED APICAL EXPOSURE TO FRESH-WATER

The electrical, structural, and permeability properties of primary cul tures of rainbow trout gill cells on permeable supports were examined after 6 days of growth in culture medium, followed by prolonged (48 hr ) apical exposure to fresh water. Permeability to the paracellular mar ker polyethylene glycol-4000 (PEG) increased significantly over 48 hr, indicating that paracellular permeability increased continuously thro ughout the freshwater exposure. The significant increases measured in the net Na+ and Cl- fluxes were attributed primarily to the opening up of the paracellular pathway. The elevated transepithelial resistance characteristic of the cultured branchial epithelium with fresh water p resent on the apical surface gradually declined during prolonged expos ure, and the decrease (increase in conductance) was greater than that expected on the basis of the increasing paracellular permeability. At any given time, PEG permeability was linearly related to conductance, but PEG permeability per unit conductance decreased significantly over 48 hr. These results are suggestive of an increase in transcellular p ermeability in addition to that in paracellular permeability. Since tr anscellular permeability appears to decrease on first exposure to fres h water, the elevated permeability during prolonged exposure was inter preted as a reopening of the transcellular pathway. A morphological ex amination revealed evidence of damage to the superficial cell layer of the multilayered culture following 48 hr of apical freshwater exposur e, accounting at least in part for the physiological changes observed. Hormonal supplementation of the basal culture medium was examined as a means of enhancing adaptation to apical fresh water. The inclusion o f teleost prolactin (200 ng/L) or teleost growth hormone (200 ng/L) in the basal culture medium had no effect on either the initial values o r the pattern of changes during prolonged exposure for TER, PEG permea bility, and net ion fluxes. (C) 1998 Wiley-Liss, Inc.