Cortisol reduces paracellular permeability and increases occludin abundance in cultured trout gill epithelia

A role for the tight junction (TJ) protein occludin in the regulation of gill paracellular permeability was investigated using primary cultured "reconstructed" freshwater (FW) rainbow trout gill epithelia composed solely of pavement cells. Cortisol treatment reduced epithelial permeability characteristics, measured as changes in transepithelial resistance (TER) and paracellular [3H]PEG-4000 flux. Cortisol also reduced net Na+ flux rates when epithelia were exposed to apical FW. cDNA encoding for the TJ protein occludin was cloned from rainbow trout and found to be particularly abundant in gill tissue. In cultured gill preparations, occludin immunolocalized to the TJ complex and transcript abundance dose-dependently increased in response to cortisol treatment in association with reduced paracellular permeability. Occludin protein abundance also increased in response to cortisol treatment. However, occludin mRNA levels did not change in response to apical FW exposure, and [3H]PEG-4000 permeability did not decrease. These data support a role for occludin in the endocrine regulation of paracellular permeability across gill epithelia of fishes.