Nuclear binding of cortisol in intestinal mucosa and liver of a teleost fish (Salmo gairdnerii irideus)

Nuclear binding of corticosteroids in fish tissues was investigated in two target organs of the trout $\text{Salmo gairdnerii irideus}$: the liver and the intestinal mucosa. Incubation of intact nuclei with ³H]-cortisol or ³H]-dexamethasone failed to demonstrate high-affinity binding of these steroids to proteins. Exchange assay of ³H]-cortisol in high-salt nuclear extracts indicated an association constant Ka = 1.9 × 10⁴ M^?1 for intestinal mucosa and 2.1 × 10⁴ M^?1 for liver. In sea water-adapted trout, the association constant remained the same as in fresh water.These results extend previous observations obtained on the cytosol which showed that no high-affinity receptors could be disclosed in fish tissues using these two corticosteroids.