REGULATION OF INTERRENAL FUNCTION IN FRESH-WATER AND SEA-WATER ADAPTED TILAPIA (OREOCHROMIS-MOSSAMBICUS)

In teleosts, cortisol is one of the key factors regulating the adaptat ion to environmental challenges, such as salinity changes. This paper compares interrenal function between fully adapted freshwater (FW) and sea water (SW) specimens of the euryhaline teleost Oreochromis mossam bicus (tilapia), combining morphometric and biosynthetic approaches. I nterrenal tissue and two tissues producing interrenal secretagogues (A CTH and ANP; atrial natriuretic peptide) were studied. The results dem onstrate that sea water adaptation concurs with a sustained stimulatio n of the interrenal cells, as evidenced by a marked hyperplasia of the cells and the higher initial ex vivo cortisol release in seawater ada pted tilapia. This difference was not reflected in ultrastructural dif ferences in the pituitary corticotropes. Plasma ACTH levels were also similar in FW and SW adapted tilapia. Moreover, in vitro data indicate that the ACTH sensitivity of the interrenal cells of both groups was also similar. A second potential interrenal secretagogue (ANP) has rec ently been implicated in teleost ionic regulation during salinity chan ges. However, plasma immunoreactive ANP levels and in vitro production of the hormone were also indistinguishable between FW and SW tilapia. ANP pretreatment of tilapia head kidneys in vitro strongly inhibited the response to ACTH, an effect previously undocumented for teleosts. Whereas the sustained stimulation of the interrenal cells under sea wa ter conditions corroborates results obtained with other teleost specie s, thereby supporting a hypoosmoregulatory role for cortisol, it also is evident that notable species differences exist regarding the regula tion of the interrenal gland under these conditions.