Hormonal Control of Osmoregulation in a Stenohaline Catfish, Ictalurus Punctatus

When channel catfish (lchtalurus punctatus) in fresh water (FW) were hypophysectomized, plasma osmolality decreased significantly after 24 hrs, and the maximum reduction was seen after 2 days. When they were transferred from FW to dilute seawater (20-40% SW), plasma osmolality of the sham-operated fish was consistently higher than the osmolality of the environmental water, whereas the osmolality of the hypophysectomized fish was equivalent to the environmental salinity. Ovine prolactin (PRL) restored the reduced osmolality of the hypophysectomized fish in FW to the level of sham-operated fish. Cortisol was also effective, but the effect was less pronounced than the effect of PRL. Injection of PRL in combination with cortisol resulted in a marked increase in plasma osmolality to a level even above that of the sham-operated fish. Ovine growth hormone (GH) was without effect. Replacement therapy in the hypophysectomized fish transferred to dilute SW revealed essentially the same results as in the fish in FW. In the stenohaline catfish, both PRL and cortisol seem to be involved importantly in the ion uptake from the environment not only in FW but also in a saline environment.