Hormonal control of osmoregulation in the channel catfish Ictalurus punctatus

Prolactin (PRL) is an important hormone for freshwater adaptation in many teleost species. In some euryhaline fishes, growth hormone (GH) and cortisol are involved in seawater adaptation by stimulating ion extrusion. When channel catfish (Ictalurus punctatus) were transferred from fresh water to dilute seawater (300-400 mOsm), their plasma osmolality was always higher than the environmental salinity. In correlation with the increase in plasma osmolality, significant increases in plasma cortisol were observed. However, no effect of ovine GH or cortisol was seen in plasma osmolality or gill Na, K-ATPase activity when the hormones were given during the course of acclimation to dilute seawater. When catfish in fresh water were hypophysectomized, plasma osmolality was significantly decreased by 24 h, reaching a minimum level after 2 days. When they were transferred to dilute seawater, the plasma osmolality of the sham-operated fish was consistently higher than that of environmental water, whereas the osmolality of the hypophysectomized fish was equivalent to the environmental salinity. Ovine PRL restored the plasma osmolality of the hypophysectomized fish in fresh water 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 additive increase in plasma osmolality to a level even above that of the sham-operated fish. Ovine GH was without effect. These treatments in hypophysectomized fish transferred to dilute seawater produced essentially the same results as those in fish in fresh water. Plasma osmolality was also increased after PRL treatment of the intact fish in fresh water. There was a synergistic effect between PRL and cortisol in hypophysectomized fish in dilute seawater as well as in intact fish in fresh water. PRL did not stimulate cortisol secretion either in hypophysectomized fish or in intact fish. In the stenohaline catfish, both PRL and cortisol seem to be involved importantly in ion uptake from the environment not only in fresh water but also in brackish water.