The hormonal control of osmoregulation in teleost fish

Hormones are critical to the physiological alterations necessary for ion homeostasis when fish move between freshwater and seawater. Cortisol promotes seawater acclimation through differentiation of salt-secreting mitochondrion-rich cells and ion transport proteins in the gill. The growth hormone/insulin-like growth factor I axis is also important in seawater acclimation and acts in synergy with cortisol. Prolactin (PRL) is important in freshwater acclimation through regulation of ion and water permeability in the gill, gut, and kidney. Cortisol also promotes ion uptake and may interact with PRL during freshwater acclimation. For many species of fish, growth hormone promotes acclimation to seawater, PRL promotes acclimation to freshwater, and cortisol interacts with both hormones, thus having a dual osmoregulatory function.