ABSTRACT
This chapter focuses on the endocrine mechanisms that control the overall capacity of the ion regulatory mechanisms in teleost fish, focusing on the osmoregulatory actions of prolactin (PRL), the growth hormone (GH)/insulin-like growth factor I (IGF-I) axis and cortisol. PRL has been shown to regulate several aspects of the ion regulatory mechanisms that are characteristic of freshwater (FW) fish. In FW, teleosts must counteract the passive loss of ions and gain of water by actively taking up ions, and removing excess water by excreting a dilute urine. Cortisol is the major corticosteroid produced by the interrenal tissue of teleost fish. This hormone has several established physiological roles related to osmoregulation, intermediary metabolism, growth, stress and immune function. In addition to the independent osmoregulatory actions of PRL, GH/IGF-I axis and cortisol, there is substantial evidence indicating the existence of synergy and antagonism of these hormones with one another.
