The endocrine system mediates many of the physiological responses to the homeostatic and acclimation demands of salt and water transport. Many of the hormones involved in the control of salt and water transport are common to all vertebrates, although their precise function and target tissues have changed during evolution. Arginine vasopressin (vasotocin), angiotensin II, natriuretic peptides, vasoactive intestinal peptide, urotensin II, insulin and non-genomic actions of corticosteroids are involved in acute (minutes and hours) alterations in ion and water transport. This rapid alteration in transport is primarily the result changes in behavior, blood Xow to osmoregulatory organs, and membrane insertion or activation (e.g., phosphorylation) of existing transport proteins, ion and water channels, contransporters and pumps. Corticosteroids (through genomic actions), prolactin, growth hormone, and insulin-like growth factor I primarily control long-term (several hours to days) changes in transport capacity that are the result of synthesis of new transport proteins, cell proliferation, and diVerentiation. In addition to the important task of establishing broad evolutionary patterns in hormones involved in ion regulation, comparative endocrinology can determine species and population level diVerences in signaling pathways that may be critical for adaptation to extreme or rapidly changing environments. 2006 Published by Elsevier Inc.Keywords: Osmoregulation; Vertebrates; Ion transport; AVP; AVT; ANP; Angiotensin; Aldosterone; Cortisol; Growth hormone; Prolactin; IGF-I
Physiological requirements for salt and water transportMaintenance of constant intracellular and extracellular ionic and osmotic environment (Bernard's constancy of 'le milieu intérieur') is critical for the normal functioning of cells. With several notable exceptions, such as hagWsh, sharks and ureotelic marine frogs, the majority of vertebrates maintain a remarkably similar salt content of their extracellular Xuid, approximately one-third that of seawater. This basic strategy results in diVerent transport demands for vertebrates depending on their external environment. In fresh water environments vertebrates must actively take up salts, whereas in seawater they must secrete excess salts. In terrestrial environments vertebrates must conserve water. The demands for ion and water transport can vary greatly, depending on both internal factors such as metabolic rate, and external factors such as salinity or water availability.Hormones play a critical role in signaling and controlling the homeostatic and acclimation demands of salt and water transport (Bentley, 1998). In spite of the diVerences in transport needs and capabilities among vertebrates (and even the organs responsible for ion transport) many of the hormones involved are remarkably similar. In addition to acting on the basic mechanisms of ion transport, natural selection will act on the underlying neuroendocrine controls. Our understanding of large evolutionary trends (e.g., evolution of...
