Molecular mechanisms of Cl^- transport in fishes: New insights and their evolutionary context

With remarkably few exceptions, aquatic vertebrates maintain internal Cl^- homeostasis despite strong and sometimes fluctuating Cl^- concentration gradients between extracellular fluids and external environments. In this "Perspective," we discuss recent advances in the understanding of epithelial Cl^- transport at the molecular level within key osmoregulatory organs in fishes. New insights into mechanisms for epithelial Cl^- transport in basal lineages are highlighted to provide an evolutionary context. We describe Cl^- transport processes that employ: cystic fibrosis transmembrane conductance regulator, cation-chloride cotransporters, voltage-gated chloride channels, and chloride-anion exchangers. As the collective understanding of Cl^- transport processes continues to expand, investigators are equipped to more precisely characterize how endocrine factors promote hydromineral balance. We, therefore, conclude our discussion by paying special attention to recently defined roles for prolactin and corticosteroids in the regulation of Cl^- transport in basal and derived clades.