Novel Discoveries in Acid-base Regulation and Osmoregulation: A Review of Selected Hormonal Actions in Zebrafish and Medaka

Abstract Maintenance of internal ionic and acid-base homeostasis is critical for survival in all biological systems. Similar to mammals, aquatic fishes have developed sophisticated homeostatic mechanisms to mitigate metabolic or environmental disruptions in ionic and acid-base status of systemic body fluids via hormone-controlled transport of ions or acid equivalents. The present review summarizes newly discovered actions of several hormones in zebrafish ( Danio rerio ) and medaka ( Oryzias latipes ) that have greatly contributed to our overall understanding of ionic/acid-base regulation. For example, isotocin and cortisol were reported to enhance transport of various ions by stimulating the proliferation and/or differentiation of ionocyte progenitors. Meanwhile, stanniocalcin-1, a well-documented hypocalcemic hormone, was found to suppress ionocyte differentiation and thus downregulate secretion of H + and uptake of Na + and Cl − . Estrogen-related receptor and calcitonin gene-related peptide also regulate the differentiation of certain types of ionocytes to either stimulate or suppress H + secretion and Cl − uptake. On the other hand, endothelin and insulin-like growth factor 1 activate the respective secretion of H + and Na + /Cl through fast actions. These new findings enhance our understanding of how hormones regulate fish ionic and acid-base regulation while further providing new insights into vertebrate evolution, mammalian endocrinology and human disease-related therapeutics.