Diverse mechanisms for body fluid regulation in teleost fishes

Abstract

Teleost fishes are the major group of ray-finned fishes and represent more than one-half of the total number of vertebrate species. They have experienced in their evolution an additional third-round whole genome duplication just after the divergence of their lineage, which endowed them with an extra adaptability to invade various aquatic habitats. Thus their physiology is also extremely diverse compared with other vertebrate groups as exemplified by the many patterns of body fluid regulation or osmoregulation. The key osmoregulatory organ for teleosts, whose body fluid composition is similar to mammals, is the gill, where ions are absorbed from or excreted into surrounding waters of various salinities against concentration gradients. It has been shown that the underlying molecular physiology of gill ionocytes responsible for ion regulation is highly variable among species. This variability is also seen in the endocrine control of osmoregulation where some hormones have distinct effects on body fluid regulation in different teleost species. A typical example is atrial natriuretic peptide (ANP); ANP is secreted in response to increased blood volume and acts on various osmoregulatory organs to restore volume in rainbow trout as it does in mammals, but it is secreted in response to increased plasma osmolality, and specifically decreases NaCl, and not water, in the body of eels. The distinct actions of other osmoregulatory hormones such as growth hormone, prolactin, angiotensin II, and vasotocin among teleost species are also evident. We hypothesized that such diversity of ionocytes and hormone actions among species stems from their intrinsic differences in body fluid regulation that originated from their native habitats, either fresh water or seawater. In this review, we summarized remarkable differences in body fluid regulation and its endocrine control among teleost species, although the number of species is still limited to substantiate the hypothesis.

Guanylin activates Cl−secretion into the lumen of seawater eel intestine via apical Cl−channel under simulated in vivo conditions

“…The presence of CFTR(inh)-172-inhibitable Cl Ϫ channel and immunoreactive CFTR has been suggested in the toadfish (28) although the inhibitor was without effect in eels. It seems that the specificity of CFTR antibody needs to be confirmed because the CFTR sequence is highly variable, and even antiserum raised against teleost CFTR is species specific (32). Furthermore, the expression of cftr gene was very small in eel intestine compared with other functional transporters such as those in the NKC system, and the expression rather decreased after seawater acclimation (5).…”

clc-2c is regulated by salinity, prolactin and extracellular osmolality in tilapia gill

“…So-called ‘fast-acting’ hormones direct acute responses such as alterations of ion transport, drinking behavior and cardiovascular function, whereas ‘slow-acting’ hormones modulate osmoregulatory tissues by altering patterns of gene expression and cell proliferation and/or differentiation (McCormick 2001, Takei et al 2014). One ‘slow-acting’ factor, the hypophyseal hormone prolactin (Prl), is considered the ‘FW-adapting hormone’ given its actions to stimulate ion-conserving and water-excreting processes (Hirano 1986).…”

Evolution of Endolymph Secretion and Endolymphatic Potential Generation in the Vertebrate Inner Ear

“…7) [Becerra and Anadon, 1993]. To our knowledge, ionocytes of the teleost inner ear have not been tested for the presence of NKA or NKCC; however, ionocytes of the teleost gill are well known to express both [Takei et al, 2014].…”

“…However, the variety of ionocytes in freshwater teleosts suggests an apomorphic (derived) condition. It has been argued that the uniform ionocyte type of the gills of saltwater fish represents the plesiomorphic condition for teleosts and that it is characterized by expressing NKA, NKCC1, CFTR and an inwardly rectifying K + channel [Hwang et al, 2011;Takei et al, 2014]. This is largely similar to dark cells (except CFTR, cystic fibrosis transmembrane regulator Cl -channel).…”