Abstract
Ca2+ is a vital element for many physiological processes in vertebrates, including teleosts, which live in aquatic environments and acquire Ca2+ from their surroundings. Ionocytes within the adult gills or larval skin are critical sites for transcellular Ca2+ uptake in teleosts. The ionocytes of zebrafish were found to contain transcellular Ca2+ transporters, epithelial Ca2+ channel (ECaC), plasma membrane Ca2+-ATPase 2 (PMCA2), and Na+/Ca2+ exchanger 1b (NCX1b), providing information about the molecular mechanism of transcellular Ca2+ transports mediated by ionocytes in fish. However, more evidence is required to establish whether or not a similar mechanism of transcellular Ca2+ transport also exists in others teleosts. In the present study, ecac, pmca2, and ncx1 were found to be expressed in the branchial ionocytes of tilapia, thereby providing further support for the mechanism of transcellular Ca2+ transport through ionocytes previously proposed for zebrafish. In addition, we also reveal that low Ca2+ water treatment of tilapia stimulates Ca2+ uptake and expression of ecac and cyp11b (the latter encodes a cortisol-synthesis enzyme). Treatment of tilapia with exogenous cortisol (20 mg/l) enhanced both Ca2+ influx and ecac expression. Therefore, increased cyp11b expression is suggested to enhance Ca2+ uptake capacity in tilapia exposed to low Ca2+ water. Furthermore, the application of cortisol receptor antagonists revealed that cortisol may regulate Ca2+ uptake through glucocorticoid and/or mineralocorticoid receptor (GR and/or MR) in tilapia. Taken together, the data suggest that cortisol may activate GR and/or MR to execute its hypercalcemic action by stimulating ecac expression in tilapia.
Highlights
The maintenance of Ca2+ homeostasis is important because Ca2+ is involved in many physiological activities, such as muscle contraction, neuron excitation, and bone formation in vertebrates (WendelaarBonga and Pang 1991)
The mRNA expression patterns of the tilapia genes encoding epithelial Ca2+ channel (ECaC), plasma membrane Ca2+-ATPase 2 (PMCA2), and NCX1 were first evaluated by RTPCR (Fig. 1)
Sections were subjected to in situ hybridization against tilapia ECaC, PMCA2,or NCX1 mRNA, and double stained with Na+/K+ ATPase α5 antibody, revealing that ecac, pmca2, and ncx1b are expressed in the ionocytes of gill filaments (Fig. 2)
Summary
The maintenance of Ca2+ homeostasis is important because Ca2+ is involved in many physiological activities, such as muscle contraction, neuron excitation, and bone formation in vertebrates (WendelaarBonga and Pang 1991). The major organ for ionoregulation in fish is the gills, which are responsible for over 95 % of Ca2+ uptake from water in freshwater-adapted species (Flik et al 1995). The skin serves as the main organ for ionoregulation at early developmental stages of fish, before the gills are fully developed (Hwang et al 1994, 2011). Ionocytes in the gills or larval skin are vital sites for ion uptake in fish (Hwang et al 2011). In an early study in trout, branchial Ca2+ uptake was demonstrated to be active and transcellular (Perry and Flik 1988). The understanding of the Ca2+ absorption mechanism in fish gills or skin progressed swiftly after the
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