LIAO, Bo-Kai; YANG, Chi-Hwa; PAN, Tien-Chien; HWANG, Pung-Pung; Institute of Fisheries Science, National Taiwan University; Institute of Fisheries Science, National Taiwan University; Institute of Fisheries Science, National Taiwan University; Institute of Zoology, Academia Sinica, Taiwan: Molecular Mechanism of Calcium Uptake in Gills of Freshwater Fish

Freshwater fish absorb Ca²⁺ predominantly from ambient water, and more than 80% of the Ca²⁺ uptake is achieved by active transporting through gill mitochondria-rich cells (MR cells). In the current model for Ca²⁺ uptake in gill MR cells, Ca²⁺ may enter passively the cytosol via apical un-identified voltage-independent Ca²⁺ channels, and then, are extruded into plasma through the basolateral Na⁺/Ca²⁺ exchanger (NCX) and Ca²⁺-ATPase (PMCA). However, there was no molecular evidence to support this model. The present study was aimed to examine the regulation of the gene expressions of these transporters in freshwater tilapia (Oreochromis mossambicus) upon acclimation to different ambient Ca²⁺ levels. Homology-based cloning strategies were used to clone the tilapia ECaC, PMCA and NCX. The phylogeny tree of ECaC suggested that tilapia has only one form, which is a homologue of mammalian CaT1, an isoform of ECaC dominantly found in small intestines. In situ hybridization showed that ECaC was expressed in the epithelial cells of gill filaments. Tilapia acclimated to low-Ca²⁺ freshwater expressed gill ECaC approximately 4-fold higher than did those acclimated to high-Ca²⁺ freshwater in quantitative-PCR analysis. However, no significant difference was found in the expressions of NCX, PMCA and sodium pump alpha-1 subunit between the 2 groups of tilapia. Furthermore, low-Ca²⁺ acclimated tilapia showed a higher Ca²⁺ influx rate compared with the counterpart. Taken together, PMCA and NCX are maintained in a basal level, while ECaC is a gatekeeper channel in MR cells and is the major regulatory target of the Ca²⁺ absorption in fish.