This study has examined whether the
calcium-sensing receptor (CaSR) plays a role in control of stanniocalcin-1 (STC-1), the dominant
calcium regulatory
hormone of fish, comparable with that demonstrated for CaSR in the mediation of ionized
calcium regulation of PTH secretion in mammals. In a previous study, we have cloned flounder STC-1 from the corpuscles of Stannius (CS). Here, we report the cloning and characterization of the CS CaSR, and the in vivo responses of this system to altered salinity,
EGTA induced
hypocalcemia, and calcimimetic administration. Quantitative PCR analysis demonstrated, for the first time, that the CS are major sites of CaSR expression in flounder. Immunoblot analysis of CS
proteins with CaSR-specific
antibodies revealed a broad band of approximately 215-300 kDa under nonreducing conditions, and bands of approximately 215-300 kDa and approximately 120-150 kDa under reducing conditions. There were no differences in CS CaSR
mRNA expression or plasma STC-1 levels between seawater and freshwater (FW)-adapted fish, although CS STC-1
mRNA expression was lower in FW animals. Immunoblots showed that glycosylated monomeric forms of the CaSR migrated at a lower molecular mass in CS samples from FW animals. The ip administration of
EGTA rapidly induced
hypocalcemia, and a concomitant lowering of plasma STC-1. Calcimimetic administration (1 mg/kg R-568) rapidly increased plasma STC-1 levels, and reduced plasma concentrations of
calcium, phosphate, and
magnesium when compared with S-568-treated controls. Together, these findings support an evolutionary conserved role for the CaSR in the endocrine regulation of
calcium before the appearance of parathyroid glands in tetrapods.