Calcium regulation in the euryhaline flounder
The concentration of calcium in extracellular fluid is precisely regulated throughout vertebrates from fish to mammals. The ability to sense changes in plasma-ionized calcium is mediated bythe calcium-sensing receptor (CaSR), which is an integral component in regulating PTH secretion from the parathyroid glands of mammals. Parathyroid-hormone Related Protein (PTHrP) is a peptide found in many different organisms, both in higher and lower vertebrates and is one of the hypercalcaemic factors in mammals. Its role in lower vertebrates has been recently established. Parathyroid hormone related peptide (PTHrP) has been implicated in calcium and osmoregulatory physiology of fish as circulating levels change in accordance with osmotic state. Flounder (Platichthysflesus) is a teleost fish and in the evolutionary scale is considered a lower vertebrate. To investigate Parathyroid-hormone Related Protein (PTHrP) and parathyroid hormone receptors (PTHRs) and their functions in teleost fishes, we have cloned and identified sites of mRNA expression of the PTHrP1,PTHrP2,PTH1R and PTH3R in the flounder. Using quantitative PCR (QPCR), PTHrP1,PTHrP2,PTH1R and PTH3R mRNA expression has been measured in gill, kidney, bladder and gut of chronically adapted seawater (SW) and freshwater (FW) flounder. QPCR measures of PTHrP1 andPTH1R mRNA expression indicated similar levels for FW and SW flounder. PTHrP2 and PTH3R mRNA expression levels in FW adapted flounder gills were approximately 2-fold higher than those in SW adapted fish. The differential expression in PTH3R in SW and FW gill implies an underlying role in osmoregulation and perhaps a calcium regulatory function in fish as the gill is the major site of ion regulation. The in vivo responses of this system to the ip administration of EGTA rapidly induced hypocalcemia, and a concomitant increasing PTH1-34 and lowering of plasma STC-1.Calcimimetic administration (1 mg/kg R-568) rapidly increased plasma STC-1 levels, and reduced plasma concentrations of calcium when compared with S-568-treated controls. Together, these findings support an evolutionary conserved role for the CaSR in the endocrine regulation of calciumbefore the appearance of parathyroid glands in tetrapods.