Endocrinology of calcium homeostasis in fishes

Abstract

Though fish live in aquatic medium from freshwater to environments of varying Ca concentrations, their plasma Ca level is precisely regulated. They possess well-developed ultimobranchial gland (UBG) and corpuscles of Stannius (CS) as well as 25-hydroxyvitamin-D-1α-hydroxylase in their renal tissue. UBG secretes calcitonin (CT) while CS elaborates two forms of stannicalcins - STC1 and SCTC2. The latter is widely distributed in various organs in the phylogeny and its level increases among women with breast cancer. Recently, four stanniocaicin genes have been isolated from CS of teleost and their physiological and phyogetrc significance are being studied. Fish do have capacity to metabolize vitamin D with the receptor (VDR) widely distributed in various organs including intestine, gill and kidney. Though fish lack functional parathyroid glands (PTG) found in tetrapods, PTH-like immunoreactivity has been identified in hypothalamus, pituitary and CS. Parathyroid hormone-related peptide (PTHrP) has also been found in elasmobranch and teleosts but its role in normal piscine Ca metabolism has not yet been defined. Recent advances in endocrine regulation of Ca metabolism in fish has been discussed.