Hormonal Regulation of Calcium Homeostasis

Abstract

A highly integrated and complex endocrine system acts to maintain calcium, phosphate, and magnesium homeostasis in all vertebrates (Figure 1). It involves an interplay between the actions of two polypeptide hormones, parathyroid hormone (PTH) and calcitonin (CT), and a sterol hormone, lα, 25-dihydroxyvitamin D [lα,25-(OH)₂D₃]. The regulation of the biosynthesis and the secretion of the polypeptide hormones is accomplished by a negative feedback mechanism involving the calcium ion activity of the extracellular fluids (Figure 2). The biosynthesis of lα,25-(OH)₂D₃ from the major circulating metabolite of vitamin D, 25 hydroxyvitamin D₃ (25-OHD₃) takes place in the kidney and is regulated by PTH and CT as well as the extracellular fluid concentrations of calcium and phosphate. Other hormones, such as Cortisol, aldosterone, growth hormone, thyroxin, epinephrine, estrogen, and testosterone, in addition to as yet unknown compounds and certain physical phenomena, undoubtedly play roles separately and in concert in conditioning organ responses to PTH, CT, and lα,25-(OH)₂D₃.