Parathyroid Hormone

Publisher Summary This chapter focuses on nature and biological function of parathyroid hormone (PTH) and describes the way cytochemical techniques may be of particular value in elucidating the role of PTH and its interaction with its target issues involved in calcium homeostasis. The fate of PTH, stored within the secretory granules of the parathyroid cell, is dependent, mainly, on the concentration of extracellular ionized calcium. The classic biological activities, both in vivo and in vitro, of intact 1–84 appear to reside in the N-terminal third of PTH. The immunoheterogeneous nature of PTH in the circulation is a result of the secretion and of the peripheral generation of C- and N-region fragments of PTH that presumably will have different biological activities and metabolic fates. PTH acts rapidly on the kidney to increase reabsorption of calcium, decrease reabsorption of phosphate, and stimulate hydroxylation systems required in the conversion of vitamin D metabolites to active forms. The effects of PTH on bone have been widely studied, and can be classified as either anabolic or catabolic with regard to the skeleton.