Aspects of Parathyroid Physiology in Mammals

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Abstract
This report summarizes the parameters of parathyroid physiology in the mammal. Emphasis is given to the part played by this hormone in the processes of growth and remodelling in bone as well as to its function in the maintenance of a relatively constant level of ionic calcium in the extracellular fluid. The importance of phosphate ions has been shown, particularly with respect to their role in the rate of Ca-transport through the extracellular fluid compartments, and their influence on the rate of parathyroid secretion. In addition, the inter-relationship of the newly discovered hormone, thyrocalcitonin, is considered. The latter hormone, by suppressing resorption of bone, may have an influence in stabilizing plasma levels of calcium; it also has a moderate, but long range effect on the rate of bone remodelling. Finally, an attempt has been made to relate the vast amount of work done with these two hormones in mammals to their phylogenetic development in vertebrates. It is suggested that the parathyroids developed at the time when the specialization of bone in vertebrates produced a solid structure which was incompatible with the fluid calcium concentration needed for the maintenance of many physiological functions. The parathyroid hormone, by its action to increase the transfer of calcium from bone against a concentration gradient, permitted these vertebrates to maintain the higher fluid calcium ion concentration needed for the normal functioning of the animal.

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