Aspects of Parathyroid Physiology in Mammals

This report summarizes the parameters of parathyroid physiologyin the mammal. Emphasis is given to the part played by thishormone in the processes of growth and remodelling in bone aswell as to its function in the maintenance of a relatively constantlevel of ionic calcium in the extracellular fluid. The importanceof phosphate ions has been shown, particularly with respectto their role in the rate of Ca-transport through the extracellularfluid compartments, and their influence on the rate of parathyroidsecretion. In addition, the inter-relationship of the newlydiscovered hormone, thyrocalcitonin, is considered. The latterhormone, by suppressing resorption of bone, may have an influencein 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 donewith these two hormones in mammals to their phylogenetic developmentin vertebrates. It is suggested that the parathyroids developedat the time when the specialization of bone in vertebrates produceda solid structure which was incompatible with the fluid calciumconcentration needed for the maintenance of many physiologicalfunctions. The parathyroid hormone, by its action to increasethe transfer of calcium from bone against a concentration gradient,permitted these vertebrates to maintain the higher fluid calciumion concentration needed for the normal functioning of the animal.