The Variation of Strontium Metabolism with Age in the Dog

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Abstract
Strontium-85 was injected intravenously into dogs of different ages. Maximum retention occurred in animals about 6 months old[long dash]approximately the age of puberty. Reduced retention in younger dogs was attributed to their high bone resorption to allow for growth. In dogs 6 months of age and greater strontium retention became progressively less up to 10 years of age. This difference was small in animals of ages greater than 5 years. Less than 200 days after injection retention was interpreted as the resultant of hetero-ionic exchange, the accretion and resorption of bone necessary for growth, and re-utilization of strontium returning to the blood especially in the young, growing animals. By about 200 days after injection, the rates of excretion (expressed as the fraction of the body burden excreted/day) in all dogs, regardless of age, approached the same value. Only small fractions of the strontium burden are released from bone after this period of time and by a mechanism largely independent of the age of the animal. It was not possible to differentiate between strontium removal resulting from actual resorption of bone and that caused by physical-chemical processes. Similarities in long-term rates of excretion suggest that physical-chemical processes may finally predominate in removing strontium from bone. Retention of strontium in dogs of various ages were described mathematically by a modified power function. The appropriate constants were evaluated for each age. This function allows more exacting estimations of strontium burdens and associated parameters than have heretofore been possible.

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