J. F. LOUTIT 



Excretion is via the kidneys and to a certain extent the gut, the percentage 

 contribution of the latter organ varying with the species of animal and with 

 the calcium status of the individual. The renal excretion at least is influenced 

 by humoral influences, notably by parathyroid and thyroid activity releasing 

 calcium from the buflfer state, the bone. Phosphate metabolism is closely 

 connected with that of calcium and disturbance of the one is reflected in the 

 other. 



Strontium and Calcium Differentiation by Tissues 

 Strontium is chemically related to calcium and in general its metabolism 

 in the animal is similar. There are certain differences, however, which 

 indicate that certain tissues distinguish between the two. Calcium is pre- 

 ferentially absorbed from the gut by a factor of about two. Strontium is 

 excreted by the kidney with greater facility by a similar factor=^ and, in the 

 experimental animal, the mammary gland^ and the placenta^ pass calcium 

 preferentially to strontium. The storehouse tissue, bone, is said to treat the 

 two elements at physiological levels without distinction'^. 



Central Differentiation of Strontium and Calcium 

 The question needing answer is — does the body control strontium metabolism 

 by a mechanism similar to but separate from that for calcium, or does the 

 mechanism not distinguish between them at, for instance, some central 

 governing locus for both, but only at the effector mechanisms? If there is 

 a controlling master mechanism for strontium alone, one would expect that 

 by markedly increasing the strontium intake one would, on analogy with 

 calcium, markedly depress the percentage uptake from the gut. However, 

 this has not been observed in man or the rat. Harrison et al.^ have shown 

 that of a single oral dose of 100 or 250 mg of stable strontium about a third 

 was absorbed by man. This dose is about 50 to 100 times that of the normal 

 daily intake of stable strontium. Nevertheless when a single tracer dose of 

 s^Sr was taken orally, the absorption was not notably different (personal 

 communication from G. E. Harrison). This ^^Sr was taken in the carrier-free 

 state, but before absorption it must have been mixed to some extent with 

 carrier from the natural diet and body fluids. One-third absorbed, therefore, 

 seems representative for strontium over a range of concentrations of at least 

 100 and possibly greater orders of magnitude. The same phenomenon has 

 been observed in rats given ^^Sr as a single dose by stomach tube and fol- 

 lowed at once by carrier strontium from to 400 [jig/g of body weight. 

 The fraction absorbed was if anything increased at the higher levels of 

 carrier «: the percentage retention by the skeleton after 24 hours was about 

 the same in all groups, but the percentage excretion in urine was greater 

 after the higher doses. The amount of strontium laid down in the skeleton 

 during this period must in absolute terms have differed betw^een the two 

 extreme groups by a factor of over 400. These results do not suggest any 

 homoeostatic mechanism operating specifically for strontium. 



If, however, the master mechanism for homoeostatic control of calcium 

 in the body cannot distinguish between strontium and calcium, then these 

 phenomena are explicable in terms of atomic concentration. The normal 

 concentration of calcium in, for instance, plasma is 100 mg or 5 mequiv. 



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