ee Disvribetion of Strontiur in tre Body strontium if depositec in the bones except for insignificant amounts in other tissues. Its metabolism is qualitatively similar to calcium but it is not a dietary substitute since calcium-starved animals develop rickets when fed strontium. Strontium is incorporated into the trabecular bone near the epinhyses and as time goes on, presumabl is incorporated in the more inert portions of bone. A great deal of research has been done on the rates of urinary and fecal excretion of strontium administered in one injection and it has been estimated that the total retention is in the order of 50% and occurs mainly in the first 2 hours. Oral retention is in the range of - 10%. strontium excretion has not been studied carefully for more than 20 days and in this time, the relationship between tne logarithm cf the percentage of the initial dose found in the daily urine and fecal excreter bears a linear relationship +. the logarithm of time. How- ever, in a few studies with Calciuml5, this relationship does not functions relating the daily excretion of calcium to time. This indicates a mumber of compartments with different exchange rates, When calrium is ahsorhed from the intestine, msculature or peritoneu or given intravenously, there is a rapid dilution into the extra cellular fluid volume which occurs in a matter of minutes. There is then an ionic exchange with a portion of the bone which is in free contact with the blood. Most of the strontium is removed from blood by two hours, Then there is also a rate of incorporation of this initially deposited material into the bony matrix and then presumably into the bony apatite crystals. There is only fragmentary data on the exchange rates for these compartments. - hold beyond 10 to 20 days, but rather there are 4 or 5 exponential Remarkably little work has been done on the chronic oral administration of radioactive strontium or calcium. However, one study with rats noted that an equilibrium excretion rate of strontium in urine was obtained about two days after the onset of feeding. The urinary excretion rate was about 3% of the daily ingested dose of radioactive of 150 microcuries of Calcium45 resulted in a stable specific activit; of piasma in about hO days at a level of 2.7 x 10-4 microcuries ’s strontium. Retention in adults approximated 4% and about double this in very young rats, Another study indicated that daily oral feeding cali5/me. Ca. In this study, the specific activity of milk was equal to that of blood and by inference urine. Presumably, the specific activity of the blood was equal to that of the ingested food under equilibrium conditions and also with a portion of the bone whose components are freely exchangeable with those in blood. Under circumstances in which a population ingested radioactive strontium at a reasonably constant rate, one might expect that the specific activity of the blood also be constant, reaching stable values in a relatively short time in relationship to duration of exposure. In this case, urine or milk would provide a useful monitorin -89- |