SKELETAL DISCRIMINATION BETWEEN SR ABD CA 71 of the tracer excreted into the gastrointestinal tract and to an error in the timing of the systematic collection of the stools. The data are insufficient to quantify this feedback from the gastrointestinal tractand it could not be included XSH64 Bs J _\ in the present model. The known gastrointestinal dis- as the various processes independent of any model for Sr - WHOLE - BODY wae Lded o 2 4 6 a 10 l2 14 16 ta 2o. 22 24 26 oy <8 30 Fic. 4. Strontium/calciumratio in whole body and knee (mean of seven subjects). The plotted values are expressed as the mean ratio of the percent of the administered doses of Sr over Ca in each individual subject. values are poorly defined and the standard deviations are relatively large compared to the values of the other parameters. The parameter values derived from the Sr data for each subject except for excretion are, with few exceptions, lower than those derived from the Ca data and the correlation is very high. The Sr values are lower than the Ca values by about the same percent as thestatistical uncertainty in the individual measurements (¢,). It is clear, therefore, that with less precise methods of analysis, the differences between the Sr and the Ca values are not readily discernible. For example, in a double tracer study on rats, Bauer and Carlsson (1) found that the accretion rate (A) and the exchange capacity (E) obtained with Ca and with Sr were notsignificantly different.. By the use of Bauer’s analytical procedure, no significant differences could be found in the A and E values of six patients studied with Sr® and Ca‘’, although an analogue computersolution of these data did suggest a smaller exchangeable pool for Sr as compared with Ca (8). It is apparent that the spread in values for each parameter among the group is large. Statistical analysis of the grouped data indicated the absence of a significant difference between the means of the Ca and the Sr parameter values. Since the present study was limited to elderly males, a greater variability probably appears in the present results than would be expected in a younger group. The importance of extending these studies to a younger and more homogeneous population is manifest Ca. The authors concluded that Sr, as a tracer for Ca, gives only approximate values, since Sr is expressed in terms of an apparentcirculating plasma volume and not in units of mass as in the case of the Ca tracer. Since the present calculated values are based on the absolute con- centration of Ca in compartment 1, to obtain the absolute Sr concentrations, a different proportionality constant would be required to determine the absolute levels of Sr based on the concentration of stable Sr in plasma. However, since it has not been demonstrated that the amount of stable Sr in plasma or boneis con- stant, it is more advantageous to continue to express Sr in terms of Ca because of the constancy of the Ca. It is obvious that long-term retention data are required to determine differences in turnover and resorption of Sr and Ca. Unfortunately, because of the short half-life of Ca‘’, this comparative study could only be carried out with sufficient accuracy for 30 days. Examination of the values for the Sr/Ca ratio for the whole-body retention obtained by whole-body counting over the 30-day period reveals the same patterns and absolute levels as previously noted (8). There is a rapid fall in the level of the ratio until an asymptote is reached at approximately 18 days and continuing to 30 days. The same pattern of Sr/Ca levels with time was found for the knee data except that the ratio leveled off at a higher value than for the whole-body retention data. This higher level may reflect the higher proportion of bone with its slower turnoverof Cain the knee field as compared to the wholebody detector’s field, which includes a larger nonosseous component. Since the Sr/Ca ratio does not continue to decrease progressively with time as a function of the differential renal discrimination, the turnover of Sr and Ca by bone must differ. The constancy of the Sr/Ca ratio after 18 days, assuming a constant renal discrimination against not only to establish more accuratelythe fine differences in the skeletal dynamics of Sr and Ca, but also to obtain data on this age group. The mean urinary excretion rate of Sr®* was approximately 3.7 times higher than that of the Ca‘, which is consistent with the known renal discrimination against Sr, indicates that Sr must be returning to compartment1 from bone less rapidly than Ca. Preceding this, in the tion of Sr®> was about 20 percent higher than that of after 18 days due to the preferential retention of ‘‘bonefixed”? Sr is complicated by the differences in excretion of Sr and Ca and the presenceof different sizes of non- Ca compared with Sr. The mean endogenousfecal excre- Ca‘?. The small systematic error in the fit of thefecal data (see Fig. 3) is probably dueto the recycling of some ile eel mee ape on ul T metabolism. Only the fecal excretion constant and the bone rate constant were found to be the same for Sr and period from 6 to 14 days, a similar leveling of the Sr/Ca ratio in urine observed in several patients (most notice- able in an osteoporotic patient) by Heaney (11) was interpreted to indicate preferential release of Sr from bone to plasma. The Sr enrichment of compartment 3 eet he FFEa TagreenTRS Br te grey a 4 roe TOT ae T Sr/Ca RATIO ~ cp ~4 o Qo uv pene crimination factor of two between Sr and Cain this recycling may also account for some of the differences in the values between the Ca and the Sr data. In a study of the comparative kinetics of Sr and Ca in man, Bronneretal. (5) measured the rate constants of