37 to years: tive ab-! other data and relates well to Marshall’s power func- Dog Age A22B 576 A22C A221 8m 4 yr 9 yr 9.5 vr Measured plasma fraction of injected dose &, of injected dose/mi plasma concentration, — in days. The coefficient ecording » obtained by extending the straight line which fits the after ininterval between 2 and 14 days to the day 1 intercept. Measured retention, wie er: tion model." | WAS @S-} In Table 12 the measured retention and plasma eposited clearance are expressed as power functions where ¢ is s still in of the retention function was TABLE 12.. PuasmMa RETENTION AND CLEARANCE OF Ba place is, The experimental slopes (the 6 term in the power funchus ob-! tion R: = at—°) were measured from the data shown spectra, the dog in Figure 32. It should be noted that “crossover,” the time at whieh the specific activity of the blood drops below 3Ba in- that of the skeleton, occurred between 14 and 100 days. | power tion vs, An average slope has been drawn for the plasma and retention eurves shown in Figure 32 to indicate a coordi- ! “crossover” point. S, The feeal and urinary excretion rates, summarized rtion of ; in Table 10, indicate a general decrease in the urimary in the excretion rate with time after injection, whereas the ; shows fecal excretion rate may have a peak somewhere beof the tween 2 and 4 days. The peak in the fecal excretion ce i — ome r rate could be due entirely or in part to a fecal delay. na conTable 11 tabulates the *Ba concentrations, as peron the cent of injected dose per gram of ash, of various bones ywever, and segments of bones. It is probably not meaningful re dur. ! to compare the actual quantity of 1**Ba retained in the val be- * dogs because of their different ages, since Decker, et d 576) | al’) have shown a marked difference in retention i con- Ire pe- s after | tivity shape ical of ; from r) with age. However, the specific activity, in terms of pereent of injected dose per gram of ashed bone seg- incnt relative to the specific activity of the whole bone, 1.30 0-3 1.00 932 0.98 {9-59 0.829-97 2 1.76 %K | 5.15 x 4.61 < | 3.13 & 1073 103 1073 1073 6477 £148 (71-47 1-8 cal bone. One, therefore, expects that trabecular bone would eventually lose activity more rapidly than cortical bone so that the ratio of the specific activity of trabecular segments to that of a whole bone should decrease with time after injection. A slight decrease is shown by 1508 and 3113 days, but the effect is surprisingly small. Thus, the analysis of tracer content in segments of various bones suggests that, while changes are occurring, these changes are not rapid. Even four and nine years after the isotope administration those bones or portions of bone usually considered to turn over rapidly still contain relatively high specific activity levels when they are compared to the regions of compact cortical bone where the turnover is known to be low. SUMMARY The whole-body retention, plasma clearance, and bone deposition of **Ba have been measured in beagle dogs. The '*Ba retention by these animals is clearly relative 3Ba retention of bone segments and whole not well described by a single power function, but does relate well to Marshall’s theory of alkaline earth metabolism. No attempt has been madeto fit these data with a series of exponential functions. gram of ash in the bone segment than in the bone as a whole. A ratio less than one indicates the opposite. At eeven days after injection the trabecular bone of the proximal and distal epiphyses has a specific activity two or three times that of the cortical bone of the midshaft. This suggests a reactivity or turnover rate of trabecular bone that is two to three times that of corti- calcium blood curves. This steep slope is typical of all barium, radium and strontium data that we have examined, but we have yet to appreeiate its significance. The excretion rates showa general decrease with time after injection. The analysis of the tracer content in various bones and segments of bones suggests that, even several years is probably a significant quantity to compare between the animals. Thus, in Table 18, we have compared the hones at short times and long times after injection. A ratio greater than one indicates more activity per TABLE 13. The plasma clearance studies indicate a steep slope of the barium blood curve when compared to normal Ratio ofr Specteic Activities OF Femur SEGMENTS TO SpPEctFic ACTIVITY OF ENTIRE FEMUR Dog 576 Dog A22B Proximal epiphysis 1.15 — Proximal shaft Proximal epiphysis plus shaft Mid-shaft Vist al shaft 0.706 0.998 0.523 0.734 — 1.24 0.90 — ldistal shaft plus epiphysis 1.117 0.95 1.11 1.19 Femurbone segment Distal epiphysis Si 1508 day 1.31 3113 day — Dog 156 Dog A22C Dog A22D 1.31 1.32 1.16 1.26 1.10 0.63 1.04 0.61 0.57 0.99 1.09 0.47 0.88 1.17 1.17 0.50 1.15 0.93 1.10 0.53 0.87 1.32 1.10 1.01 1.19 1.12 1.14 7 day 1.39 7 day 1.39 7 day ; j 1.10 Avg. 7-day value 1.29 Avg. 7 day/ 1508 day 0.98 1.02