Existence of Non-Exchangeable Calcium Compartments in Plasma

Stable Calcium Isotopes as Tracers in Studies of Mineral Metabolism

“…Similar apparent differences in '"Ca S.A. in starved rats were noted by Likins and Craven (1960). In both the rats studied by Likins and Craven (1960) and the sheep studied by Giese and Comar (1964) the total calcium of urine was determined by oxalate precipitation. Since magnesium may interfere in these methods, and since starved animals may excrete many times as much magnesium as calcium in urine (Fiske and Logan, 1931), these two factors may explain these results in starved animals.…”

“…Animal experiments on normal dogs by ] and by Giese and Comar (1964) on normal sheep showed the same S.A. of serum and urine '"Ca.…”

“…Their observation that urine S.A. was only 55-70 % that of plasma ultrafil-trate depended upon oxalate precipitation of calcium-with possible errors in over-estimation of total calcium in urine due to magnesium interf erence. Giese and Comar (1964) reported similar Ca* S.A. in the serum and urine of 2 normal sheep, but found lower urine Ca* S.A. in the same 2 animals when starved 48 hours despite the fact that total plasma calcium changed only slightly. The lower S.A. was associated with increased '"a in the urine in one sheep, and increased 'OCa in the other when compared with the values in the fed control experiment.…”

“…Ca* S.A. differences (Likins and Craven, 1960) and starved sheep (Giese and Comar, 1964) as well as in the specialized stop-flow studies of renal calcium exchange reported by Miller et d., 1964. Magnesium interference in the methods used for urine calcium in the starved animals may account for the apparent lower Ca* S.A. in these studies.…”

“…Others have reported variation in S.A. ratios of blood and urine but have been unable to state whether these were real or due to systematic analytic errors (Dow and Stanbury, 1960). Profound variations in bloodlurine Ca* specific activity ratios have been related to starvation in both rats (Likins and Craven, 1960) and sheep (Giese and Comar, 1964).…”