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Magnesium-28 Studies in Lambs¹

University of Tennessee-Atomic Energy Commission, Agricultural Research Laboratory, Oak Ridge, Tennessee, and Department of Animal Science, University of Illinois, Urbana

Magnesium-deficient and control lambs were dosed both orally and intravenously with Mg²⁸. Distribution of the isotope in the various tissues was measured in addition to the rate of disappearance of Mg²⁸ from the plasma and whole blood. The excretory pathways were also investigated in these studies.

1. Plasma disappearance of intravenously administered Mg²⁸ was very rapid during the first two hours after dosing followed subsequently by a slower exponential disappearance until approximately 8 to 10 hours when most of the blood activity had disappeared.

2. Uptake of this isotope by the red blood cells was very small even for the deficient lambs. The shape of the Mg³³ plasma and whole blood disappearance curves was the same for both the control and deficient lambs with smaller amounts of Mg²⁸ occurring in the blood of the deficient group.

3. Plasma Mg²⁸ reached a maximum 12 to 14 hours after oral dosing. The amounts of isotope in the blood of the deficient lambs were less than that of controls due to the greater uptake by the tissues from the deficient group.

4. Tissues from the deficient animals had higher concentration and total amounts of the isotope as compared with control lambs. Although the heart, liver, spleen, lung and kidney were the most active sites of Mg²⁸ activity, the ubiquitous character of this ion in the animal body was shown by its widespread distribution in other tissues.

5. Although a higher concentration of Mg²⁸ was found in the teeth of the deficient lambs than of controls, the amounts occurring in the latter show the remarkable avidity of this tissue for magnesium. These results indicate the dynamic character of teeth in young animals and a possible requirement for magnesium in normal tooth development.

6. The long bones from the control group, particularly the shank portion, contained the highest isotope activity. Only negligible amounts were observed in the bone marrow of either deficient or control lambs.

7. Absorption of magnesium was much greater, and urinary excretion was much less, by deficient lambs than by control lambs.

² Present address: Dublin University, Ireland.

Manuscript received 2 March 1961.