FEASIBILITY OF MEASURING ORGAN MAGNESIUM TURNOVER INVIVO BY CONTINUOUS FEEDING OF A STABLE ISOTOPE

The feasibility of measuring organ endogenous magnesium (Mg(en)) turnover in vivo by continuous feeding of a single stable isotope of Mg was demonstrated in this investigation. Adult CD-1 mice were fed a Mg deficient diet and deionized water with (+Mg) or without (-Mg) added Mg-24 (290-mu-g/mL) for 16 days. The change in organ Mg-25 content over time was then accurately determined by in vitro isotope dilution with Mg-26 as spike. Organ endogenous Mg content was then calculated as Mg-25(en)/0.1028 and exogenous Mg (Mg(ex)) content from the expression Mg(ex) = Mg(total) - Mg(en). All soft tissues examined in the + Mg group showed significant turnover of Mg(en) and accumulation of Mg(ex). The rate at which this occurred was organ specific. Apparent half-lives for Mg(en) turnover were 3.83, 4.13, 5.87, and 8.77 days for liver, heart, brain, and skeletal muscle, respectively. Mg restriction resulted in a dramatic decrease in the rate of Mg(en) turnover with apparent half-lives ranging from 60.3 to 146 days. Brain showed the smallest decrease in Mg(en) turnover with Mg restriction, and was the only tissue observed to lose a significant amount of total Mg.