Tissue distribution, kinetics, and biologic half-life of Mg²⁸ in the dog

Mg 28 of a high specific activity was used in these studies. It was rapidly injected intravenously into 12 dogs and the concentration-time course curves in plasma were obtained. Urinary and fecal excretion was followed in seven animals. The dogs were killed 7 min to 68 hr following injection. Seventy-four tissues were sampled and assayed for Mg 28 concentration. Plasma Mg 24 was determined by flame spectrophotometry. With these data it was possible to follow the concentration-time course of Mg 28 in the various tissues as well as to calculate their "exchanging" Mg 24 content and effective rates of exchange. A variation in the pattern of uptake and in concentration of exchanging Mg 24 was noted among the different tissues. During the interval from 24–68 hr, body exchanging Mg 24 space and mass were found to range from 3.2–3.8 liters/kg and 4.9–5.7 mEq/kg, respectively. The biologic half-life of Mg 28 in the dog estimated from excretion data was 11 days.