Distribution and tissue uptake of magnesium related to its pharmacological effects

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Abstract
Atomic absorption spectrophotometry was used to measure the Mg and Ca content of blood, of different parts of the central nervous system, peripheral nerve, and skeletal muscle of rabbits. Intravenously administered Mg did not enter blood cells; the proportion of Mg bound to plasma colloids remained essentially unchanged. During iv administration of MgCl2, the Ca concentration in whole blood rose, that in plasma diminished, indicating release of Ca from tissues and uptake by, or adsorption to blood cells. The fraction of Ca bound to plasma colloids decreased, indicating displacement of Ca from protein binding sites by the excess of Mg. Data concerning the clearance of a large Mg load are also presented. Following the administration of 4.3 mEq/kg of MgCl2, the net uptake of Mg in skeletal muscle and in peripheral nerve was 4-7 times greater than in the brain and brain stem. These results, seen in conjunction with other reports, support the view that the central nervous system is protected either by a barrier, or by a regulating mechanism, which keeps the physiologically effective concentrations of Mg++ within limits.