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Parenteral Magnesium Load Testing with ²⁸Mg in Weanling and Young Adult Rats^1,2,

USDA, SEA, Human Nutrition, Beltsville Human Nutrition Research Center, Beltsville, MD 20705; Department of Pediatrics, St. Louis University School of Medicine, St. Louis, MO 63104 and Trace Element Research Laboratory, Veterans Administration Medical Center, 50 Irving Street, N.W., Washington, DC 20422

A sound diagnostic test for Mg deficiency is needed. This is a report of the parenteral Mg load test conducted in weanling and young adult rats fed a purified basal diet containing 3 mg magnesium/100 g with 150 mg of added magnesium/100 g (control) or 0 added magnesium (deficient). Weanlings were studied at about 1 week of dietary treatment and young adults at 2 weeks. The protocol included: a) a 6-hour preload urinary collection; b) an intraperitoneal load of 15 mg of magnesium/kg (weanlings) or 12 mg/kg (young adults) with 2 µCi ²⁸Mg given simultaneously with each load; c) a 6-hour postload urinary collection; d) chemical analysis of selected tissues and urine for Mg; and e) ²⁸Mg counting 6 and 24 hours postload. Controls all excreted large amounts of Mg pre- and postload, retaining less than 26% of nonradioactive loads. They had high urinary ²⁸Mg counts. In Mg-deficient animals, the concentration of Mg in bone more than halved. These animals avidly conserved Mg and retained over 85% of nonradioactive Mg loads. Their ²⁸Mg activity in vital organs was 3–6 times greater than in controls. We concluded that the parenteral Mg load test reliably identifies severe Mg deficiency.

KEY WORDS: • Magnesium deficiency • parenteral magnesium load tests

¹ Supported by the Life Seekers of St. Louis, MO; by the Research Development Fund and the Fleur de Lis Fund of the Cardinal Glennon Memorial Hospital for Children and the Department of Pediatrics, St. Louis University; and by the Veterans Administration Research Fund, Washington, DC.

² Presented in part at the meetings of the American Institute of Nutrition, the 64th Annual Meeting of the Federation of American Societies for Experimental Biology, Anaheim, CA, 17 April 1980. Fed. Proc. 39: 4086, 1980 (abs.) and Pediatr. Res. 14: 722, 1980 (abs.).

Manuscript received 23 October 1980.