Design of Parenteral Magnesium Load Tests in Weanling and Young Adult Rats

Abstract

Abstract

There is great difficulty in assessing the nutritional status of magnesium (Mg) in young, living mammals. The parenteral Mg load test has been advocated as a useful indicator, but requires further study, first of all, in an animal model. The present study was conducted to (a) design an appropriate Mg load test for Mg-deficient and Mg-sufficient rats in two age groups—weanling and young adults, and (b) to learn whether or not the test can be effectively conducted by giving the parenteral load of Mg by the intramuscular (im) route instead of the intraperitoneal (ip) or intravenous (iv). To design the parenteral Mg load test, all animals were fed identical purified diets, except for the addition of 150 mg of magnesium/100 g of diet in the control diets and zero magnesium in the deficient diet which contained only 0.3 mg/100 g. Plasma and urinary Mg values were followed. The final test included: (a) a fasting 6-hr preload urinary collection, (b) an 18-hr refeeding period, (c) intraperitoneal loading with 15 mg of magnesium/kg body wt for weanlings or 12 mg/kg for young adults, and (d) a fasting, 6-hr postload urinary collection. Weanling and young adult rats fed Mg-sufficient diets excreted substantial amounts of Mg pre- and postload, retaining less than a mean of 25% of the load; rats fed the Mg-poor diet excreted little Mg preload, retaining a mean of at least 85% of the load. We compared muscle Mg values from the Mg-injected anterior thigh muscle versus the contralateral noninjected muscle taken from the same animals at the end of the 6-hr load test and found no difference; apparently, Mg was completely absorbed from the muscle, even in very sick weanling rats. Mg load test results using im and ip loading showed that the two routes of Mg loading gave essentially the same Mg load test data.