EXCESS DIETARY MAGNESIUM INCREASES EXPRESSION OF TRPM7, BUT DOES NOT IMPROVE THE AMOUNT OF MAGNESIUM IN RAT SKELETAL MUSCLE OF SWIMMING EXERCISED RATS　

19th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, Amsterdam, Netherland

Introduction
Magnesium (Mg) is an abundant mineral in the human body, and plays a role as a co-factor to activate various enzymes, particularly gluco-regulated kinases. We have observed that swimming exercise increased the apparent absorption of Mg, expression of TRPM7 and amount of Mg content in rat skeletal muscle, revealing significant positive correlations between these variables. These results suggested that swimming exercise would enhance Mg requirements in rats. Therefore, the aim of this study is to examine whether the increase in dietary Mg content improves the nutritional Mg-status measuring these variables in swimming rats.
Methods
Forty-four 6-week-old male Wistar-Hannover rats were randomly assigned to two equal groups. Rats in one group (n=22) were housed under sedentary conditions during the experimental period, and rats in another swam continuously for 2 h without a load (6 days/week). Rats in each group divided into two subgroup and one-half of the rats were fed a purified diet of AIN-93G composition (Mg; 530 ppm) and the remaining half were fed high Mg diet (Mg; 1194 ppm) according to AIN-93G for 15 days (17.5 g/day).
On day 15, rats were sacrificed and their blood and gastrocnemius muscle were quickly removed. Mg levels in serum and muscle were determined using a sequential plasma spectrometer. Amount of TRPM7 expression in muscle was detected with Western blotting analysis. Apparent Mg absorption was calculated from amounts of Mg intake and fecal excretion.
Results
Activities of citrate synthase were higher in the swimming rats than in the sedentary rats. Apparent Mg absorption in swimming rats was also higher than in sedentary rats, but not significantly. Serum Mg level in sedentary rats with high Mg diet was higher compared to rats fed normal diet. Mg levels in skeletal muscle were not affected by both amount of dietary Mg and swimming exertion. The expression of TRPM7 in skeletal muscle was improved the effect of dietary Mg contents and swimming exertion. 
Discussion
The present investigation demonstrated that excess dietary Mg did not improve the amount of Mg in rat skeletal muscle regardless of swimming exercise. However, expression of TRPM7 was elevated with increasing dietary Mg content. In the previous study, we observed that the elevation of TRPM7 expression and amounts of Mg contents in swimming rats skeletal muscle. Therefore, exercise-derived stimulation and high Mg concentration in blood might be enhanced the expression of TRPM7 independently. Also, it might be existed some efflux system to regulate intramuscular Mg concentration. Further study would be needed to elucidate the mechanism of Mg influx into skeletal muscle derived from exercise stimulation in rats.