International Journal of Sport and Exercise Science, 1(2): 19-26 19 Effects of Magnesium Sulfate on Dynamic Changes in Blood Glucose Levels and Glucose Transporter-3 Expression in the Striatum during Short-term Forced Swimming in Gerbils 1 2,* I-Ju Chen , Shiu-Min Cheng 1Department of Physical Education, Asia University, Taichung, Taiwan, 413, ROC 2Department of Psychology, Asia University, Taichung , Taiwan, 413, ROC Received 20 Jan 2010; Accepted 28 Feb 2010 Abstract In a previous study, we reported that when gerbils were subjected to a period of forced swimming, glucose levels and magnesium levels in the plasma became elevated. In addition, a pretreatment with magnesium sulfate increased the expression of glucose transporter-3 (GLUT-3) in the cortex. The goal of the present study was to evaluate dynamic changes in blood glucose, lactate and magnesium levels in gerbils that had been subjected to short-term forced swimming using an auto-blood-sampling system. A second goal was to investigation the effects of forced swimming and magnesium sulfate on GLUT-3 expression, glucose and magnesium levels in the striatum, which is involved in energy delivery and motor function. Data collection involved the use of a microdialysis analyzer, a flame atomic absorption spectrometer and western blotting assays to detect glucose, lactate, magnesium and GLUT-3 levels, respectively. Gerbils were subjected to a blood sampling assay and pretreated with saline (n=6) or magnesium sulfate (n=6, 90 mg·kg -1, ip) 30 min before the period of forced swimming. Whole blood glucose, lactate, and magnesium levels increased to 175% and 235%, 305% and 190%, and 102% and 128% of basal levels during swimming in both the control and magnesium sulfate-treated groups (p<0.05). The glucose and magnesium levels during swimming were 60% and 26% greater in the magnesium sulfate-treated group (p<0.05), respectively, whereas the lactate levels were attenuated at 115% (p<0.05). Another group of gerbils was subjected to a brain sampling assay. The gerbils were separated into four groups (n=4, each group): control group (C group) pretreated only with saline; magnesium sulfate group (Mg group) pretreated only with magnesium sulfate; saline control and forced swimming group (C+S group) pretreated with saline 15 min prior to the forced swimming period then rested for 30 min; the magnesium sulfate plus the forced swimming group (Mg+S group) was pretreated with magnesium sulfate 15 min prior to the forced swimming period then rested for 30 min. The results indicated that GLUT-3 and magnesium levels were slightly, but not significantly higher in the Mg group, the C+S group and the Mg+S group. These data suggest that magnesium increased whole blood glucose and magnesium levels during forced swimming; however, the effects of magnesium and swimming on GLUT-3 protein expression varied in different regions of the brain. Keywords : Forced swimming, Glucose transporter, Magnesium, Striatum Accumulating evidence indicates the existence of a direct Introduction relationship between the redistribution of magnesium in the Magnesium (Mg), the second most abundant intracellular body and the type of exercise and that the status of magnesium cation, functions as a cofactor in more than 300 enzymatic influences the nature of this redistribution [5,6]. reactions. Magnesium is also involved in cellular energy In general, long-term exercise or exercise until exhaustion production, neuromuscular functions, glycogen breakdown, and results in hypomagnesaemia and hypoglycaemia [7,8], whereas modulating the activity of adenosine triphosphatase, providing short-term and high-intensity exercise induces energy for cells [1-3]. Thus, the relationship between hypermagnesaemia and hyperglycaemia [9-11]. Conflicting magnesium status and exercise has received significant findings regarding on the effect of exercise on erythrocyte research attention. Magnesium depletion may lead to changes concentration have been reported. A mechanism for the in neuromuscular function and reductions in physical observed decrease in plasma magnesium concentration after performance and the efficiency of energy metabolism [4]. long term physical exercise could be a shift of Mg into erythrocytes. However, in several studies a decrease in plasma * Corresponding author: Shiu-Min Cheng Mg was not accompanied by an increase in erythrocyte Mg Tel: +886-4-23323456 ext. 3252 [9,12]. In our previous study, we reported that short-term Fax: +886-4-23321057 forced swimming results in an increase in plasma glucose and E-mail: [email protected] magnesium levels in gerbils [13]. Short-term, high intensity 20 International Journal of Sport and Exercise Science, Vol. 2. No.1 2010 exercise transiently increased plasma magnesium Forced swimming concentrations as the consequence of a decrease in plasma The gerbils were randomly divided into a control (saline) volume [6,9]. The two major fractions of whole blood are group or a magnesium sulfate supplement group. Saline or plasma and erythrocytes. Therefore, the aim of this study was magnesium sulfate (90 mg ·kg -1, intraperitoneal injection) was to measure the dynamic levels of whole blood magnesium, injected 30 min prior to the forced swimming task. Each gerbil glucose and lactate levels in gerbil subjected to forced was placed on a polystyrene board floating in a Plexiglas swimming using an auto-blood-sampling system coupled to a cylinder that was 40 cm in diameter, 35 cm in height, and filled microdialysis analyzer. with warm water (about 35 °C, no heat loss or gain occurred) to Glucose is an essential energy source for most cells and is a height of 18 cm [13]. The polystyrene board was then the primary energy source for brain tissue during exercise. As removed gently, and each gerbil was forced to swim for 15 min Glucose is a hydrophilic molecule, the facilitated diffusion of (if a gerbil was judged to be immobile, i.e., when it floated glucose across the plasma membrane is mediated by a family of passively with the head above water, it was allowed to rest for specific glucose transporters (GLUTs). To date, 13 members of 10 sec, and then forced to continue with the swimming this family of transporters have been identified [14-17]. exercise). After the forced swimming period, each gerbil was GLUT-3 and GLUT-5 are major glucose tansporter isoforms in removed from the water, and then carefully and rapidly dried the brain. GLUT-3 (a 496-amino-acid isoform) is a with a towel and a hair blower. In the first experiment, each high-affinity isoform largely expressed in neurons where it is gerbil was then placed on the polystyrene board again, and believed to be the predominant glucose transporter isoform; blood samples were collected during the 3 h recovery period. In GLUT-5 is expressed in microglia. On the other hand, the second experiment, each gerbil was placed on the magnesium regulates glycolysis [1], and we have reported that polystyrene board again, and allowed to rest for 30 min before magnesium supplementation enhanced glucose and GLUT-3 sacrificing. The gerbils were quickly decapitated and their levels in the cortex of gerbil [18]. Moreover, using a brains were removed and frozen (a diagram is shown in Fig. 1). microdialysis technique, we found that magnesium sulfate increased extracellular glucose levels in the striatum of gerbils that had been subjected a short-term period of forced swimming [19]. As the striatum is involved in the control of motor function and energy delivery [20], magnesium sulfate supplementation was also employed to evaluate changes in the striatum levels of GLUT-3, glucose, and magnesium during short-term forced swimming in this study. Materials and Methods Animals Adult and naïve male gerbils (n=28), weighing between 70 and 80 g, were obtained from the Laboratory Animal Center of the Taichung Veterans General Hospital. All animals were Figure 1. Brain sampling time procedures housed at a temperature of 25 °C in a light (12:12 h light-dark cycle)-controlled room with free access to rat chow and tap Auto-blood- sampling water. Animal care and experimental procedures (# La-95225, In the first experiment, blood samples were automatically TCVGH, Taiwan) were in accordance with the Guide for the collected via a catheter implanted in the jugular vein by a Care and Use of Laboratory Animals by the U. S. Department computer aided auto-blood sampling system (DR-II, Eicom, of Health and Human Services and with the policy statement Kyoto, Japan) in conscious, freely moving gerbils. Thirty µl regarding the care and use of experimental animals by the samples of whole blood were collected at 15 min intervals. In American College of Sports Medicine. The gerbils were order to preserve glucose, 15 µl of a NaF solution and 90 µl of randomly separated into two groups and subjected to two 1N HClO 4 were added to the whole blood sample, which was experiments. In the first experiment, gerbils (n=12) were then centrifuged at 1,000 g for 10 min at 4°C. 10 µl of Tris subjected to blood sampling during the forced swimming, and buffer was added to the aliquots of the supernatant (40 µl) and in the second experiment, the gerbils (n=16) were subjected to directly injected into the microdialysis analyzer (CMA/600, brain sampling. For each gerbil in the blood-sampling group, a Carnegie Medicin), equipped with a flame atomic absorption blunt dissection to clear the fat and connective tissue beside the spectrometer for detecting the concentrations of glucose, lactate jugular vein was performed. A length of PUC-40 tube was then and magnesium, respectively. connected to the jugular vein of each gerbil and blood was withdrawn according to the method described by Harms and Brain homogenization Ojeda [21]. The PUC-40 was connected to the DR-II, the In the second experiment, the gerbils (n=16) were randomly auto-blood-sampling device through a subcutaneous vein in the separated into four groups: saline control (C group, n=4), neck from the back.