Showing papers in "Magnesium Research in 1999"

Magnesium deficiency-induced osteoporosis in the rat: uncoupling of bone formation and bone resorption.

Abstract: Magnesium (Mg) intake has been linked to bone mass and/or rate of bone loss in humans. Experimental Mg deficiency in animal models has resulted in impaired bone growth, osteopenia, and increased skeletal fragility. In order to assess changes in bone and mineral homeostasis that may be responsible, we induced dietary Mg deficiency in adult Simonsen albino rats for 16 weeks. Rats were fed either a low Mg diet (0.002 percent) or a normal control Mg diet (0.063 percent). Blood was obtained at baseline, 4 weeks, 8 weeks, 12 weeks and 16 weeks in both groups for serum Mg, calcium, PTH, and 1.25(OH)2-vitamin D determinations. Femora were harvested at 4 weeks and 16 weeks for mineral analysis and histomorphometry. Serum Mg fell in the Mg depleted group to 0.6 mg/dl (mean) by 16 weeks (controls = 2.0 mg/dl). The serum calcium (Ca) concentration was higher in the Mg depleted animals at 16 weeks, 10.8 mg/dl (controls = 8.9 mg/dl). Serum PTH concentration fell progressively in the Mg deficient rats to 30 pg/ml by week 16 (control = 96 pg/ml). Serum concentration of 1.25(OH)2-vitamin D also fell progressively in the Mg deficient animals by 16 weeks to 14 pg/ml (control = 30 pg/ml). While the percent ash weights of Ca and phosphorus in the femur were not different at any time point, the percent ash weight of Mg progressively fell to 0.54 percent vs control (0.74 percent) by 16 weeks. The percent ash weight of potassium also fell progressively in the Mg deficient group to approximately 30 percent of control by 16 weeks. Histomorphometric analyses showed a significant drop in trabecular bone volume in Mg deficient animals by 16 weeks (percent BV/TV = 13.2 percent vs 17.3 percent in controls). Evaluation of the endosteal bone surface features showed significantly greater bone resorption in the Mg depleted group as reflected in increased number of tartrate-resistant positive osteoclasts/mm bone surface (7.8 vs 4.0 in controls) and an elevated percent of bone surface occupied by osteoclasts (percent OcS/BS = 12.2 percent vs 6.7 percent in controls. This increased resorption occurred in the presence of an inappropriate lowered bone forming surface relative to controls; a decreased number of osteoblasts per mm bone surface (0.23 vs 0.94 in control) and a decrease in percent trabecular surface lined by osteoid (percent OS/BS = 0.41 vs 2.27 percent in controls) were also noted. Our findings demonstrate a Mg-deficiency induced uncoupling of bone formation and bone resorption resulting in a loss of bone mass. While the fall in PTH and/or 1.25(OH)2-D may explain a decrease in osteoblast activity, the mechanism for increased osteoclast activity is unclear. These data suggest that Mg deficiency may be a risk factor for osteoporosis.

Influence of low magnesium concentrations in the medium on the antioxidant system in cultured human arterial endothelial cells.

Abstract: Using cultured human endothelial cells, we investigated the contribution of concentrations of magnesium to the antioxidant system and oxidative stress. Cells were cultured at decreasing magnesium levels (569, 380, 190 and 95 microM) for 72 h. We then measured the amount of released hydrogen peroxide (H2O2) from the cells, the consumption of exogenous H2O2, the intracellular reduced glutathione (GSH) and the oxidized glutathione (GSSG) contents and the activities of glutathione reductase and catalase. Magnesium at a level of 949 microM was used as a control. The effect of magnesium deficiency on cellular membrane permeability was determined by measurement of the amount of [14C] amino acid mixture released from the cells. The results showed that during 72 h of magnesium-deficient treatment, the H2O2 release from the cells gradually increased and consumption of exogenous H2O2 was enhanced during the first 48 h of treatment. GSH content gradually decreased but GSSG was not affected. The activity of glutathione reductase was first stimulated and then inhibited. Catalase activity was gradually reduced. [14C]Amino acid mixture release from the cells continuously increased. We suggest that magnesium deficiency affected the intracellular antioxidant system in cultured endothelial cells.

Prevalence of magnesium and zinc deficiencies in nursing home residents in Germany.

Abstract: In a multicentric study with 345 seniors over 70 years old we investigated magnesium and zinc levels in serum together with the prevalence of their typical symptoms of deficiency in nursing home residents (NHR) and non-nursing home residents (nNHR). In addition calcium, sodium and potassium levels in serum were determined as well as creatinine and albumin. Considering all seniors 33 per cent exhibited hypomagnesemia and 19 per cent hypozincemia. Zinc levels of female and male NHR were significantly lower than levels of nNHR. Hypomagnesemia was significantly associated with calf cramps and with diabetes mellitus. Hypozincemia was significantly associated with impaired wound healing.

Brain free magnesium concentration is predictive of motor outcome following traumatic axonal brain injury in rats.

Abstract: A number of studies have supported a role for brain free magnesium as an important secondary injury factor in the development of neurologic deficits following traumatic brain injury. Despite this, few studies have characterised free magnesium changes in diffuse models of brain injury relevant to clinical trauma, and none have critically examined the association between brain free magnesium concentration and degree of neurologic deficit following graded trauma. In the present study, a combination of nuclear magnetic resonance spectroscopy and rotarod motor function tests were used to characterise the relationship between brain free magnesium concentration and neurologic motor function following graded traumatic axonal brain injury in rats. Induction of moderate or severe impact-acceleration induced traumatic brain injury resulted in a profound decline (p < 0.01) in brain free magnesium concentration that persisted for a minimum of 4 days post-trauma in both injury groups. Posttraumatic rotarod deficits assessed on a daily basis after injury were linearly correlated with brain free magnesium concentration measured in the same animals immediately after the motor tests were performed (r = 0.87; p < 0.001). These results suggest that brain free magnesium declines following graded diffuse axonal brain injury and that the concentration of the ion after trauma may be a prognostic indicator of motor outcome following.

Magnesium in drinking water and the risk of death from diabetes mellitus.

Abstract: This report examines whether magnesium in drinking water is protective against the probability of dying from diabetes mellitus. All eligible deaths from diabetes (6781 cases) of Taiwan residents from 1990 through 1994 were compared with deaths from other causes (6781 controls), and the levels of magnesium in the drinking water of these residents was determined. Data on magnesium levels in drinking water throughout Taiwan were obtained from the Taiwan Water Supply Corporation (TWSC). Controls were pair matched to the cases by sex, year of birth, and year of death. The results of the present study show that there seems to be a significant protective effect of magnesium intake from drinking water on the risk of dying from diabetes mellitus. This is an important finding for the Taiwan water industry and human health.

The possible role of magnesium in protection of premature infants from neurological syndromes and visual impairments and a review of survival of magnesium-exposed premature infants.

Abstract: The survival rate of very preterm, low birth weight infants (weighing less than 1500 g) is 85 per cent in the USA and is ever increasing, while 42 to 75 per cent of extremely premature infants (weighing 751-1000 g) survive. Of great concern is the lack of consistent decrease in neurological syndromes and associated visual impairments. Because of short gestations, these infants have not had time to accrue up to 80 per cent of magnesium normally present at term. These very preterm infants are at highest risk for cerebral hypoxia/ischemia (H/I), intracranial hemorrhage (ICH), periventricular leukomalacia (PVL) or cystic PVL (CPVL), and possible sequelae, cerebral palsy (CP) and mental retardation (MR). These syndromes are associated with damage to optic structures and the visual pathways which traverse the brain. Visual defects are common in surviving preterm infants. Increased levels of harmful neurochemical mediators that have been reported in these conditions include oxygen free radicals, excitatory amino acids, tumor necrosis factor-alpha (TNF-a), and thromboxane A2 (TXA2) which are aggravated in magnesium deficiency and may be ameliorated by magnesium. We review the published data concerning the effects of prenatal magnesium supplementation on ICH, CPVL, CP and MR and available reports concerning survival. Further considerations on the safety and efficacy of magnesium sulphate administration given prenatally to the preterm neonate await the outcome of three trials that are continuing for more than a year on three continents.

Importance of timing of magnesium administration: a study on the isolated ischemic-reperfused rat heart

Abstract: The present study attempted to explore the antioxidant potential of magnesium (2 and 10 mM) in the postischemic isolated rat heart. Magnesium demonstrated a significant antioxidant effect, as deduced by thiobarbituric acid reactive material and glutathione levels. However, this effect was evident only when magnesium was administered prior to ischemia but not when administered during reperfusion. On the other hand, attenuation of calcium accumulation following reperfusion was noted in both treatment schedules and showed dose dependency when administered during reperfusion. Thus, the results of this study strongly suggests an antioxidant effect of magnesium which is distinct from its calcium antagonising effects and it also stresses the critical role played by the time of administration of magnesium.

Platelet magnesium depletion in normotensive and hypertensive obese subjects: the role of salt-regulating hormones and catecholamines.

Abstract: We measured plasma and platelet magnesium concentrations, plasma epinephrine and norepinephrine, and plasma aldosterone and renin concentrations in normotensive (NT-Ob, n = 19, BMI 35.7 +/- 7.4 kg/m2, WHR 0.92 +/- 0.05) and hypertensive (HT-Ob, n = 11, BMI 35.2 +/- 3.6 kg/m2, WHR 0.93 +/- 0.07) obese subjects, and in a group of age- and sex-matched lean controls (n = 14, BMI 23.1 +/- 1.8 kg/m2, WHR 0.79 +/- 0.05). Plasma aldosterone and renin concentrations were significantly higher in obese subjects with respect to controls. Moreover, plasma norepinephrine and epinephrine levels were significantly increased in obese subjects, and plasma norepinephrine was higher in HT-Ob when compared to NT-Ob group. Platelet magnesium concentrations were significantly reduced in both normotensive and hypertensive obese subjects with respect to controls (controls 2.65 +/- 0.35 mumol/10(8) cells, NT-Ob 2.02 +/- 0.19 mumol/10(8) cells--p < 0.001, HT-Ob 1.98 +/- 0.18 mumol/10(8) cells--p < 0.001), while a slightly significant decrease in plasma magnesium levels was only detectable in HT-Ob group. Urinary magnesium and magnesium fractional excretion were significantly increased in hypertensive obeses. Pearson's correlation analysis, separately performed in each group of subjects, showed that plasma aldosterone, renin, epinephrine, norepinephrine and magnesium fractional excretion were negatively correlated to platelet magnesium levels in NT-Ob and HT-Ob groups, but not in lean controls. The multiple linear regression analysis performed in the whole group of obese subjects considering platelet magnesium as a dependent variable showed that platelet magnesium decrease together with the increase in plasma epinephrine (p = 0.046) and norepinephrine (p = 0.020), also after adjusting for age, sex, BMI, WHR, HOMA IR and diagnosis of hypertension. Furthermore, platelet magnesium showed a trend for negative association (p < 0.1) to plasma aldosterone and magnesium fractional excretion in multivariate analysis. The impairment in platelet magnesium handling observed in normotensive and hypertensive obese patients seems to be associated to a rise in renin-angiotensin-aldosterone and sympathetic systems activity. Our results suggest that platelet magnesium depletion, together with disturbances of salt-regulating hormones and catecholamines, may be involved in the pathophysiology of cardiovascular complications from obesity.

Biotechnological implications of the interactions between magnesium and calcium

Abstract: At the biochemical level, magnesium and calcium are known to act antagonistically towards each other. For example, many enzymes whose activities critically depend on sufficiency of intracellular magnesium, especially transphosphorylases, will be detrimentally affected by small increments in levels of cellular calcium ions. Growth of cells, cell division cycle progress and intermediary metabolism are also absolutely dependent on the bioavailability of magnesium, which can be compromised if excess calcium is present. Many biotechnological processes, therefore, which fundamentally exploit cellular growth and metabolism, will be influenced strongly by relative concentrations of the two cations and the ratio of bioavailable magnesium to calcium in external growth media. This paper surveys the interactive effects of magnesium and calcium at the bioinorganic level and discusses some examples of how these interactions may play important roles in governing the physiology of cells which are widely exploited in modern industrial bioprocesses.

Magnesium therapy in type 1 diabetes. A double blind study concerning the effects on kidney function and serum lipid levels.

Abstract: To test the hypothesis that magnesium depletion might be of importance for the development of vascular complications in diabetes mellitus we performed a randomized, double-blind, placebo-controlled study during 12 months with 20-30 mmol/day of oral magnesium hydroxide in 28 type 1 diabetic patients Urinary albumin excretion, Cr-EDTA-clearance and certain blood cardiovascular risk factors were measured At the end of the study there were no significant differences of these parameters between the two groups, except that serum triglyceride values increased in three magnesium treated patients who either showed an increase in blood glycosylated hemoglobin values or body weight during the study

Do forestomach epithelia exhibit a Mg2+/2H(+)-exchanger?

Abstract: In ruminants the forestomachs (reticulum, rumen and omasum) represent the main site of Mg absorption. Readily fermentable carbohydrates enhance Mg availability for ruminants in vivo. The beneficial effect of carbohydrate addition on Mg absorption occurs preintestinally and is associated with changes in the rumen fluid (short chain fatty acids, SCFA, luminal pH, carbon dioxide, NH3, lactate, osmolarity). SCFA and HCO3-/CO2 increase Mg efflux from the isolated reticulorumen in vivo and stimulate 28Mg flux from mucosal to serosal across rumen epithelium in vitro. The stimulatory effect of SCFA on Mg absorption differs between acids, being a function of SCFA absorption and possibly SCFA metabolism. The stimulatory effect of HCO3-/CO2 is mediated by carbonic anhydrase activity. Experimental data suggest that the availability of protons inside the cell might be involved in the stimulation of Mg absorption. The contribution of Mg/H exchange to Mg2+ uptake across the apical membrane of rumen epithelium is discussed in comparison with other possible ways of stimulation (Mg/Na exchange, Mg/anion cotransport, electrogenic and metabolic effects of SCFA). It remains to be shown that Mg2+ uptake into rumen epithelium is directly linked to an efflux of protons. However, an apical Mg/H exchange (in contrast to other suggested Mg transporters) would explain a variety of in vivo and in vitro observations on ruminal Mg absorption.

Pharmacological effects of magnesium on arterial thrombosis : mechanisms of action?

Abstract: At present no consensus exists on the role of magnesium in acute myocardial infarction and this is primarily due to conflicting results from recent clinical trials. The ISIS-4 trial clearly showed that magnesium infusion is without benefit when given after thrombolysis or many hours after symptom onset. In contrast, the LIMIT-2 study provided strong evidence that early magnesium administration, given before any thrombolytic therapy protects the myocardium and improves long-term survival. Debate on the interpretation of the trial results is still ongoing, and is particularly focused on the time-dependency of magnesium, which has emerged from recent experimental studies. Candidate mechanisms, by which magnesium might modify the outcome of acute myocardial infarction, includes antiarrhythmic properties, improved coronary perfusion and haemodynamics, protection of the ischaemic myocardium, and an antithrombotic effect. So far animal models have shown a time-dependent effect of magnesium when given for both myocardial protection in experimental ischaemia-reperfusion injury and for antithrombotic purposes. Additional clinical trials are warranted and these must be carefully designed and implemented in the light of the laboratory evidence now available. In the present review magnesium's antithrombotic properties is discussed with respect to its effect on platelets, coagulation, fibrinolysis, and endothelial mediators with vasodilating and antithrombotic qualities. Observations from studies in patients with preeclampsia, another clinical condition where platelet hyperreactivity is well-recognized and where magnesium therapy is well-established, are briefly discussed in the present paper.

Tissue concentrations of magnesium in rats receiving various dosages of ethanol.

Abstract: Ethanol exerts various influences on living organisms. The experiments were conducted in order to determine the influence of ethanol on changes of magnesium concentration in various tissues. The experiment was conducted on white Wistar rats of both sexes. The animals were divided into groups of ten rats each. They received ethanol in their drinking water for 6 weeks. The first group received 5 per cent ethanol, the second received 20 per cent ethanol, the third was the control group and received water. All animals were fed with the same laboratory feed and watered ad libitum. After the duration of the experiment the animals were sacrificed using ketamine. The brain, liver, heart, lung, kidney, spleen, femoral muscle and blood serum were then harvested for further testing. The magnesium concentration was tested in these tissues by using atomic absorption spectrophotometry (AAS). Ethanol in feed causes a shift in the tissue concentration of magnesium. The range of these concentration differences is determined by the dosages as well as by the type of tissue.

Magnesium in stomach cancer.

Abstract: Experiments were conducted in order to determine magnesium concentrations in stomach cancer tissue. As a control, non-cancerous stomach tissue of the same patients was used. Mg concentration in blood serum was also tested in all patients undergoing surgical operation. It has been determined that Mg concentration was higher in cancerous stomach tissue than in control tissue. Average Mg concentration in the blood serum of patients with stomach cancer fluctuates near the lower limit of normal. An analysis of the relationship between Mg concentration and the degree of clinical advancement of cancer has shown that Mg concentration in cancerous stomach tissue increases with the clinical stage. The results obtained were statistically significant. They show that there is a relationship between Mg concentration and the development of stomach cancer.

Correlation of serum HDL-cholesterol and LCAT levels with the fraction of ionized magnesium in children.

Abstract: Correlation of serum lipids and apolipoprotein levels with serum total magnesium concentration and whole blood ionized magnesium level was determined in 47 children (14 female and 33 male; mean age, 8.7 +/- 4.2 years). Mean serum concentration of magnesium was 2.19 +/- 0.19 mg/dl, whole blood concentration of ionized magnesium 1.23 +/- 0.08 mg/dl, and fraction of ionized magnesium (ratio of whole blood ionized magnesium to serum total magnesium) 0.56 +/- 0.04. Neither serum total magnesium level nor whole blood ionized magnesium level had any correlation with serum albumin, lipid, and apolipoprotein levels. However, the fraction of ionized magnesium was significantly correlated with HDL-cholesterol (n = 46, r = 0.31, p = 0.0345), apolipoprotein A-1 (n = 41, r = 0.39, p = 0.0124), and lecithin-cholesterol acyltransferase (LCAT) (n = 20, r = 52, p = 0.0184). These results suggest that fraction of ionized magnesium is more closely linked to serum HDL-cholesterol and LCAT level than with the serum total magnesium level or whole blood ionized magnesium.

Influence of age and hormonal treatment on intestinal absorption of magnesium in ovariectomised rats.

Abstract: This study was designed to assess the effect of age, ovariectomy and estrogen treatment on the absorption and the balance of Mg in the rat. Three groups of fifteen 6- (mature), 12- (old), and 30-month-old female rats (senescent), fed a diet containing 1.5 g of Mg/kg were used in the present study. Within each group, 10 rats were surgically ovariectomized (OVX). From day 2 until day 60 after OVX, they were s.c. injected with either solvant or 17 beta-estradiol (E: 10 mg/kg bw/48 h, n = 5). Five other rats were sham operated (SH, n = 5) and received solvent alone. Animals were pair fed 6 g/100 g bw/day and distilled water was available ad libitum. Food intake, urine and feces from each individual rat were measured 1 day per week, during the last 5 weeks. The results clearly showed that apparent Mg absorption (per cent) was not significantly altered with aging. Moreover, whatever the age, neither OVX nor E treatment had any significant effect on Mg apparent absorption.

Cardiovasoprotective foods and nutrients: possible importance of magnesium intake.

Abstract: Various highly efficient cardiovasoprotective diets associate reduced intake of total and saturated fats, reasonable supply of monounsaturated fat and omega-3 fatty acids, moderate consumption of alcohol, increased intake of cereals, fruits, vegetables, fish and low fat dairy products. Among several protective nutrients magnesium should be given particular consideration because of the very frequent occurrence of chronic primary magnesium deficiency which appears to act as a cardiovascular risk factor and also reversely, because of the noticeable high level of the magnesium content in cardiovasoprotective diets.

Preliminary communication on intralymphocyte ionized magnesium in hypertensive patients under treatment with beta-blockers.

Abstract: We measured free intralymphocyte magnesium (Mgi) in 10 hypertensive subjects (HS) before and after treatment with atenolol (A) and in 16 normotensive control subjects (NC). We also carried out in vitro studies on human lymphocytes to test the effect of catecholamines and beta-antagonists on Mgi. Mgi in HS was not statistically different before and after the treatment with A (M +/- SD, basal: 227 +/- 31 + mumol/l; A: 236 +/- 47 mumol/l; NS). Mgi in NC proved not statistically different as compared to HS without treatment (M +/- SD, NC: 232 +/- 35 mumol/l; HS before treatment: 227 +/- 31 mumol/l; NS). The Mgi of the lymphocytes treated in vitro with noradrenaline (NA) significantly decreased in comparison with the control (M +/- SD, basal: 244 +/- 26 mumol/l; NA: 198 +/- 25 mumol/l; p = 0.0001), whereas it did not change after incubation with NA and propranolol (P) (M +/- SD, basal: 238 +/- 32 mumol/l; NA + P: 239 +/- 30 mumol/l; NS). On the other hand Mgi decreased significantly after incubation with NA and A (M +/- SD, basal: 255 +/- 21 mumol/l; NA + A: 202 +/- 34 mumol/l). Our in vivo data show that the Mgi of HS remains unvaried after treatment with A, and that there is not statistical difference between Mgi of NC and of HS before treatment. The in vitro study demonstrated that catecholamines cause a significant reduction in Mgi. This effect is only inhibited by a non-selective beta-blocker such as P. These results seem to explain the ineffectiveness of treatment with atenolol on our patients' Mgi.

Free and total circulating magnesium following glucagon injection in humans.

Abstract: It has been postulated that parathormone, calcitonine, insulin and catecholamines are involved in extracellular magnesium homeostasis. Yet, there is still a rudimentary knowledge of the endocrine factors that control circulating magnesium homeostasis. The effects of exogenous glucagon injection on circulating total and ionized magnesium were investigated in 11 healthy humans (five females and six males, aged between 21 and 30, median 26 years). As compared with a control study, intravenous injection of a bolus of 1 mg of glucagon was associated with the expected raised glucose (at 5, 10, 20 and 30 min) and with decreased potassium (at 20 and 30 min) and inorganic phosphate (at 20 and 30 min) levels. Intravenous glucagon was not followed by significant changes in plasma total and ionized magnesium. Consequently, there is still little evidence for a glucagon-dependent control of the extracellular magnesium concentration after acute administration of glucagon.

Improvement of muscle protein functionality in processed meats by magnesium and other divalent chloride salts.

Abstract: Concern over dietary fat in processed meats led to the passage of the '40 per cent' rule in the United States. Substitution of NaCl, linked to hypertension, with divalent chloride salts such as MgCl2 and CaCl2 has shown limited success. Early studies showed that these divalent salts had a deleterious effect on the functional properties of meat when used at product levels that resulted in high aqueous phase ionic strengths (0.4-0.6). However, our research focus has been to determine the utility of low levels (0.05 per cent) of MgCl2, CaCl2 and ZnCl2 in improving the functional properties of processed meats. Effects of divalent salts have been evaluated in turkey breast and thigh minces, beef model systems, and frankfurter formulations containing heart muscle. To determine if time postmortem affects muscle's response to divalent cations, salts were added to broiler thigh muscle in the early postmortem period. The important findings were (1) MgCl2 increased myosin solubility, (2) CaCl2 enhanced gel forming ability in cooked batters, and (3) ZnCl2 dramatically decreased myosin solubility in the absence of food-grade phosphate (sodium tripolyphosphate).

Associated thyrogastric autoimmunity increases the prevalence of low erythrocyte magnesium in type 1 diabetes

Abstract: A group of 230 type 1 (insulin-dependent) diabetic patients were screened for the presence of thyrogastric autoantibodies (aTPO and PCA) and magnesium depletion. Thirty-seven per cent presented with significant levels of 1 autoantibody and 7 per cent were positive for both. Exactly 25 per cent of the subjects had low levels of erythrocyte Mg (RBC-Mg < 5.5 mg/dl). Female patients were more prone to lower RBC-Mg and had a significant higher prevalence of aTPO and, although actually euthyroid, had a more pronounced history of thyroid disease. The presence of both antibodies was accompanied with the highest prevalence of low RBC-Mg. However, PCA positivity with hypergastrinaemia alone did not show a significant increase of the Mg depletion. The mechanisms involved in this phenomenon remain unclear but besides gender, duration of diabetes and the metabolic consequences of the disease, the presence of associated thyroid and gastric dysfunction can play a supplementary role in the maintenance of the chronic Mg problems in type 1 diabetes.

Effects of parenteral pharmacological magnesium loading on insulin secretion in experimental thyrotoxicosis.

Abstract: Thyrotoxicosis is characterised by decreased magnesium pool and also insulin resistance. The present study is evaluating the parameters of glucose metabolism under pharmacological magnesium loading in experimentally induced thyrotoxicosis, in rats. Insulin secretion was significantly increased in thyrotoxicosis compared to controls, expressing probably the status of insulin resistance due to thyroxine excess. After intraperitoneal magnesium infusion, plasma magnesium reached pharmacologically high concentrations and insulin secretion decreased significantly, but this decrease was not accompanied by alterations of glucose homeostasis. In controls, we also found a tendency towards the decrease of insulin secretion after magnesium loading, but it did not reach statistical significance. Thus, insulin secretion seems more sensitive to the inhibitory effects of magnesium overload in experimental thyrotoxicosis.

A macrocyclic reporter ligand for Mg2+: analytical implications for clinical magnesium determinations.

Abstract: A new approach is presented for measuring Mg in plasma using the macrocyclic reporter ligand, NOTMP (1, 4, 7-triazacyclononane- 1, 4, 7-tris (methylene methylphosphinate)) and 31P nuclear magnetic resonance spectroscopy (NMR). By virtue of its intermediate binding constant for Mg (Kd = 0.35 mM), measurements of Mg using NOTMP allows one to discriminate between Mg bound to the high and low affinity ligands present in plasma, when combined with more conventional measurements of Mg. We used this approach in conjunction with measurements of total Mg using atomic absorption spectroscopy (AAS) and ionized Mg using an ion selective electrode (ISE) to characterize the distribution of Mg in the plasma of 16 normal adults. The percentage of Mg distributed among high and low affinity ligands and in an ionized fraction was 31, 14 and 55 per cent respectively. Similar measurements on plasma following equilibrium dialysis suggest that the high and low affinity ligands in plasma correspond to high and low molecular weight compounds, respectively. Measurement of Mg by NOTMP, AAS and ISE were not affected by the storage of blood samples for up to 48 h at 4 degrees C. The addition of MgSO4 to plasma and its subsequent analysis by these three methods suggests that the added Mg is primarily distributed among the high affinity ligands (mostly likely proteins) and ionized fractions. The approach presented here may offer novel insights into assessing the distribution of Mg in clinical samples.