Renal magnesium transport

Pathophysiology of Drug-Induced Hypomagnesaemia

Magnesium (Mg²⁺) is the second most abundant intracellular and fourth extracellular cation found in the body and is involved in a wide range of functions in the human cell and human physiology. Its role in most of the enzyme processes (ATP-ases)-stabilisation of nucleic acids (DNA, RNA), regulation of calcium and potassium ion channels, proliferation, glucose metabolism and apoptosis-make it one of the most important cations in the cell. Three pathogenetic mechanisms are mainly implicated in the development of hypomagnesaemia: reduced food intake, decreased intestinal absorption and increased renal excretion of Mg²⁺. This review presents the function of Mg²⁺, how it is handled in the kidney and the drugs that cause hypomagnesaemia. The frequency and the number of drugs like diuretics and proton-pump inhibitors (PPIs) that are used daily in medical practice are discussed in order to prevent and treat adverse effects by providing an insight into Mg²⁺ homeostasis.

Magnesium deficiency in critical illness

Magnesium (Mg) deficiency commonly occurs in critical illness and correlates with a higher mortality and worse clinical outcome in the intensive care unit (ICU). Magnesium has been directly implicated in hypokalemia, hypocalcemia, tetany, and dysrhythmia. Moreover, Mg may play a role in acute coronary syndromes, acute cerebral ischemia, and asthma. Magnesium regulates hundreds of enzyme systems. By regulating enzymes controlling intracellular calcium, Mg affects smooth muscle vasoconstriction, important to the underlying pathophysiology of several critical illnesses. The principle causes of Mg deficiency are gastrointestinal and renal losses; however, the diagnosis is difficult to make because of the limitations of serum Mg levels, the most common assessment of Mg status. Magnesium tolerance testing and ionized Mg2+ are alternative laboratory assessments; however, each has its own difficulties in the ICU setting. The use of Mg therapy is supported by clinical trials in the treatment of symptomatic hypomagnesemia and preeclampsia and is recommended for torsade de pointes. Magnesium therapy is not supported in the treatment of acute myocardial infarction and is presently undergoing evaluation for the treatment of severe asthma exacerbation, for the prevention of post-coronary bypass grafting dysrhythmias, and as a neuroprotective agent in acute cerebral ischemia.

Whole blood ionized magnesium: age-related differences in normal values and clinical implications of ionized hypomagnesemia in patients undergoing surgery for congenital cardiac disease

OBJECTIVES

We sought to (1) determine reference values for whole blood ionized magnesium concentrations in newborns, children, and young adults and (2) evaluate the frequency and clinical implications of ionized hypomagnesemia in patients undergoing surgery for congenital heart disease.

METHOD

We prospectively measured ionized magnesium concentrations in 299 subjects (113 control subjects and 186 patients undergoing surgery for congenital heart disease). Subjects were categorized by age. In the surgical group blood samples were obtained before bypass, during bypass (cooling and rewarming), after bypass, and during admission to the intensive care unit. Ionized hypomagnesemia was defined as ionized magnesium level 2 standard deviations below the mean of control subjects in the same age group. Patients were analyzed, controlling for cardiopulmonary bypass time.

RESULTS

In the control group ionized magnesium concentrations differed by age. Neonates and adults showed lower ionized magnesium concentrations compared with those of other age groups. Infants exhibited the highest ionized magnesium concentration. In the surgical group patients older than 1 month showed a higher proportion of ionized hypomagnesemia compared with that found in neonates at baseline (P <.001), after bypass (P =. 03), and at admission to the intensive care unit (P =.02). Controlling for cardiopulmonary bypass time, patients older than 1 month who were hypomagnesemic during bypass showed longer intubation time (P =.001) and longer intensive care stay (P =.01) and tended to have a higher pediatric severity of illness score on intensive care admission (P =.14) compared with patients without ionized hypomagnesemia.

CONCLUSIONS

There are age-related differences in normal ionized magnesium concentrations. Ionized hypomagnesemia is a common and clinically relevant occurrence among patients older than 1 month of age undergoing surgery for congenital heart disease.

Serum ionized magnesium monitoring during orthotopic liver transplantation

During orthotopic liver transplantation (OLT) citrate accumulates and magnesium can be chelated, which can lead to ionized hypomagnesemia and cardiovascular dysfunction. Our aim was to study the serum ionized magnesium (Me2+) evolution and establish its relation to serum total Mg and citrate levels during OLT. We studied 58 adult patients undergoing OLT. The serum Me2+ level dropped significantly at the end of the preanhepatic phase, and remained low until the end of the procedure. Furthermore, the Me2+ levels remained below the range of reference from the beginning of the anhepatic phase onward. There was an inverse correlation between Me2+ and citrate for all patients. Me2+, like ionized calcium (Ca2+), is chelated by citrate and its evolution is a mirror image of that of citrate. In our patients, we did not observe any significant dysrhythmias that could be directly attributed to ionized hypomagnesemia. In conclusion, low preoperative levels, together with the massive transfusion of blood products and the increase in renal losses, cause progressive ionized hypomagnesemia in OLT patients. We propose that it he routinely monitored and treated accordingly, as is already done with Ca2+.

Hypocalcaemia and chronic alcohol intoxication: transient hypoparathyroidism secondary to magnesium deficiency

The authors report the observation of an alcoholic patient admitted for tetanic manifestations, in whom severe hypocalcaemia associated with hyperphosphatemia were suggestive of hypoparathyroidism. Administration of magnesium supplementation alone improved the clinical features and led to the correction of the calcium abnormalities. The mechanisms of hypomagnesemia in alcohol intoxication are reviewed as well as the links with hypocalcaemia.

Nutritional support of critically ill patients

Severe depletion of body protein stores can result from prolonged starvation or from hormonal and cytokine-mediated effects during critical illness. Recent advances in the understanding of cytokine actions have substantially refined the interpretation of the nutritional assessment of critically ill patients. In addition, the design of nutritional programs for hospitalized patients has changed considerably during the past decade. Although nutritional support of critically ill patients will not lead to positive nitrogen balance, nutrition can increase protein synthesis, enhance immune function, and beneficially modify the body's response to an illness.

Double-blind, placebo-controlled trial of alternate-day furosemide therapy in infants with chronic bronchopulmonary dysplasia

To test the hypothesis that alternate-day administration of furosemide will result in a sustained improvement in pulmonary function without causing alterations in electrolyte or mineral homeostasis, we conducted a randomized, double-blind, placebo-controlled study of 11 hospitalized, oxygen-dependent, spontaneously breathing infants with chronic bronchopulmonary dysplasia. Infants were randomly selected to receive either furosemide, 4 mg/kg in two divided doses on alternate days orally, or placebo for 8 days, followed by crossover to the alternate-therapy for an additional 8-day period. The two study periods were separated by a 48-hour washout period. Dynamic compliance, total pulmonary resistance, the concentration of electrolytes in serum, and the concentrations of calcium and creatinine in urine were measured on nontreatment days. Alternate-day furosemide therapy increased dynamic lung compliance by 76 +/- 112% and decreased total pulmonary resistance by 20 +/- 39%, compared with placebo (both variables p = 0.032). Alternate-day furosemide therapy did not result in increased urine output, electrolyte abnormalities, or increased urinary calcium excretion. We conclude that this simplified treatment regimen may be useful in the management of infants with chronic bronchopulmonary dysplasia. The results support our previous speculation that furosemide improves pulmonary function by mechanisms unrelated to its diuretic properties.

Cytosolic free magnesium in cardiac myocytes: identification of a Mg2+ influx pathway

Regulation of intracellular Mg2+ activity in the heart is not well characterized. Cardiac myocytes were prepared as primary cultures from 7 day old chick embryo hearts and intracellular Mg2+ concentration [( Mg2+]i) was determined in single ventricular cells with mag-fura-2. Basal [Mg2+]i was 0.48 +/- 0.03 mM in normal culture medium. There was no correlation of basal [Mg2+]i with cellular contraction or intracellular [Ca2+]i (determined with fura-2). Cardiocytes cultured (16 hr) in low Mg (0.16 mM) media contained 0.21 +/- 0.05 mM Mg2+ which returned to normal levels when placed in Mg media with a refill time of 20 min. Basal [Ca2+]i (121 +/- 11 nM) and stimulated [Ca2+]i (231 +/- 41 nM) was similar to control cells. Verapamil, 25 microM, reversibly blocked Mg2+ refill. In conclusion, the basal [Mg2+]i of isolated cardiomyocytes is considerably below the Mg2+ electrochemical equilibrium allowing passive Mg2+ influx. The influx pathway for Mg2+ is inhibited by verapamil and appears to be independent of Ca2+ as assessed by fura-2.

Magnesium levels in the body fluids of dam and fetal rabbits during the reproductive phase

Gestation- and lactation-dependent variations of magnesium concentrations were measured in the maternal and fetal body fluids and tissues of rabbits. Samples of serum, milk, and mammary gland were investigated. A decrease in magnesium in the maternal serum during the early gestation period from 0.80 mmol/l in the first week to 0.73 mmol/l in the third week was observed. In the last week of gestation a rapid increase of magnesium concentrations both in maternal (0.83 mmol/l) and fetal (2.84 mmol/l) serum was determined. An assumed active diaplacental transfer of magnesium from the doe to the fetus was indirectly established. An active transport mechanism for magnesium from the maternal serum into the mammary glands was enhanced by strong concentration gradients of approximately 1:33 between serum and milk.