Effect of 1,25-dihydroxyvitamin D3 on calcium and magnesium absorption in the healthy human jejunum and ileum

Total serum and ionized magnesium concentrations in healthy and hospitalized dogs

OBJECTIVE

To determine if an association between ionized magnesium (iMg) and total magnesium (tMg) exists in healthy and hospitalized dogs admitted through an emergency service and to assess the associations between iMg and tMg with total protein, albumin, ionized calcium, and total calcium.

DESIGN

Prospective cohort study.

SETTING

Veterinary teaching hospital.

ANIMALS

Sixty-nine dogs were enrolled. The healthy control group (group 1) included 24 dogs, and the hospitalized group (group 2) included 45 dogs.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

For both groups, signalment, a venous blood gas, and serum biochemistry were obtained. In addition, the presumptive diagnosis was recorded for group 2. Blood was obtained prior to any therapeutic interventions. Group 1 tMg was within the reference interval (RI), and the values for iMg were used to provide a healthy group range (HGR) of 0.44-0.50 mmol/L. Group 2 tMg was within the RI, but iMg was below the calculated HGR range (group 2 median iMg = 0.4 mmol/L; range = 0.27-0.70). There was a significant positive correlation between iMg and tMg in each group (group 1: r = 0.6713, P = 0.0003; group 2: r = 0.5312, P = 0.0002). Ionized Mg and tMg were not significantly associated with any of the other evaluated variables in either group.

CONCLUSIONS

Ionized Mg and tMg were significantly associated in both healthy and hospitalized dogs, but the relationship was weaker in the hospitalized dogs compared with the healthy population. For hospitalized dogs, the relationship was weak enough to question the validity of using iMg and tMg interchangeably to track magnesium status.

Unapparent systemic effects of regional anticoagulation with citrate in continuous renal replacement therapy: a narrative review

The use of citrate, through reversible binding of calcium, has become the preferred choice for anticoagulation in continuous renal replacement therapy in the critically ill patient. Though generally considered as very efficacious in acute kidney injury, this type of anticoagulation can cause acid-base disorders as well as citrate accumulation and overload, phenomena which have been well described. The purpose of this narrative review is to provide an overview of some other, non-anticoagulation effects of citrate chelation during its use as anticoagulant. We highlight the effects seen on the calcium balance and hormonal status, phosphate and magnesium balance, as well as oxidative stress resulting from these unapparent effects. As most of these data on these non-anticoagulation effects have been obtained in small observational studies, new and larger studies documenting both short- and long-term effects should be undertaken. Subsequent future guidelines for citrate-based continuous renal replacement therapy should take not only the metabolic but also these unapparent effects into account.

Intestinal Calcium Absorption

In this article, we focus on mammalian calcium absorption across the intestinal epithelium in normal physiology. Intestinal calcium transport is essential for supplying calcium for metabolism and bone mineralization. Dietary calcium is transported across the mucosal epithelia via saturable transcellular and nonsaturable paracellular pathways, both of which are under the regulation of 1,25-dihydroxyvitamin D₃ and several other endocrine and paracrine factors, such as parathyroid hormone, prolactin, 17β-estradiol, calcitonin, and fibroblast growth factor-23. Calcium absorption occurs in several segments of the small and large intestine with varying rates and capacities. Segmental heterogeneity also includes differential expression of calcium transporters/carriers (e.g., transient receptor potential cation channel and calbindin-D_9k ) and the presence of favorable factors (e.g., pH, luminal contents, and gut motility). Other proteins and transporters (e.g., plasma membrane vitamin D receptor and voltage-dependent calcium channels), as well as vesicular calcium transport that probably contributes to intestinal calcium absorption, are also discussed. © 2021 American Physiological Society. Compr Physiol 11:1-27, 2021.

Acute Tissue Mineral Deposition in Response to a Phosphate Pulse in Experimental CKD

Pathogenic accumulation of calcium (Ca) and phosphate (PO₄ ) in vasculature is a sentinel of advancing cardiovascular disease in chronic kidney disease (CKD). This study sought to characterize acute distribution patterns of radiolabeled ³³ PO₄ and ⁴⁵ Ca in cardiovascular tissues of rats with CKD (0.25% dietary adenine). The disposition of ³³ PO₄ and ⁴⁵ Ca was assessed in blood and 36 tissues after a 10-minute intravenous infusion of one of the following: (i) PO₄  pulse + tracer ³³ PO₄ ; (ii) PO₄  pulse + tracer ⁴⁵ Ca; or (iii) saline + tracer ⁴⁵ Ca in CKD and non-CKD animals. After the infusion, ³³ PO₄ in blood was elevated (2.3× at 10 minutes, 3.5× at 30 minutes, p < 0.05) in CKD compared with non-CKD. In contrast, there was no difference in clearance of ⁴⁵ Ca from the blood. Compared with controls, CKD rats had a markedly increased ³³ PO₄ incorporation in several tissues (skeletal muscle, 7.8×; heart, 5.5×), but accrual was most pronounced in the vasculature (24.8×). There was a significant, but smaller, increase in ⁴⁵ Ca accrual in the vasculature of CKD rats (1.25×), particularly in the calcified rat, in response to the acute phosphate load. Based on the pattern of tissue uptake of ³³ PO₄ and ⁴⁵ Ca, this study revealed that an increase in circulating PO₄ is an important stimulus for the accumulation of these minerals in vascular tissue in CKD. This response is further enhanced when vascular calcification is also present. The finding of enhanced vascular mineral deposition in response to an acute PO₄  pulse provides evidence of significant tissue-specific susceptibility to calcification. © 2018 American Society for Bone and Mineral Research.

Serum Concentration of Macro-, Micro-, and Trace Elements in Silver Fox (Vulpes vulpes) and Their Interrelationships with Morphometric, Densitometric, and Mechanical Properties of the Mandible

The optimal content of macro-, micro-, and trace elements in tissues ensures proper systemic growth and development and optimal health status in animals and humans. However, very little is known on the elemental content in the plasma compartment in Silver fox. The aim of this study was to determine the content of selected elements in serum obtained from 8-month-old female (N = 8) and male (N = 7) silver foxes. Moreover, relationships of the evaluated elements with the morphological, densitometric, and mechanical parameters of the mandible were determined. Serum content of 12 different elements was measured using inductively coupled plasma-atomic emission spectrometry. The morphometric and densitometric properties of the mandible were determined using quantitative computed tomography method, while mechanical endurance was tested using a three-point bending test. Serum concentration of calcium was significantly higher by 20% in male foxes (P = 0.01), while manganese concentration was significantly lower in males by over 17% (P = 0.03). Positive correlations of serum concentration of calcium, phosphorus, and magnesium with the morphological traits of the mandible such as weight, length, and bone volume were stated (P < 0.05). In the group of elements playing regulatory functions, the positive relationships between serum concentrations of selenium, chromium, manganese, copper, and cobalt were found (P < 0.05). The elaborated experimental model may serve for further studies on foxes, especially focused on nutritional factors affecting elemental homeostasis, whole-body metabolism, and systemic growth and development. Daily diet formulation and precise delivery for farm foxes, together with relatively large animal population maintained at the same environmental conditions, regularly subjected to slaughter procedure, enable economical experimentation with various dietary and pharmacological manipulations.

Vitamin D and Ageing

One hundred years has passed since the discovery of vitamin D as the active component of cod-liver oil which cured the bone disease rickets. Since then our knowledge of vitamin D has expanded tremendously and has included recognition of the importance of UV radiation as a source of the vitamin as well as the discovery of the vitamin as a nutrient, a pro-hormone and a potent steroid hormone with a major role in calcium and bone metabolism. In the last 25 years or so, the discovery of the vitamin D receptor in over 30 different body tissues together with the existence of the alpha-1-hydroxylase enzyme in these tissues provided evidence of a pleiotropic role of vitamin D outside its classical role in the skeleton. These important discoveries have provided the basis for the increasing interest in vitamin D in the context of nutritional requirements for health including the prevention of chronic diseases of ageing. The recent publication of the Dietary Reference Intake report on vitamin D and calcium by the North American Institute of Medicine (IOM) is the most comprehensive report to date on the basis for setting nutritional requirements for vitamin D. This chapter will summarize the nutritional aspects of vitamin D and discuss the changes in vitamin D metabolism and requirements with ageing. It will summarize key evidence on the relationship between vitamin D status and some of the main ageing related health outcomes including bone, muscle and cognitive health as well as survival focusing on the published literature in very-old adults (those >= 85 years of age).

The Importance of Magnesium in Clinical Healthcare

The scientific literature provides extensive evidence of widespread magnesium deficiency and the potential need for magnesium repletion in diverse medical conditions. Magnesium is an essential element required as a cofactor for over 300 enzymatic reactions and is thus necessary for the biochemical functioning of numerous metabolic pathways. Inadequate magnesium status may impair biochemical processes dependent on sufficiency of this element. Emerging evidence confirms that nearly two-thirds of the population in the western world is not achieving the recommended daily allowance for magnesium, a deficiency problem contributing to various health conditions. This review assesses available medical and scientific literature on health issues related to magnesium. A traditional integrated review format was utilized for this study. Level I evidence supports the use of magnesium in the prevention and treatment of many common health conditions including migraine headache, metabolic syndrome, diabetes, hyperlipidemia, asthma, premenstrual syndrome, preeclampsia, and various cardiac arrhythmias. Magnesium may also be considered for prevention of renal calculi and cataract formation, as an adjunct or treatment for depression, and as a therapeutic intervention for many other health-related disorders. In clinical practice, optimizing magnesium status through diet and supplementation appears to be a safe, useful, and well-documented therapy for several medical conditions.

Role of Vitamin D in Inflammatory Bowel Disease

Vitamin D is a secosteroid hormone that possesses immunomodulatory properties and has been demonstrated to potentially influence inflammatory bowel disease (IBD) pathogenesis and activity. Epidemiologic data have associated vitamin D deficiency with an increased risk of IBD, hospitalizations, surgery, and loss of response to biologic therapy. Conversely, IBD itself can lead to vitamin D deficiency. This bidirectional relationship between vitamin D and IBD suggests the need for monitoring and repletion of vitamin D, as needed, in the IBD patient. This review discusses the role of vitamin D in IBD and provides practical guidance on vitamin D repletion.

Vitamin D and bone health outcomes in older age

The aim of this review is to summarise the evidence linking vitamin D to bone health outcomes in older adults. A plethora of scientific evidence globally suggests that large proportions of people have vitamin D deficiency and are not meeting recommended intakes. Older adults are at particular risk of the consequences of vitamin D deficiency owing to a combination of physiological and behavioural factors. Epidemiological studies show that low vitamin D status is associated with a variety of negative skeletal consequences in older adults including osteomalacia, reduced bone mineral density, impaired Ca absorption and secondary hyperparathyroidism. There seems to be inconsistent evidence for a protective role of vitamin D supplementation alone on bone mass. However, it is generally accepted that vitamin D (17·5 μg/d) in combination with Ca (1200 mg/d) reduces bone loss among older white subjects. Evidence for a benefit of vitamin D supplementation alone on reducing fracture risk is varied. According to a recent Agency for Healthcare Research and Quality review in the USA the evidence base shows mixed results for a beneficial effect of vitamin D on decreasing overall fracture risk. Limitations such as poor compliance with treatment, incomplete assessment of vitamin D status and large drop-out rates however, have been highlighted within some studies. In conclusion, it is generally accepted that vitamin D in combination with Ca reduces the risk of non-vertebral fractures particularly those in institutional care. The lack of data on vitamin D and bone health outcomes in certain population groups such as diverse racial groups warrants attention.

The Na⁺/H⁺ exchanger isoform 3 is required for active paracellular and transcellular Ca²⁺ transport across murine cecum

Intestinal calcium (Ca²⁺) absorption occurs via paracellular and transcellular pathways. Although the transcellular route has been extensively studied, mechanisms mediating paracellular absorption are largely unexplored. Unlike passive diffusion, secondarily active paracellular Ca²⁺ uptake occurs against an electrochemical gradient with water flux providing the driving force. Water movement is dictated by concentration differences that are largely determined by Na⁺ fluxes. Consequently, we hypothesized that Na⁺ absorption mediates Ca²⁺ flux. NHE3 is central to intestinal Na⁺ absorption. NHE3 knockout mice (NHE3-/-) display impaired intestinal Na⁺, water, and Ca²⁺ absorption. However, the mechanism mediating this latter abnormality is not clear. To investigate this, we used Ussing chambers to measure net Ca²⁺ absorption across different segments of wild-type mouse intestine. The cecum was the only segment with net Ca²⁺ absorption. Quantitative RT-PCR measurements revealed cecal expression of all genes implicated in intestinal Ca²⁺ absorption, including NHE3. We therefore employed this segment for further studies. Inhibition of NHE3 with 100 μM 5-(N-ethyl-N-isopropyl) amiloride decreased luminal-to-serosal and increased serosal-to-luminal Ca²⁺ flux. NHE3-/- mice had a >60% decrease in luminal-to-serosal Ca²⁺ flux. Ussing chambers experiments under altered voltage clamps (-25, 0, +25 mV) showed decreased transcellular and secondarily active paracellular Ca²⁺ absorption in NHE3-/- mice relative to wild-type animals. Consistent with this, cecal Trpv6 expression was diminished in NHE3-/- mice. Together these results implicate NHE3 in intestinal Ca(2+) absorption and support the theory that this is, at least partially, due to the role of NHE3 in Na⁺ and water absorption.

Defining hypercalciuria in nephrolithiasis

The classic definition of hypercalciuria, an upper normal limit of 200  mg/day, is based on a constant diet restricted in calcium, sodium, and animal protein; however, random diet data challenge this. Here our retrospective study determined the validity of the classic definition of hypercalciuria by comparing data from 39 publications analyzing urinary calcium excretion on a constant restricted diet and testing whether hypercalciuria could be defined when extraneous dietary influences were controlled. These papers encompassed 300 non-stone-forming patients, 208 patients with absorptive hypercalciuria type I (presumed due to high intestinal calcium absorption), and 234 stone formers without absorptive hypercalciuria; all evaluated on a constant restricted diet. In non-stone formers, the mean urinary calcium was well below 200  mg/day, and the mean for all patients was 127±46  mg/day with an upper limit of 219  mg/day. In absorptive hypercalciuria type I, the mean urinary calcium significantly exceeded 200  mg/day in all studies with a combined mean of 259±55  mg/day. Receiver operating characteristic curve analysis showed the optimal cutoff point for urinary calcium excretion was 172  mg/day on a restricted diet, a value that approximates the traditional limit of 200  mg/day. Thus, on a restricted diet, a clear demarcation was seen between urinary calcium excretion of kidney stone formers with absorptive hypercalciuria type I and normal individuals. When dietary variables are controlled, the classic definition of hypercalciuria of nephrolithiasis appears valid.

Addressing vitamin D deficiency in Canada: a public health innovation whose time has come

There is disturbing evidence of widespread vitamin D deficiency in many population groups, particularly within nations at high latitude. Numerous recent studies in the scientific literature associate vitamin D deficiency with a colossal increase in morbidity and mortality. Since Canada is at higher latitude, this review assesses the vitamin D status within the Canadian population. This review was prepared by assessing available medical and scientific literature from Medline, as well as by reviewing several books and conference proceedings. A standard 25(OH)D level of 75-80nmol/l or more was used to indicate vitamin D sufficiency. Between 70% and 97% of Canadians demonstrate vitamin D insufficiency. Furthermore, studies assessing 25(OH)D levels of vitamin D at 25-40nmol/l reveal that many Canadians have profoundly deficient levels. Repletion of vitamin D3 with 2000IU/day for those not receiving judicious sun exposure and those with no contra-indications would likely achieve normalized levels in more than 93% of patients, without risk of toxicity. Explicit directives regarding vitamin D assessment and management are urgently required.

Intestinal vitamin D receptor is required for normal calcium and bone metabolism in mice

BACKGROUND & AIMS

Vitamin D receptor (VDR)-knockout mice develop severe hypocalcemia and rickets, accompanied by disruption of active intestinal calcium absorption. To specifically study the effects of VDR in intestinal calcium absorption, we investigated whether restoration of intestinal VDR is sufficient to recover the phenotype of VDR-knockout mice.

METHODS

We generated mice with intestine-specific transgenic expression of human VDR and crossed them to VDR knockout mice. The intestine, kidney, and bone phenotypes of the VDR- knockout mice with intestine-specific expression of human VDR (knockout/transgenic [KO/TG]) were analyzed.

RESULTS

Transgenic expression of VDR in the intestine of VDR-knockout mice normalized duodenal vitamin D-regulated calcium absorption as well as vitamin D-regulated calcium binding protein D9k and TRPV6 gene expression in the duodenum and proximal colon. As a result, animal growth and the serum levels of calcium and parathyroid hormone were normalized in KO/TG mice. Other phenotypes were revealed when calcium metabolism was normalized in KO/TG mice: serum 1,25 dihydroxyvitamin D levels were higher in KO/TG mice than normal mice owing to reduced renal expression of the vitamin D-degrading enzyme CYP24, urinary calcium excretion was higher and associated with lower renal calcium binding protein D9k and calcium binding protein D28k than normal mice, and bone density and volume increased in KO/TG compared with normal mice owing to increased mineral apposition rate and osteoblast number.

CONCLUSIONS

Intestinal VDR and vitamin D-regulated intestinal calcium absorption are critical for controlling whole-body calcium metabolism in growing mice. Normalizing intestinal calcium absorption and metabolism reveals essential roles for VDR in control of bone formation and renal control of serum 1,25(OH)2D and urinary calcium excretion.

Metabolic bone disease in gastrointestinal illness

Metabolic bone disease is often silent, often undiagnosed, and occurs frequently in patients with chronic gastrointestinal illnesses. Potentially modifiable risk factors, such as malnutrition, malabsorption, prolonged use of glucocorticoids, and a sedentary lifestyle, can lead to low bone mass, an increased rate of bone loss, and debilitating bone disease. This article explores common gastrointestinal illnesses that place patients at risk for developing metabolic bone disease. Concepts are presented to assist the practitioner in identifying patients at risk; clinical evaluation and diagnostic test selection are discussed, and therapeutic options for the prevention and treatment of metabolic bone disease in gastrointestinal illness are presented.

Action of green tea catechin on bone metabolic disorder in chronic cadmium-poisoned rats

The purpose of this study was to investigate the effects of green tea catechin on bone metabolic disorders and its mechanism in chronic cadmium-poisoned rats. Sprague-Dawley male rats weighing 100+/-10 g were randomly assigned to one control group and three cadmium-poisoned groups. The cadmium groups included a catechin free diet (Cd-0C) group, a 0.25% catechin diet (Cd-0.25C) group and a 0.5% catechin diet (Cd-0.5C) group according to their respective levels of catechin supplement. After 20 weeks, the deoxypyridinoline and crosslink values measured in urine were significantly increased in the Cd-0C group. Cadmium intoxication seemed to lead to an increase in bone resorption. In the catechin supplemented group (Cd-0.5C group), these urinary bone resorption marks, were decreased. The serum osteocalcin content in the cadmium-poisoned group was significantly increased as compared with the control group. In the catechin supplemented group serum osteocalcin content values were lower than the control group. The cadmium-intoxicated group (Cd-0C group), had lower bone mineral density than the control group (total body, vertebra, pelvis, tibia and femur). The catechin supplement increased bone mineral density to about the same as the control group. Bone mineral content showed a similar trend to total bone mineral density. Therefore, the bone mineral content of the Cd-0C group at the 20th week was significantly lower than the control group. The catechin supplemented group (Cd-0.5C group) was about the same as the control group. The cause of decreasing bone mineral density and bone mineral content by cadmium poisoning was due to the fast bone turnover rate, where bone resorption occurred at a higher rate than bone formation. The green tea catechin aided in normalizing bone metabolic disorders in bone mineral density, bone mineral content and bone calcium content caused by chronic cadmium intoxication.

Effect of green tea catechin on arachidonic acid cascade in chronic cadmium-poisoned rats

The purpose of this study was to investigate the effect of green tea catechin on the cyclooxygenase and lipoxygenase pathways in chronic cadmium-poisoned rats. Sprague-Dawley male rats weighing 100 +/- 10 g were randomly assigned to one normal and three cadmium-poisoned groups. The cadmium groups were classified as catechin-free diet group (Cd-0C), 0.25% catechin diet group (Cd-0.25C) and 0.5% catechin diet group (Cd-0.5C), in accordance with the level of catechin supplement. The phospholipase A2 activity was remarkably increased 117% in the Cd-0C group and 60% in the Cd-0.25C group compared with the normal group, and the level in the Cd-0.5C group was the same as the normal group. Activity of platelet cyclooxygenase increased 284% in the Cd-0C group, 147% in the Cd-0.25C group and 193% in the Cd-0.5C group. The synthesis of platelet thromboxane A2 (TXA2) increased 157% in the Cd-0C group and 105% in the Cd-0.25C group, compared with the normal group. The Cd-0.5C group showed the same level as the normal group. Prostacyclin (PGI2) formation in the aorta decreased 24% in the Cd-0C group and 18% in the Cd-0.25C group. The ratio of PGI2/TXA2, the thrombocyte synthesis index, decreased 70% in the Cd-0C group and 59% in the Cd-0.25C group. The activity of 5'-lipoxygenase in the polymorphonuclear leukocyte was increased 40% in the Cd-0C group as compared with the normal group. Catechin-supplemented Cd-0.25C and Cd-0.5C groups showed the level of the normal group. In this study, the observed content of leukotriene B4, which induces the inflammatory process, increased 54% in the Cd-0C group, and in catechin-supplemented groups, showed the same level as in the normal group. The serum peroxide value increased 60% in the Cd-0C group compared with the normal group; but in the Cd-0.5C group, it showed the level of the normal group. These results indicate that chronic cadmium poisoning in rats accelerates arachidonic acid metabolism. Inhibition of arachidonic acid metabolism due to catechin supplementation, however, decreases platelet aggregation and inflammatory action. In conclusion, it would appear that green tea catechin supplementation in chronic cadmium-poisoned rats inhibits the arachidonic acid cascade by regulating the activity of phospholipase A2.

Dose-effect relations of loop- and thiazide-diuretics on calcium homeostasis: a randomized, double-blinded Latin-square multiple cross-over study in postmenopausal osteopenic women

BACKGROUND

Thiazide diuretics (TDs) reduce whereas loop diuretics (LDs) increase urinary calcium. We studied the effects of different doses of a TD and LD on electrolytes, calcitropic hormones and biochemical bone markers.

SUBJECTS AND METHODS

In a five-period crossover study, comparing four active doses with placebo, 40 postmenopausal women with osteopenia were treated with different doses of LD bumetanide (n = 20, 0.5-2.0 mg per day) or TD bendroflumethiazide (n = 20, 2.5-10 mg per day). Each treatment period lasted 1 week.

RESULTS

Urinary calcium decreased dose-dependently in response to the bendroflumethiazide. The best hypocalciuric effect was achieved by 5 mg day-1 of bendroflumethiazide. Total plasma calcium levels increased, whereas ionised calcium at ambient pH-values decreased because of increased pH-values in response to the bendroflumethiazide. Plasma PTH levels did not change, whereas a slight dose-dependent increase occurred in plasma 1,25(OH)2D levels. As a marker of bone formation, plasma osteocalcin levels increased. Conversely, bumetanide dose-dependently increased renal calcium losses with a concomitant increase in plasma PTH and 1,25(OH)2D levels. Plasma osteocalcin levels increased and bone-specific alkaline phosphatase levels decreased dose-dependently.

CONCLUSION

Whether a LD or TD is chosen as diuretic therapy affects calcium homeostasis. The effects of LDs are potentially harmful to bone. Further studies are needed to evaluate whether long-term treatment with LDs causes osteoporosis. Until then, we suggest using, if possible, a TD rather than a LD as diuretic therapy in order not to risk deleterious effects on bone metabolism.

Magnesium in disease: a review with special emphasis on the serum ionized magnesium

This review deals with the six main clinical situations related to magnesium or one of its fractions, including ionized magnesium: renal disease, hypertension, pre-eclampsia, diabetes mellitus, cardiac disease, and the administration of therapeutic drugs. Issues addressed are the physiological role of magnesium, eventual changes in its levels, and how these best can be monitored. In renal disease mostly moderate hypermagnesemia is seen; measuring ionized magnesium offers minimal advantage. In hypertension magnesium might be lowered but its measurement does not seem relevant. In the prediction of severe pre-eclampsia, elevated ionized magnesium concentration may play a role, but no unequivocal picture emerges. Low magnesium in blood may be cause for, or consequence of, diabetes mellitus. No special fraction clearly indicates magnesium deficiency leading to insulin resistance. Cardiac diseases are related to diminished magnesium levels. During myocardial infarction, serum magnesium drops. Total magnesium concentration in cardiac cells can be predicted from levels in sublingual or skeletal muscle cells. Most therapeutic drugs (diuretics, chemotherapeutics, immunosuppressive agents, antibiotics) cause hypomagnesemia due to increased urinary loss. It is concluded that most of the clinical situations studied show hypomagnesemia due to renal loss, with exception of renal disease. Keeping in mind that only 1% of the total body magnesium pool is extracellular, no simple measurement of the real intracellular situation has emerged; measuring ionized magnesium in serum has little added value at present.

Different mechanism of magnesium and calcium transport across rat duodenum

Concentration and voltage dependence of Mg transport across the rat duodenum was measured in an Ussing chamber. Mucosa (m) to serosa (s) Mg flux exhibits a cellular fraction comparable to that found for Ca. Mg sm flux is purely diffusive and probably restricted to the paracellular pathway. At all concentrations between 0.5 and 5 mmol/liter, Mg is secreted. Diffuse sm Mg flux is 3.5 times higher than the diffusive component of ms Mg transport. This prevalence of diffusive sm Mg flux over that from mucosa to serosa, which may be explained by an "anomalous solvent drag effect," is responsible for the Mg secretion observed. Mg 5 mmol/liter decreases ms Ca flux and abolishes Ca absorption. The voltage clamp experiments reveal that Mg has no effect on the cellular transport but only decreases diffusive ms Ca flux. 1 alpha,25-dihydroxyvitamin D3 has only a small effect on cellular Mg transport but remarkably stimulates ms Ca flux and increases Ca absorption by about 85%. Dexamethasone increases ms Mg flux but decreases ms Ca transport and hereby abolished Mg secretion or Ca absorption. In conclusion, Mg is secreted across the short-circuited duodenum whereas Ca is absorbed. There is evidence that both earth alkalines are transported by distinct cellular mechanisms. The data also demonstrate that diffusive movement across the paracellular pathway plays an important role on net Mg transport.

Effect of physical activity on the metabolism of magnesium in the rat

The influence of exercise and immobilization on magnesium (Mg) balance, as influenced by vitamin D, was evaluated in female Sprague-Dawley rats that were divided into four groups: exercised fed ad libitum, pair-fed exercised, immobilized, and control. After 9 weeks, Mg absorption and retention were higher in the exercised groups than in the control group; however, immobilization resulted in an increase in the urinary excretion of Mg and a negative Mg balance. Serum levels of 1,25-dihydroxyvitamin D [1,25(OH)2D] were higher in the exercised group than in the control group. The tibial content of Mg was higher in both exercise groups and lower in the immobilized than in the control group. In an additional experiment to evaluate the Mg balance during early adaptation to exercise training, no change was seen in Mg absorption and balance during the first 5 weeks of exercise, although serum levels of 1,25(OH)2D increased significantly. Immobilization resulted in a decrease in Mg retention without any effect on intestinal absorption. Urinary Mg was consistently higher in the exercised and immobilized groups than in the control group. Prolonged exercise training, after adaptation, increases and longer immobilization decreases intestinal absorption and retention of Mg. The influence of physical activity on Mg absorption is not necessarily mediated by vitamin D. Physical activity may influence mineral absorption by a mechanism associated with growth.

The role of magnesium in clinical biochemistry: an overview

Magnesium is the second most abundant intracellular cation. It is essential for a wide variety of metabolically important reactions, in particular those involving ATP. Hypomagnesaemia is surprisingly common in hospital populations but is sometimes either undetected or overlooked. Serum magnesium concentrations provide a guide to magnesium status but while hypomagnesaemia is a reliable indicator of magnesium deficiency, normomagnesaemia does not exclude magnesium depletion. A wide variety of conditions predispose to magnesium depletion. Clinical magnesium deficiency has potentially fatal consequences in vulnerable groups of patients and should be excluded in all such cases. Magnesium deficiency may result in hypokalaemia, hypocalcaemia or other disturbances of electrolyte homeostasis, refractory cardiac arrhythmias, or increased sensitivity to digoxin. The capacity to measure serum and urine magnesium concentrations rapidly, regularly and reliably should be part of the repertoire of all clinical chemistry laboratories involved in the care of critically ill patients.

Cellular and paracellular magnesium transport across the terminal ileum of the rat and its interaction with the calcium transport

Concentration and voltage dependence of unidirectional magnesium fluxes across the stripped mucosa of the rat terminal ileum were measured in an Ussing chamber. The mucosa-to-serosa magnesium flux exhibited a curvilinear concentration dependence, whereas serosa-to-mucosa flux of magnesium was linearly related to magnesium concentration between 0.25 and 5 mM. At low concentrations magnesium was absorbed, whereas at the magnesium concentration of 5 mM the serosa-to-mucosa magnesium flux was higher than the mucosa-to-serosa flux, resulting in magnesium secretion. Only the mucosa-to-serosa flux of magnesium had a voltage-independent (i.e., nondiffusive) cellular component. Due to the high capacity of this cellular mucosa-to-serosa transport of magnesium, which was about 7.5 times greater than that of calcium, absorption of magnesium was performed in the terminal ileum in contrast to calcium, which was secreted under the same conditions. However, magnesium serosa-to-mucosa flux was totally voltage dependent (i.e., diffusive) and probably restricted to the paracellular pathway. The diffusive serosa-to-mucosa flux of magnesium was about two times greater than the diffusive fraction of the mucosa-to-serosa transport of magnesium. The prevalence of the diffusive serosa-to-mucosa flux of magnesium over that from mucosa to serosa, responsible for magnesium secretion observed at the magnesium concentration of 5 mM, may be explained by an "anomalous solvent drag effect." Voltage clamp experiments showed that magnesium had no effect on the cellular mucosa-to-serosa transport of calcium. However, it decreased the diffusive calcium flux in this direction. 1 alpha,25-dihydroxyvitamin D3 did not influence the unidirectional or net magnesium transport but increased the calcium flux in both directions to the same degree. In conclusion, magnesium is absorbed in the terminal ileum at least partially by a cellular, vitamin D3-insensitive process that is different from the calcium transport mechanism.

Magnesium transport across colon ascendens of the rat

Concentration and voltage dependence of unidirectional Mg fluxes across the rat colon ascendens were measured in a modified Ussing chamber. Mucosa (M) to serosa (S) Mg flux exhibits a cellular component, whereas SM flux is totally diffusive. At all the concentrations between 0.125 and 8 mmol/liter MS Mg transport is higher than the flux in the opposite direction, resulting in Mg absorption. In contrast to Mg, in Ca transport a cellular component is involved in both directions across the tissue, 1 alpha, 25-Dihydroxyvitamin D3 has no influence on the Mg transport. Mg (5 mmol/liter) remarkably decreases MS Ca flux and reduces Ca absorption by 70%. The parallel decrease in MS Ca flux with that of the simultaneously measured paracellular marker mannitol and the voltage clamp experiments reveal that Mg has no influence on cellular Ca transport but only reduces diffusive MS Ca flux, possibly by decreasing transepithelial fluid absorption. The experiments demonstrate that the colon ascendens of the rat is capable of absorbing Mg at rates comparable to that found for Ca. There is evidence that Mg and Ca are transported by separate cellular mechanisms. Diffusive movement across the paracellular route plays an important role on net transport of both earth alkali ions.

Calcium metabolism

In normal individuals, 1,25-dihydroxyvitamin D (1,25-D) levels regulate calcium (Ca) absorption according to Ca intake; its synthesis is stimulated by low Ca intake, probably via increased parathyroid hormone (PTH) secretion, to increase Ca absorption, and suppressed during high intake to reduce Ca absorption. The body also adapts Ca absorption in response to renal Ca excretion, and phosphate absorption in response to phosphate intake. These adaptations may fail or be impaired in certain diseases. In disorders of overadaptation, the intestinal tract absorbs excessive amounts of Ca due to overproduction of 1,25-D, as in absorptive hypercalciuria, sarcoidosis, primary hyperparathyroidism, and tumoral calcinosis. Intestinal hyperabsorption and hypercalciuria may occur on both low- and high-Ca diets. Primary hyperparathyroidism and hypoparathyroidism are bihormonal, related to over- and underproduction, respectively, of both 1,25-D and PTH. Underadaptation disorders are typically related to low 1,25-D synthesis or resistance to this metabolite; examples include postmenopausal osteoporosis, chronic renal failure, and osteomalacia. Many of these adaptational disorders can be relieved or improved by manipulating Ca, phosphate, sodium, or protein intake or by administering exogenous 1,25-D. Overabsorption of Ca and other substances, such as oxalate, may be responsible for Ca nephrolithiasis. Hypocitraturia (which may be a complication of certain diseases or the result of unbalanced diet or excessive exercise), diets high in readily metabolizable sugars and purine-rich proteins (meat, poultry, and fish), and low fluid intake can all contribute to stone formation. Various regimens may reduce the risk of Ca nephrolithiasis.

Primary infantile hypomagnesaemia; report of two cases and review of literature

We describe two male infants suffering from primary hypomagnesaemia, diagnosed at 3 months and 2.5 months of age. They both presented with generalised convulsions, with case 2 exhibiting hypocalcaemia which did not respond to calcium and case 1 having normocalcaemia at first but hypocalcaemia 3 days after admission. Both improved dramatically after initiation of magnesium therapy. A carrier-mediated transport defect is the most likely cause of this disease. It is of the utmost importance that a correct and prompt diagnosis be made as therapy is simple and effective. Failure in diagnosing this condition could prove fatal as demonstrated in the family history of case 2.

Distribution of the 10,000 molecular weight calcium binding protein along the small and large intestine of man

The distribution of the 10,000 molecular weight calcium binding protein along the human small and large intestine was studied using an enzyme linked immunoadsorbent assay. Small intestinal mucosal samples were obtained from the duodenal bulb, the second and third part of the duodenum and at about 50 cm intervals from jejunum and ileum of five whole small intestines of necro-kidney donors. Mucosal samples of caecum, colon ascendens, and transversum were also investigated. The amount of calcium binding protein per milligram of cytosolic protein increased throughout duodenum to reach the maximum in the proximal segment of jejunum and then declined steadily to nearly undetectable levels in ileum. In the colon no 10,000 molecular weight CaBP was detectable. The distribution of CaBP along the small and large intestine of man is thus parallel to the efficiency of the active calcium absorption of human intestine.

The acute biochemical effects of four proprietary calcium preparations

Changes in serum and urine biochemical indices have been studied in ten normal subjects in the four hours following the ingestion of four proprietary calcium supplements. Each was taken in a dose containing 1 gram of elemental calcium. The four preparations were ranked according to the amount of calcium absorbed in the order Spar-Cal and Calcium Sandoz greater than Os-Cal greater than Ossopan. There were no significant differences between the four preparations in the changes in parathyroid hormone (PTH) and urine hydroxyproline levels. For this reason, the four results from each subject were averaged. Following the calcium load there was a reduction in mean PTH from 0.16 +/- 0.01 to 0.10 +/- 0.02 micrograms/l (p less than 0.001) and a decline in urine hydroxyproline/creatinine ratio from 20 +/- 1 to 17 +/- 1 (p less than 0.02), suggesting that bone resorption responds immediately to dietary calcium intake. There was a rise in urine sodium excretion which correlated with the indices of calcium absorption (r = 0.63, p less than 0.01) but not with the sodium content of the calcium preparations. This effect could be important, particularly in elderly patients on borderline sodium intakes.