Studies on mechanisms of hypocalcemia of magnesium depletion

Effects of magnesium supplementation on post-thyroidectomy hypocalcemia: a prospective single-center study

BACKGROUND

Severe and/or symptomatic hypocalcemia due to hypoparathyroidism is the main contraindication for discharge in patients who have undergone thyroid surgery. Hypomagnesemia may contribute to the onset of hypoparathyroidism and is frequently observed after thyroid surgery in hypocalcemic patients. The impact of prophylactic and postoperative Magnesium supplementation on postoperative hypocalcemia and hypomagnesemia was prospectively evaluated by comparing patients undergoing prophylactic supplementation to a control group of patients who had only received magnesium after evidence of postoperative hypomagnesemia.

METHODS

One hundred and twenty patients who underwent a total thyroidectomy participated in the study. Seventy-three patients were included in the study group, 47 in the control group. Prior to surgery, patients in the study group were given magnesium orally for 5 days; postoperatively, calcium and magnesium was administered to all patients who displayed hypocalcemia and hypomagnesemia.

RESULTS

Postoperative biochemical hypocalcemia (serum calcium <8.5 mg/dL, regardless of its clinical severity) was found in 60 patients (50%) on D1 and in 58 patients (48.4%) on D2. Among hypocalcemic patients, hypomagnesemia was recorded in 29 at D1 (48%), and in 46 at D2 (79%). A significant positive correlation was found between magnesium, calcium, and parathyroid hormone in the first two postoperative days, while a significant inverse correlation occurred for these same parameters and length of hospital stay (P<0.001). One hundred and five patients (87.5%) were discharged as expected on the second postoperative day (65 in the study group, 40 in the control group, P=0.724), whereas 15 patients (12.5%) required prolonged hospitalization (eight in the study group, seven in the control group, P=0.721). The Study group only showed significantly higher magnesium levels on the first postoperative day (P=0.03).

CONCLUSIONS

Although magnesium and calcium levels showed the same trend after thyroidectomy, neither Magnesium prophylaxis nor Magnesium treatment influenced the clinical course of postoperative hypocalcemia.

Effects of feeding level, milking frequency, and single injection of cabergoline on blood metabolites, hormones, and minerals around dry-off in dairy cows

This study aimed to investigate the effect of the different dry-off strategies based on reducing feeding level (normal vs. reduced energy density), reducing milking frequency (twice vs. once daily), and administration of a dopamine agonist after last milking (i.e. saline vs. cabergoline injection) on blood metabolites, hormones, and minerals around dry-off. In this experiment, 119 Holstein dairy cows were used in a 2 × 2 × 2 factorial arrangement. In the last week before dry-off, cows were allocated to 1 of the 4 possible dry-off strategies based on feeding level and milking frequency. Within 3 h after last milking, cows were injected with either saline or a D₂ dopamine agonist (cabergoline; Velactis, Ceva Santé Animale, Libourne, France; labeled for use only with abrupt dry-off, e.g., no preceding reduction in feeding level or milking frequency before last milking). After dry-off, all cows were fed the same dry cow diet and data collection continued for a week. Blood samples were collected from the coccygeal vein on d -9, -6, -5, -2, 1, 2, 5, and 7 relative to dry-off. Additionally, blood was sampled at 0, 3, and 6 h relative to injection of either cabergoline or saline, equivalent to d 0.125, 0.250, and 0.375 relative to last milking (dry-off). The reduced feeding level before dry-off caused reduced glucose and insulin concentrations as well as increased free fatty acid concentrations, particularly when reduced feeding level was combined with milking the cows 2× daily. The intramuscular injection of cabergoline caused the expected reduction in circulating prolactin concentrations. In addition, dopamine-agonist cabergoline induced an atypical simultaneous pattern of plasma metabolites (i.e., increased glucose and free fatty acid concentrations), hormones (i.e., reduced insulin and increased cortisol concentrations), and minerals (i.e., reduced calcium concentration), indicating that normal metabolic and mineral homeostatic regulations were hindered after the injection of ergot alkaloid cabergoline. In conclusion, reducing milking frequency seems the best management strategy to reduce milk production at dry-off among those tested in this study.

Possible role of low magnesium levels in the onset of postoperative hypoparathyroidism following thyroidectomy

BACKGROUND

A positive correlation between calcium and magnesium serum levels is well known and depends upon various factors. This study aims at verifying the existence of this association in a retrospective series of patients who underwent thyroid surgery.

METHODS

Two hundred and eighty-five consecutive patients (202 female, 83 male, mean age 57 years) who underwent at least total thyroidectomy (TT) and had a complete clinical and biochemical pre- and postoperative evaluation were included in the study. Patients were evaluated with regard to: sex, age, indications for surgery, operative time, number of accidentally removed parathyroids, extent and time of surgery, thyroiditis, final histology, pre- and postoperative levels of calcium, magnesium, vitamin D, and creatinine, presence of symptoms of hypocalcemia. Statistical analysis was performed using the R software.

RESULTS

In the postoperative period, biochemical hypocalcemia (<8.5 mg/dL) was observed in 126 patients (44%) and severe hypocalcemia (<7.5 mg/dL, a level indicating the need for longer hospitalization) was seen in 40 (14%). When analyzing patients with and without postoperative hypocalcemia, the factors affecting postoperative biochemical hypocalcemia were: older age (P=0.019), longer operative time (P=0.039), and a highly significant correlation between postoperative calcium and magnesium levels (r=0.432; P<0.001).

CONCLUSIONS

The only factor among the ones we analyzed in this retrospective study that would appear to be linked to the onset of clinically relevant hypocalcemia is low magnesium levels in the postoperative period. A prospective randomized study with a group of patients undergoing magnesium replacement in the postoperative period can clarify the possible role of magnesium repletion on hypocalcemia.

Efficacy of pamidronate in pediatric osteosarcoma patients with low bone mineral density

PURPOSE

Most surviving pediatric osteosarcoma patients experience osteoporosis, bone pain, and pathologic fracture during and after therapy. The aim of this study was to evaluate the efficacy and side effects of pamidronate therapy in these patients.

METHODS

Nine osteosarcoma patients (12.8±1.6 years of age; 5 boys and 4 girls) who had a history of nontraumatic fracture or severe pain after completing chemotherapy were included. Intravenous pamidronate (1.5 mg/kg) was given every 6 weeks for 4 to 6 cycles. Bone mineral density (BMD) of the lumbar spine was measured by dual-energy x-ray absorptiometry. Clinical outcomes including acute side effects were also evaluated.

RESULTS

After pamidronate treatments, all patients experienced decreased pain. Seven of 9 patients could walk without a crutch. The BMD of lumbar spine was increased by 0.108±0.062 mg/cm(2) after 8.4±1.0 months (n=8, P=0.017) and the mean z-score improved from -2.14±0.94 to -1.76±0.95 (P=0.161). Six patients (67%) had an acute-phase reaction, and 2 patients had symptomatic hypocalcemia.

CONCLUSION

Pamidronate appears to be safe and effective for the treatment of osteosarcoma in children with low BMD and bone pain.

Essential and nonessential elements in nestling rooks Corvus frugilegus from eastern Poland with a special emphasis on their high cadmium contamination

Concentration of minerals (sodium, potassium, calcium [Ca], magnesium, iron [Fe], copper, zinc [Zn], manganese [Mn], and cobalt) as well as toxic metals (cadmium [Cd], lead [Pb]) were determined in five tissues (liver, lung, kidney, muscle, and bone) of nestling rooks (Corvus frugilegus; 1 to 13 days old) found dead in seven breeding colonies in eastern Poland. Cd concentration in all analyzed tissues was in the narrow range of 17.0-17.2 mg/kg dry weight (dw) Cd, which in the light of the literature data indicates acute contamination by this toxic metal. Similarly, we found increased levels of Pb, which in all tissues ranged between 5.0 and 6.2 mg/kg dw. Results of multivariate general linear model (GLM) testing of the effect of three variables (tissue type, colony, and nestling age) on tissue concentrations of various metals showed significance for Fe, Cu, Zn, and Mn. Only concentrations of Ca, Fe, and Zn differed significantly between the analyzed tissues. GLM analysis did not show any statistically significant differences in tissue levels of minerals and both toxic metals among examined rookeries, which indicates the widespread presence of nonpoint Cd and Pb pollution linked to agricultural activity and similar levels of these inorganic contaminants on crop fields (feeding grounds) around breeding colonies. We concluded that high levels of both toxic metals, Cd and Pb, probably resulting from the diet of nestling rooks, are based mainly on a diet of ground-dwelling beetles gathered on crop fields.

Reference intervals of plasma calcium, phosphorus, and magnesium for African grey parrots (Psittacus erithacus) and Hispaniolan parrots (Amazona ventralis)

Calcium (Ca), phosphorus (P), and magnesium (Mg) are important elements for body homeostasis in several diseases associated with imbalances in the plasma concentration of these ions. This is the first published report of reference intervals for Mg in association with Ca and P levels for psittacine species. One milliliter of blood was collected from 26 Hispaniolan parrots (Amazona ventralis) and 24 African grey parrots (Psittacus erithacus). The plasma concentrations of Ca, P, and Mg were determined for each sample. Statistical analyses were performed including all data (analysis 1) and after exclusion of the subjects with Ca > or = 14.00 mg/dl (3.5 mmol) (analysis 2). The data from analysis 1 have a narrower interval than that observed in analysis 2. Following the normality test (Shapiro-Wilk, alpha = 0.05), the univariate and mean procedures were run. For the reference intervals, the lower and upper values were used, after elimination of the outliers calculated by Blom scores from the ranked variables. The analysis 1 references for the Hispaniolans were Ca = 8.80-10.40 mg/dl (2.20-2.60 mmol/L), P = 1.80-4.40 mg/dl (0.58-1.42 mmol/L), Mg = 1.80-3.10 mg/dl (0.74-1.27 mmol/L), and Ca:P ratio = 2.62-5.39; for the African greys analysis 1 references were Ca = 8.20-20.20 mg/dl (2.05-5.05 mmol/L), P = 2.50-5.90 mg/dl (0.81-1.91 mmol/L), Mg = 2.10-3.40 mg/dl (0.82-1.4 mmol/L), and Ca:P ratio = 1.81-3.77. The analysis 2 references for the Hispaniolans were Ca = 8.80-10.30 mg/dl (2.20-2.58 mmol/L), P = 1.80-3.80 mg/dl (0.58-1.23 mmol/L), Mg = 1.90-3.00 mg/dl (0.82-1.07 mmol/L), Ca:P ratio = 2.62-5.39; for the African greys analysis 2 references were Ca = 1.07 mmol/L), Ca:P ratio = 1.67-3.50. The results of this study are important for evaluating Mg concentrations in relation to the Ca and P parameters in psittacines. This information will be particularly helpful for veterinarians evaluating the hypocalcemic syndrome in African grey parrots and other disease processes associated with Ca, P, and Mg physiologic imbalances.

Electrolytes and whole body potassium in acute leukemia

In a consecutive series of 22 patients with acute leukemia, the total body potassium was studied in 18 patients on 39 occasions during relapse and remission. Total body water was also determined. A control group consisting of 88 age-matched healthy volunteers was also studied. The patients had a significantly lower mean potassium concentration, per kg body weight, per kg lean body mass and per kg water, than the controls (p less than 0.001). Individually, 11 out of the 18 patients had at least one value below the lower 95% confidence limit. Hypokalemia was frequent both in the patients with low (7/11) and normal (3/6) potassium per kg lean body mass. Five of 13 investigated patients showed laboratory indications of secondary hyperaldosteronism, which might be partly responsible for the hypokalemia. Increased serum or urine levels of lysozyme were found in 62% of the patients.

Magnesium in Animal Nutrition

Magnesium (Mg) supplementation remarkably improves the digestibility of feed. In cows and sows, it has improved the reproduction and shortened the service period. In broilers it increased weight gain, and it has increased egg production of laying hens. In addition, increasing Mg intake benefits the quality of breeding eggs and improves hatching yield. However, increasing Mg intake has not altered visceral composition of embryos, although brain and liver might have the capacity to store Mg at intake above the requirement.

Growth, thyroid function, and serum macromineral levels in magnesium-deficient chicks

Growth and thyroid function were studied in Mg-deficient chicks. Dietary levels of 80 to 315 ppm Mg were compared with control levels of 578 to 787 ppm Mg. Signs of Mg deficiency appeared rapidly and acutely within 2 to 5 days at dietary levels of 250 to 260 ppm or lower. Growth and feed intake decreased progressively as the deficiency became more severe. Control chicks pair-fed with the deficient chicks gained significantly more weight. Serum Mg decreased at all levels of Mg below control, but at 260 and 315 ppm it returned to control values after 21 days on treatment. Serum Ca diminished only when dietary Mg was 250 ppm or less. Serum K increased in severely deficient chicks but decreased over time in milder deficiencies. Thyroid gland weights were unchanged. However, very young chicks fed a Mg-deficient diet had lower serum 3,5,3'-triiodothyronine (T3) whereas serum thyroxine (T4) was generally unaffected. Beyond 1 wk of age chicks that had prior access to a Mg-sufficient diet had low serum T4 levels whereas serum T3 was unchanged. Therefore, peripheral thyroid hormone metabolism is altered in a Mg deficiency, but this effect is dependent on the age at which the deficiency occurs.

Magnesium status in freshwater fish, common carp (Cyprinus carpio, L.) and the dietary protein-magnesium interaction

Common carp juveniles were fed for 9 weeks one of the eight semipurified diets containing graded levels of magnesium, 0.08, 0.6, 1.1, 3, 2 g Mg kg(-1) and 25 or 44% protein.Fish growth and feed utilization were significantly affected by both Mg and protein levels in the diets. Significant interaction between these two studied variables existed in relation to the fish performance as well as to mean deposition rate of several minerals in common carp body. The fish fed diets containing 0.08 g Mg kg(-1) had reduced growth and developed deficiency signs such as muscle flaccidity and skin hemorrhages.Results indicated that a minimum Mg level of 0.6 g Mg kg(-1) was required to elevate plasma and bone magnesium content and to reduce the whole body Ca concentration (hypercalcinosis symptom). Further increase of dietary Mg up to 3.2 g Mg kg(-1) improved growth rate of fish insignificantly, but the deposition rate of dietary Mg fell to as low as 7.4 and 10.7 percent in low- and high-protein diet fed fish, respectively. In Mg-deficient fish, considerable amount of magnesium was absorbed via extra-oral routes, however, this way of the covering magnesium need becomes insufficient in fast growing fish.

Effects of a magnesium-free dialysate on magnesium metabolism during continuous ambulatory peritoneal dialysis

While the use of magnesium-containing compounds is usually contraindicated in dialysis patients, the risk of toxicity from hypermagnesemia can be reduced by lowering the magnesium concentration in dialysate. We examined the effects of a magnesium-free dialysate on both serum magnesium level and the peritoneal removal rate of magnesium over 12 weeks in 25 stable patients undergoing continuous ambulatory peritoneal dialysis (CAPD). After 2 weeks, the serum magnesium level decreased from 2.2 to 1.9 mg/dL (0.9 to 0.8 mmol/L) (P less than .02) and the peritoneal removal rate increased from 66 to 83 mg/d (2.8 to 3.5 mmol/d) (P less than .05), with both values remaining stable thereafter. There was a strong association between these parameters (r = -0.62, P less than .05), suggesting that the serum magnesium level decreased as a result of the initial increased peritoneal removal rate. For an additional 4-week period, a subgroup of nine patients received magnesium-containing, phosphate binding agents instead of those containing only aluminum. During this phase, serum inorganic phosphorus was well controlled. The serum magnesium level increased only from 1.8 to 2.5 mg/dL (0.7 to 1.0 mmol/L) (P less than .05), due in great part to the concomitant 41% rise in peritoneal magnesium removal from 91 to 128 mg/d (3.8 to 5.3 mmol/d) (P less than .05). No toxicity was noted during the entire 16-week study period, nor did serum calcium change. Thus, serum magnesium levels remained within an acceptable range as magnesium-containing phosphate binders were given through the use of magnesium-free peritoneal dialysate.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary calcium and phosphorus and seizure susceptibility of magnesium deficient rats

Convulsions are characteristic of magnesium deficiency and hypocalcemia. In this study, weanling rats were fed magnesium deficient diets with varying concentrations of calcium and phosphorus. Diets were either normal (Mg =) or low (Mg-) in magnesium and were either low (Ca- or P-), normal (Ca = or P =) or high (Ca+ or P+) in calcium or phosphorus. After consuming the diets for 17 days, the rats were tested for audiogenic seizures and blood was then drawn for serum mineral analyses. Rats fed Mg-Ca = P =, Mg-Ca = P-, Mg-Ca+P = or Mg-Ca+P+ diets had high incidences of seizures. Those fed Mg-Ca-P =, Mg-Ca-P-, Mg-Ca = P+, Mg-Ca-P+ or Mg-Ca+P- diets had low incidences of seizures. In general, animals with low serum magnesium and calcium levels and high serum potassium levels were susceptible to audiogenic seizures. In this model, serum magnesium level is the most important determinant of seizure susceptibility, followed by calcium and potassium.

Electrocardiographic finding simulating acute myocardial infarction in a compound metabolic aberration

A patient with hypokalemic metabolic alkalosis, hypophosphatemia, and hypomagnesemia/hypocalcemia is described. Electrocardiography demonstrated the pattern of acute anterior myocardial infarction. Further evaluation revealed that the patient had not actually had the acute myocardial infarction and that the electrocardiographic change was a mere simulation. The possible role of hypomagnesemia in the pathogenesis of the electrocardiographic change and the interrelation between the metabolic disturbances noted are discussed.

Effects of magnesium and lactose supplementation on bone metabolism in the X-linked hypophosphatemic mouse

Besides rickets and osteomalacia, the X-linked hypophosphatemic male mouse (Hyp/Y) presents with low serum calcium (Ca) and increased urinary hydroxyproline (OH-Pro) excretion, suggesting a parathyroid hormone (PTH)-stimulated bone resorption despite reduced magnesium (Mg) bone content. In this study, we have investigated by histochemical methods the state of bone resorption in 50-day-old untreated Hyp/Y mice and the effects of 4 wk of Mg therapy or dietary lactose supplementation on bone formation and resorption. Mineral and skeletal changes were evaluated on serum, urinary and bone ash concentrations of Ca, phosphorus (P) and Mg, and by histomorphometric analysis of tetracycline double labeled undeclalcified caudal vertebrae. The number of acid phosphatase stained chondroclasts and osteoclasts was lower than normal in untreated Hyp/Y and was restored after Mg therapy while the osteoclastic surface was increased above normal. Accordingly, serum P and urinary Ca, P, Mg, cAMP and OH-Pro were increased while TmP/GFR was unchanged. On the other hand, dietary lactose corrected serum Ca which probably suppressed PTH secretion since the renal P conservation was improved and the osteoclast number and the osteoclastic surface were decreased. Both treatments reduced the growthplate and osteoid seam thickness and increased the bone calcification rate. The results indicate that the low skeletal Mg present in Hyp/Y partially impairs bone responsiveness to PTH since Mg therapy restored the osteoclastic bone resorption which secondarily provided new minerals for bone mineralization. The greater than normal bone resorption found in Mg treated-Hyp/Y and the decreased bone resorption observed in lactose treated animals indicate that the chronic hypocalcemia induces secondary hyperparathyroidism in Hyp/Y mice.

Effect of magnesium deficiency on renal magnesium and calcium transport in the rat

Recollection of micropuncture experiments were performed on acutely thyroparathyroidectomized rats rendered magnesium deficient by dietary deprivation. Urinary magnesium excretion fell from a control of 15 to 3% of the filtered load after magnesium restriction. The loop of Henle, presumably the thick ascending limb, was the major modulator for renal magnesium homeostasis. The transport capacity for magnesium, however, was less in deficient rats than control animals. Absolute magnesium reabsorption increased with acute infusions of magnesium chloride but was always less in magnesium-deficient rats than control rats for any given filtered load, which suggests either a defect of a resetting of the reabsorption mechanism. Recollection micropuncture demonstrated that this was a characteristic of the loop of Henle. Proximal magnesium reabsorption remained unchanged at 15% of the filtered load and was unaffected by magnesium deficiency or acute magnesium repletion. Distal tubular magnesium reabsorption was limited during depletion and increased to a similar extent in control and deficient rats with enhanced magnesium delivery. Calcium reabsorption was not altered in magnesium deficiency; however, elevations of extracellular magnesium resulted in a specific inhibition of calcium reabsorption within the loop of Henle. These data suggest that overall control of renal magnesium reabsorption occurs within the loop of Henle and that the proximal tubule reabsorbs a constant fraction of the filtered load despite variations in body magnesium status.

Evidence for skeletal resistance to parathyroid hormone in magnesium deficiency. Studies in isolated perfused bone

Hypocalcemia during magnesium (Mg) depletion has been well described, but the precise mechanism(s) responsible for its occurrence is not yet fully understood. The hypocalcemia has been ascribed to decreased parathyroid hormone (PTH) secretion as well as skeletal resistance to PTH. Whereas the former is well established, controversy exists as to whether or not Mg depletion results in skeletal resistance to PTH. These studies examine the skeletal response to PTH in normal dogs and dogs fed a Mg-free diet for 4-6 mo. Isolated tibia from normal (serum Mg 1.83+/-0.1 mg/100 ml) and experimental dogs (serum Mg 1.34+/-0.15 mg/100 ml) were perfused with Krebs-Henseleit buffer during a constant infusion of 3 ng/ml of synthetic bovine PTH 1-34 (syn b-PTH 1-34). The arteriovenous (A-V) difference for immunoreactive PTH (iPTH) across seven normal bones was 37.5+/-3%. In contrast, the A-V difference for iPTH was markedly depressed to 10.1+/-1% across seven bones from Mg-depleted dogs. These findings correlated well with a biological effect (cyclic AMP [cAMP] production) of syn b-PTH 1-34 on bone. In control bones, cAMP production rose from a basal level of 5.8+/-0.2 to 17.5+/-0.7 pmol/min after syn b-PTH 1-34 infusion. In experimental bones, basal cAMP production was significantly lower than in controls, 4.5+/-0.1 pmol/min, and increased to only 7.1+/-0.4 pmol/min after syn b-PTH 1-34 infusion. Even when PTH concentrations were increased to 20 ng/ml, cAMP production by experimental bones was lower than in control bones perfused with 3 ng/ml. Histological examination of bones from Mg-deficient dogs showed a picture compatible with skeletal inactivity. These studies demonstrate decreased uptake of iPTH and diminished cAMP production by bone, which indicates skeletal resistance to PTH in chronic Mg deficiency.

Influence of magnesium on the secretion and action of parathyroid hormone

The interactions of serum levels of magnesium and parathyroid hormone (PTH) have been studied in a patient with hypomagnesemia and hypocalcemia following intestinal bypass surgery for obesity. When serum magnesium was low serum PTH was not stimulated by hypocalcemia. With correction of magnesium deficiency hypocalcemia was associated with elevation of serum PTH levels. Infusion of exogenous PTH induced a clearly detectable renal response in the presence of hypomagnesemia but the response was diminished when serum magnesium was elevated. In this patient it appears that hypomagnesemia suppressed parathyroid gland activity, leaving the renal action of PTH intact.

Hypocalcaemia: differential diagnosis and investigation

Hypocalcaemia accounts for many of the telephone inquiries made to the Department of Biochemical Medicine in Dundee. About half of these cases can be explained by hypoalbuminaemia; the causes of “true” hypocalcaemia are given in Table 1.

Chronic renal failure is probably the most frequent cause of hypocalcaemia in Britain. In the world as a whole it is likely that vitamin D deficiency, causing rickets in children and osteomalacia in adults, is the commonest cause of hypocalcaemia.

Functional hypoparathyroidism and parathyroid hormone end-organ resistance in human magnesium deficiency

Hypocalcaemia is a well-recognized manifestation of magnesium deficiency. We have studied seventeen patients with this syndrome in an attempt to determine the pathogenesis of the hypocalcaemia. Mean initial serum calcium concentration was 5-6 mg/dl and mean initial serum magnesium concentration was 0-75 mg/dl. Serum immunoreactive parathyroid hormone (IPTH) was measured in sixteen patients in the untreated state. Despite severe hypocalcaemia, serum IPTH was either undetectable (less than 150 pg/ml) or normal (less than 550 pg/ml) in all but two patients. Serial measurements made during the initial 4 days of magnesium therapy in four patients showed an increase in serum IPTH within 24h, but a delayed increase in serum calcium, which required approximately 4 days to reach normal values. The effect of the rapid normalization of serum magnesium on serum IPTH and serum calcium concentration was studied in three patients. Within 1 min after 144-300 mg of elemental magnesium was administered i.v., serum IPTH had risen from undetectable to 3600 pg/ml and 1725 pg/ml in two patients and from 425 pg/ml to 937 pg/ml in the third. Serum calcium concentrations were unchanged after 30-60 min. These data provide evidence for impaired parathyroid gland function in most of the magnesium deficient patients. The rapidity with which serum IPTH rose in response to magnesium therapy indicates that this may reflect a defect in parathyroid hormone (PTH) secretion rather than its biosynthesis. The failure of serum calcium concentration to increase during the initial days of magnesium repletion, at a time when serum IPTH concentrations were normal or elevated, suggests end-organ resistance to PTH in these patients. The renal response to PTH was examined in two magnesium deficient patients by measurement of urinary cyclic AMP excretion following administration of parathyroid extract. In both patients there was a minimal increase in urinary cyclic AMP concentrations. In contrast, when the hepatic response to glucagon was tested on the same patients by measurement of plasma cyclic AMP concentrations following administration of glucagon, normal increases were observed. These results suggest that adenylate cyclase systems of various organs may be affected differentially by a state of magnesium deficiency. It is suggested that magnesium deficiency may result in defective cyclic AMP generation in the parathyroid glands and in the PTH target organs. This could be the principal mechanism operative in both impaired PTH secretion and end-organ resistance to PTH which together contribute to the development of hypocalcaemia.

Recent advances in calcium metabolism. I. Mechanisms of calcium homeostasis

Within recent years newly acquired knowledge has provided a clearer understanding of some aspects of the complex mechanisms that collectively maintain calcium homeostasis within body fluids and it is our intent to define current concepts of the interrelationship of these various factors to the end that fuller understanding may be available concerning the maintenance of calcuim homeostasis in health as well as features which result in its disruption and the consequent effects of imbalances of calcium in various disease states. In this first section dealing with the physiologic state, there are included descriptions of: (1) the metabolism of vitamin D, the synthesis of its active metabolites, 25 OHD3 and1.25(OH)2D3, and the metabolic actions of the active vitamin D metabolite and analogues upon gastrointestinal, bone, and kidney functions; (2) the synthesis, secretion, and metabolic activity of parathyroid hormone and the difficulties with the radioimmunoassay of PTH related to the number of PTH-like peptides in the circulation; and (3) the chemistry, metabolism, and biologic activities of calcitonin, a hypocalcemic principle derived from the parafollicular cells of the thyroid gland. It is emphasized that under normal circumstances these humoral mechanisms act in an integrated manner to maintain serum concentrations of total and ionized calcium within narrowly defined limits.

Effect of age and magnesium depletion on bone magnesium pools in rats

In vivo and in vitro studies were carried out to characterize the exchangeable bone magnesium pool and determine what effect age and magnesium depletion has on bone magnesium. A highly significant correlation was found between the size of the in vitro elutable and in vivo exchangeable bone magnesium (r=0.97). To show that the exchangeable bone magnesium was the surface-limited bone magnesium, elution studies were performed 4 h after the in vivo administration of radiomagnesium. Specific activity in the eluant was 85% of that found in the serum at time of death, suggesting that the elutable and exchangeable bone magnesium pools were largely the same pool. Bone magnesium concentration fell with increasing age. The entire fall in bone magnesium was a result of a decrease in the surface-limited fraction. Since bone crystals have been shown to enlarge with aging with resulting contraction of the surface area, this would be the most apparent explanation for this finding. During magnesium depletion, magnesium concentration in both the exchangeable and nonexchangeable pools decreased. The fractional change in the exchangeable pool was much larger than the change in total or nonexchangeable bone magnesium, suggesting that the surface-limited magnesium pool is available during magnesium depletion. The change in size of the nonexchangeable bone magnesium pool appeared to be more related to the duration of magnesium depletion than the change in serum magnesium levels. The fall in magnesium concentration in this pool is probably a consequence of continuing formation of low magnesium bone during the depletion period.