Renal and systemic magnesium metabolism during chronic continuous PTH infusion in normal subjects

Fibroblast growth factor-23 and parathyroid hormone suppress small intestinal magnesium absorption

In the present study, we examined the systemic and direct effects of parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF-23) on duodenal, jejunal, and ileal Mg²⁺ absorption. The rats were injected with FGF-23 or PTH for 5 h before collecting the duodenum, jejunum, and ileum for Mg²⁺ transport analysis in Ussing chambers. The duodenum, jejunum, and ileum were directly exposed to FGF-23, PTH, or FGF-23 plus PTH with or without cell signaling inhibitors for 150 min in Ussing chambers prior to performing the Mg²⁺ transport study. The small intestinal tissues were also subjected to western blot analyses for FGF receptor (FGFR), PTH receptor (PTHR), Klotho, transient receptor potential melastatin 6 (TRPM6), and cyclin as well as the cystathionine β-synthase domain divalent metal cation transport mediator 4 (CNNM4) expression. The small intestine abundantly expressed FGFR and PTHR proteins, whereas, Klotho was not expressed in rat small intestine. Systemic PTH or FGF-23 injection significantly suppressed transcellular Mg²⁺ transport in the duodenum and jejunum. Direct FGF-23-, PTH-, or FGF-23 plus PTH exposure also suppressed transcellular Mg²⁺ absorption in the duodenum and jejunum. There was no additional inhibitory effect of PTH and FGF-23 on intestinal Mg²⁺ absorption. The inhibitory effect of PTH, FGF-23, or FGF-23 plus PTH was abolished by Gö 6850. Systemic PTH- or FGF-23-injection significantly decreased membranous TRPM6 expression, but increased cytosolic CNNM4 expression in the duodenum, jejunum, and ileum. In the present study, we propose a novel magnesiotropic action of PTH and FGF-23 by modulating small intestinal Mg²⁺ absorption.

Effect of parathyroid hormone on serum magnesium levels: the neglected relationship in hemodialysis patients with secondary hyperparathyroidism

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is an important complication in patients with end-stage renal disease. Since recent studies have shown that magnesium (Mg) disturbance plays an important role in CKD-MBD and cardiovascular mortality, the interest on magnesium has grown recently. Although much concern focused on the effect of Mg on parathyroid hormone (PTH) levels, however, the influence of PTH on serum Mg levels is nearly unexplored. To evaluate the effect of PTH on serum Mg levels, we first described the relationship between serum Mg and PTH in secondary hyperparathyroidism. Besides, we also monitored the changes of serum Mg concentration after parathyroidectomy (PTX) in 23 patients. In our study, we found that hypermagnesemia (>2.5 mg/dL) occurred in up to 44% of cases and hypomagnesemia did not present. No statistically significant correlations were found between serum Mg levels and PTH (r = -0.143, p = 0.134). Correlation analysis and regression analysis suggested that the derangement of magnesium homeostasis was consistent with the derangement of calcium/phosphorus homeostasis. However, after PTX, serum magnesium levels dropped immediately after the surgery, minimally at the first day and gradually restored from the third day. The changes of serum Mg after surgery was positive correlated with the changes of serum phosphate (r = 0.558, p = 0.003). Taken altogether, our data suggested that the therapeutic strategies to achieve optimum serum magnesium levels in CKD-MBD should take into account the varying stages of disease development since PTH could also influence magnesium metabolism and this problem might be important in severe secondary hyperparathyroidism.

Calcium paradox of aldosteronism and the role of the parathyroid glands

The hypercalciuria and hypermagnesuria that accompany aldosteronism contribute to a fall in plasma ionized extracellular Ca2+ and Mg2+ concentrations ([Ca2+]o and [Mg2+]o). Despite these losses and the decline in extracellular levels of these cations, total intracellular and cytosolic free Ca2+ concentration ([Ca2+]i) is increased and oxidative stress is induced. This involves diverse tissues, including peripheral blood mononuclear cells (PBMC) and plasma. The accompanying elevation in plasma parathyroid hormone (PTH) and reduction in bone mineral density caused by aldosterone (Aldo)-1% NaCl treatment (AldoST) led us to hypothesize that Ca2+ loading and altered redox state are due to secondary hyperparathyroidism (SHPT). Therefore, we studied the effects of total parathyroidectomy (PTx). In rats receiving AldoST, without or with a Ca2+-supplemented diet and/or PTx, we monitored urinary Ca2+ and Mg2+ excretion; plasma [Ca2+]o, [Mg2+]o, and PTH; PBMC [Ca2+]i and H2O2 production; plasma α1-antiproteinase activity; total Ca2+ and Mg2+ in bone, myocardium, and rectus femoris; and gp91phox labeling in the heart. We found that 1) the hypercalciuria and hypermagnesuria and decline ( P < 0.05) in plasma [Ca2+]o and [Mg2+]o that occur with AldoST were not altered by the Ca2+-supplemented diet alone or with PTx; 2) the rise ( P < 0.05) in plasma PTH with AldoST, with or without the Ca2+-supplemented diet, was prevented by PTx; 3) increased ( P < 0.05) PBMC [Ca2+]i and H2O2 production, increased total Ca2+ in heart and skeletal muscle, and fall in bone Ca2+ and Mg2+ and plasma α1-antiproteinase activity with AldoST were abrogated ( P < 0.05) by PTx; and 4) gp91phox activation in right and left ventricles at 4 wk of AldoST was attenuated by PTx. AldoST is accompanied by SHPT, with parathyroid gland-derived calcitropic hormones being responsible for Ca2+ overload in diverse tissues and induction of oxidative stress. SHPT plays a permissive role in the proinflammatory vascular phenotype.

Magnesium: metabolism and hormonal regulation in different species

1. The magnesium ion is of great importance in physiology by its intervention in 300 enzymatic systems, its membrane role and its function in neuromuscular excitability. 2. The skeleton is the first pool of magnesium in the animal body. 3. For intestinal absorption, renal metabolism, bone accretion and resorption, magnesium shows analogies with calcium. 4. Magnesium exchange between extracellular, cellular and skeletal compartments are very precisely controlled. 5. Parathyroid hormone, 1 alpha, 25-dihydroxy-vitamin D3, calcitonin and estrogens are the principal hormone systems implicated in magnesium metabolism. 6. The kidney is the principal site of magnesium excretion and shows important magnesium regulation mechanisms.

Magnesium. An update on physiological, clinical and analytical aspects

There is an increased interest in the role of magnesium ions in clinical medicine, nutrition and physiology. The characteristics of the binding of magnesium and calcium ions to various components, macromolecules and biological membranes are described. Magnesium affects many cellular functions, including transport of potassium and calcium ions, and modulates signal transduction, energy metabolism and cell proliferation. The mechanism of cellular uptake and efflux of magnesium, its intracellular transport, intestinal absorption, renal excretion and the effect of hormones on these are reviewed. Magnesium deficiency is not uncommon among the general population: its intake has decreased over the years especially in the western world. The magnesium supplementation or intravenous infusion may be beneficial in various diseased states. Of special interest is the magnesium status in alcoholism, eclampsia, hypertension, atherosclerosis, cardiac diseases, diabetes, and asthma. The development of instrumentation for the assay of ionized magnesium is reviewed, as are the analytical procedures for total magnesium in blood and free magnesium in the cytosol. The improved procedures for the assay of different magnesium states are useful in understanding the role of magnesium in health and disease.

Total thyroparathyroidectomy in patients with thyroid cancers and changes in erythrocytic Mg levels following parathyroautotransplantation

Parathyroid hormone (PTH) rapidly decreases in patients with thyroid cancers, even after autotransplantation following total thyroparathyroidectomy. Simultaneously, serum calcium (Ca) and erythrocytic Ca levels also decrease, and in some cases, tetanic symptoms caused by hypocalcemia may occur. Assuming that magnesium (Mg) might participate in the alteration of Ca and PTH levels, the postoperative changes in intracytoplasmic Mg levels of erythrocyte were determined for several days. In the present paper, red blood cells (RBC) were used as a substitute for general somatic cells and controls were taken from patients with breast cancers. Although the erythrocytic Mg levels of the patients with thyroid cancers showed almost no change until day 7, an apparent decrease was noted in the patients with breast cancers during the period from day 1 to day 7, with a significant difference being seen between the two groups. Assuming that the operative invasions were compatible between these two groups of patients, the difference may be explained by a rapid decrease of postoperative serum PTH in the patients with thyroid cancers.