Second messenger role of magnesium in pancreatic acinar cells of the rat

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.

Magnesium in Combinatorial With Valproic Acid Suppressed the Proliferation and Migration of Human Bladder Cancer Cells

Magnesium, the second most predominant intracellular cation, plays a crucial role in many physiological functions; magnesium-based biomaterials have been widely used in clinical application. In a variety of cancer types, the high intracellular concentration of magnesium contributes to cancer initiation and progression. Therefore, we initiated this study to investigate the likelihood of confounding magnesium with cancer therapy. In this study, the anti-tumor activity of magnesium and underlying mechanisms were assessed in bladder cancer both in vitro and in vivo. The results indicated that the proliferation of bladder cancer cells was inhibited by treatment with a high concentration of MgCl₂ or MgSO₄. The apoptosis, G0/G1 cell cycle arrest, autophagy, and ER stress were promoted following treatment with MgCl₂. However, the migratory ability of MgCl₂ treated cells was similar to that of control cells, as revealed by the trans-well assay. Besides, no significant difference was observed in the proportion of CD44 or CD133 positive cells between the control and MgCl₂ treated cells. Thus, to improve the therapeutic effect of magnesium, VPA was used to treat cancer cells in combination with MgCl₂. As expected, combination treatment with MgCl₂ and VPA could markedly reduce proliferation, migration, and in vivo tumorigenicity of UC3 cells. Moreover, the Wnt signaling was down-regulated, and ERK signaling was activated in the cells treated with combination treatment. In conclusion, the accurate utilization of MgCl₂ in targeting autophagy might be beneficial in cancer therapy. Although further studies are warranted, the combination treatment of MgCl₂ with VPA is an effective strategy to improve the outcome of chemotherapy.

Signal Transduction: Magnesium Manifests as a Second Messenger

Mg²⁺ is an essential ion for the cell but whether it can act as a bona fide second messenger has long been questioned. A recent study supports this hypothesis and shows a signalling role for Mg²⁺ in GABA-mediated neuronal maturation.

Magnesium in man: implications for health and disease

Magnesium (Mg(2+)) is an essential ion to the human body, playing an instrumental role in supporting and sustaining health and life. As the second most abundant intracellular cation after potassium, it is involved in over 600 enzymatic reactions including energy metabolism and protein synthesis. Although Mg(2+) availability has been proven to be disturbed during several clinical situations, serum Mg(2+) values are not generally determined in patients. This review aims to provide an overview of the function of Mg(2+) in human health and disease. In short, Mg(2+) plays an important physiological role particularly in the brain, heart, and skeletal muscles. Moreover, Mg(2+) supplementation has been shown to be beneficial in treatment of, among others, preeclampsia, migraine, depression, coronary artery disease, and asthma. Over the last decade, several hereditary forms of hypomagnesemia have been deciphered, including mutations in transient receptor potential melastatin type 6 (TRPM6), claudin 16, and cyclin M2 (CNNM2). Recently, mutations in Mg(2+) transporter 1 (MagT1) were linked to T-cell deficiency underlining the important role of Mg(2+) in cell viability. Moreover, hypomagnesemia can be the consequence of the use of certain types of drugs, such as diuretics, epidermal growth factor receptor inhibitors, calcineurin inhibitors, and proton pump inhibitors. This review provides an extensive and comprehensive overview of Mg(2+) research over the last few decades, focusing on the regulation of Mg(2+) homeostasis in the intestine, kidney, and bone and disturbances which may result in hypomagnesemia.

Molecular characterization and subcellular localization of Arabidopsis class VIII myosin, ATM1

Land plants possess myosin classes VIII and XI. Although some information is available on the molecular properties of class XI myosins, class VIII myosins are not characterized. Here, we report the first analysis of the enzymatic properties of class VIII myosin. The motor domain of Arabidopsis class VIII myosin, ATM1 (ATM1-MD), and the motor domain plus one IQ motif (ATM1-1IQ) were expressed in a baculovirus system and characterized. ATM1-MD and ATM1-1IQ had low actin-activated Mg(2+)-ATPase activity (Vmax = 4 s(-1)), although their affinities for actin were high (Kactin = 4 μM). The actin-sliding velocities of ATM1-MD and ATM1-1IQ were 0.02 and 0.089 μm/s, respectively, from which the value for full-length ATM1 is calculated to be ∼0.2 μm/s. The results of actin co-sedimentation assay showed that the duty ratio of ATM1 was ∼90%. ADP dissociation from the actin·ATM1 complex (acto-ATM1) was extremely slow, which accounts for the low actin-sliding velocity, low actin-activated ATPase activity, and high duty ratio. The rate of ADP dissociation from acto-ATM1 was markedly biphasic with fast and slow phase rates (5.1 and 0.41 s(-1), respectively). Physiological concentrations of free Mg(2+) modulated actin-sliding velocity and actin-activated ATPase activity by changing the rate of ADP dissociation from acto-ATM1. GFP-fused full-length ATM1 expressed in Arabidopsis was localized to plasmodesmata, plastids, newly formed cell walls, and actin filaments at the cell cortex. Our results suggest that ATM1 functions as a tension sensor/generator at the cell cortex and other structures in Arabidopsis.

Unitary Ca(2+) current through recombinant type 3 InsP(3) receptor channels under physiological ionic conditions

The ubiquitous inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) channel, localized primarily in the endoplasmic reticulum (ER) membrane, releases Ca(2+) into the cytoplasm upon binding InsP(3), generating and modulating intracellular Ca(2+) signals that regulate numerous physiological processes. Together with the number of channels activated and the open probability of the active channels, the size of the unitary Ca(2+) current (i(Ca)) passing through an open InsP(3)R channel determines the amount of Ca(2+) released from the ER store, and thus the amplitude and the spatial and temporal nature of Ca(2+) signals generated in response to extracellular stimuli. Despite its significance, i(Ca) for InsP(3)R channels in physiological ionic conditions has not been directly measured. Here, we report the first measurement of i(Ca) through an InsP(3)R channel in its native membrane environment under physiological ionic conditions. Nuclear patch clamp electrophysiology with rapid perfusion solution exchanges was used to study the conductance properties of recombinant homotetrameric rat type 3 InsP(3)R channels. Within physiological ranges of free Ca(2+) concentrations in the ER lumen ([Ca(2+)](ER)), free cytoplasmic [Ca(2+)] ([Ca(2+)](i)), and symmetric free [Mg(2+)] ([Mg(2+)](f)), the i(Ca)-[Ca(2+)](ER) relation was linear, with no detectable dependence on [Mg(2+)](f). i(Ca) was 0.15 +/- 0.01 pA for a filled ER store with 500 microM [Ca(2+)](ER). The i(Ca)-[Ca(2+)](ER) relation suggests that Ca(2+) released by an InsP(3)R channel raises [Ca(2+)](i) near the open channel to approximately 13-70 microM, depending on [Ca(2+)](ER). These measurements have implications for the activities of nearby InsP(3)-liganded InsP(3)R channels, and they confirm that Ca(2+) released by an open InsP(3)R channel is sufficient to activate neighboring channels at appropriate distances away, promoting Ca(2+)-induced Ca(2+) release.

P2X(7) receptors in rat parotid acinar cells: formation of large pores

1. Permeabilization of cells mediated by P2X(7) receptors occurs to varied degrees in native and heterologous expression systems. Previous studies on P2X(7) receptors in parotid acinar cells suggested that ATP does not permeabilize these cells. 2. Modification of the assay conditions showed that ATP permeabilizes freshly dissociated rat parotid acinar cells to the fluorescent dye YOPRO-1. 3. The pharmacological and physiological properties of this effect indicate that permeabilization is mediated by the P2X(7) receptor. Adenosine 5'-triphosphate (ATP) and 3'-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (BzBzATP) were effective agonists with EC(50) values of 49.3 and 0.6 microM, respectively. 4. Permeabilization was best observed in low divalent cation concentrations and at physiological temperatures. Previous studies failed to detect permeabilization because of the sensitivity of this effect to temperature and divalent cations. 5. An important consideration in understanding the effect of divalent cations is that the fluorescence of YOPRO-1/nucleic acid complexes is directly quenched by addition of divalent cations. This must be considered if quantitative study of the interaction of divalent cations with P2X(7) receptors is carried out using fluorescent DNA-binding dyes. 6. In summary, our data show that P2X(7) receptors in parotid acinar cells can form large pores in the plasma membrane. This property likely contributes to signalling and may be cytotoxic and have particular significance in damaged or inflamed salivary glands.

Adaptation of the exocrine pancreas to dietary fats

This article reviews studies on the adaptation of the exocrine pancreas to dietary fat. We include all the latest information about the mechanisms that underlie the adaptation of the secretory mechanism of the exocrine pancreas to the amount and the type of dietary fat. We review the kinetics of pancreatic adaptation and the mediators of the adaptive response of the pancreas including cellular and molecular mechanisms (modulation of intracellular messengers and gene expression of the different enzymes and secretagogues involved in the adaptation process). At the same time we include our results in this field in dogs and humans.

Modulation of 2-oxoglutarate dehydrogenase complex by inorganic phosphate, Mg(2+), and other effectors

The interplay of inorganic phosphate (Pi) with other ligands such as Mg(2+), ADP, ATP, and Ca(2+) on the activation of 2-oxoglutarate dehydrogenase complex (2-OGDH) in both isolated enzyme complex and mitochondrial extracts was examined. Pi alone activated the enzyme, following biphasic kinetics with high (K(0.5) = 1.96+/-0.42 mM) and low (K(0.5) = 9.8+/-0.4 mM) affinity components for Pi. The activation by Pi was highly pH-dependent; it increased when the pH raised from 7.1 to 7.6, but it was negligible at pH values below 7.1. Mg-Pi and Mg-ADP, but not Mg-ATP, were more potent activators of 2-OGDH than free Pi and free ADP. ATP inhibited the 2-OGDH activity by chelating the free Mg(2+) and also as a Mg-ATP complex. With or without Mg(2+), ADP, and Pi activated the 2-OGDH by increasing the affinity for 2-OG and the V(m) of the reaction; ATP diminished the V(m), but it increased the affinity for 2-OG in the mitochondrial extract. Pi did not modify the 2-OGDH activation by Ca(2+). The results above mentioned were similar for both preparations, except for hyperbolic kinetics in the isolated enzyme and sigmoidal kinetics in the mitochondrial extracts when 2-oxoglutarate was varied. The data of this study indicated that physiological concentrations of Pi may exert a significant activation of 2-OGDH, which was potentiated by Mg(2+) and high pH, but surpassed by ADP.

Effects of divalent cations on single-channel conduction properties of Xenopus IP3 receptor

The effects of Mg2+ and Ba2+ on single-channel properties of the inositol 1,4,5-trisphosphate receptor (IP3R) were studied by patch clamp of isolated nuclei from Xenopus oocytes. In 140 mM K+ the IP3R channel kinetics and presence of conductance substates were similar over a range (0-9.5 mM) of free Mg2+. In 0 mM Mg2+ the channel current-voltage (I-V) relation was linear with conductance of approximately 320 pS. Conductance varied slowly and continuously over a wide range (SD approximately 60 pS) and sometimes fluctuated during single openings. The presence of Mg2+ on either or both sides of the channel reduced the current (blocking constant approximately 0.6 mM in symmetrical Mg2+), as well as the range of conductances observed, and made the I-V relation nonlinear (slope conductance approximately 120 pS near 0 mV and approximately 360 pS at +/-70 mV in symmetrical 2.5 mM Mg2+). Ba2+ exhibited similar effects on channel conductance. Mg2+ and Ba2+ permeated the channel with a ratio of permeability of Ba2+ to Mg2+ to K+ of 3.5:2.6:1. These results indicate that divalent cations induce nonlinearity in the I-V relation and reduce current by a mechanism involving permeation block of the IP3R due to strong binding to site(s) in the conduction pathway. Furthermore, stabilization of conductance by divalent cations reveals a novel interaction between the cations and the IP3R.

Extracellular Mg2+ inhibits both histamine-stimulated Ca(2+)-signaling and exocytosis in human tracheal secretory gland cells

The effects of extracellular Mg2+ concentration have been investigated on the histamine-stimulated exocytotic process of human tracheal secretory gland (HTG) cells. The exocytosis of secretory granules (SG) was observed concomitantly with dynamic changes of intracellular Ca2+ ([Ca2+]i) and Mg2+ concentrations ([Mg2+]i). The rate of SG exocytosis was appraised by the decrease of quinacrine fluorescence emission. Dynamic changes of [Mg2+]i and [Ca2+]i in HTG cells were determined by the combined use of UV-microspectrofluorometry with Mag-Indo-1 and Indo-1 probes, respectively. High Mg2+ medium significantly inhibited the histamine-stimulated secretion. The influence of the extracellular and intracellular Mg2+ concentrations on [Ca2+]i was analyzed. Basal [Mg2+]i increased from 0.8 mM in a Mg(2+)-free medium to 1.7 mM in 10 mM Mg2+ medium. Histamine induced a [Mg2+]i increase which is dependent on extracellular Mg2+ concentration. The histamine stimulated [Ca2+]i rise was reduced in the presence of elevated Mg2+ extracellular medium and inhibitory effects of extracellular Mg2+ were concomitant with changes in [Mg2+]i. Our data suggest that the inhibition by extracellular Mg2+ of stimulated exocytosis is dependent on both the increase of [Mg2+]i and the inhibition of cytosolic Ca2+ influx.

Modulation of oxidative phosphorylation by Mg2+ in rat heart mitochondria

The effect of varying the Mg2+ concentration on the 2-oxoglutarate dehydrogenase (2-OGDH) activity and the rate of oxidative phosphorylation of rat heart mitochondria was studied. The ionophore A23187 was used to modify the mitochondrial free Mg2+ concentration. Half-maximal stimulation (K0.5) of ATP synthesis by Mg2+ was obtained with 0.13 +/- 0.02 mM (n = 7) with succinate (+rotenone) and 0.48 +/- 0.13 mM (n = 6) with 2-oxoglutarate (2-OG) as substrates. Similar K0.5 values were found for NAD(P)H formation, generation of membrane potential, and state 4 respiration with 2-OG. In the presence of ADP, an increase in Pi concentration promoted a decrease in the K0.5 values of ATP synthesis, membrane potential formation and state 4 respiration for Mg2+ with 2-OG, but not with succinate. These results indicate that 2-OGDH is the main step of oxidative phosphorylation modulated by Mg2+ when 2-OG is the oxidizable substrate; with succinate, the ATP synthase is the Mg2+-sensitive step. Replacement of Pi by acetate, which promotes changes on intramitochondrial pH abolished Mg2+ activation of 2-OGDH. Thus, the modulation of the 2-OGDH activity by Mg2+ has an essential requirement for Pi (and ADP) in intact mitochondria which is not associated to variations in matrix pH.