Phosphorylation-independent modulation of L-type calcium channels by magnesium-nucleotide complexes

Colorimetric and Fluorometric N-Acylhydrazone-based Chemosensors for Detection of Single to Multiple Metal Ions: Design Strategies and Analytical Applications

The development of optical sensors for metal ions has gained significant attention due to their broad applications in biology, the environment, and medicine. Colorimetric and fluorometric detection methods are particularly valued for their simplicity, cost-effectiveness, high detection limits, and analytical power. Among various chemical probes, the hydrazone functional group stands out for its extensive study and utility, owing to its ease of synthesis and adaptability. This review provides a comprehensive overview of N-acylhydrazone-based probes, serving as highly effective colorimetric and fluorometric chemosensors for a diverse range of metal ions. Probes are categorized into single-ion, dual-ion, and multi-ion chemosensors, each further classified based on the detected metal(s). Additionally, the review discusses detection modes, detection limits, association constants, and spectroscopic measurements.

Fluorescent deactivation behaviors based on ESIPT and TICT of novel double target fluorescent probe and its sensing mechanism for Al3+/Mg2+: A TD-DFT study

Based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods with integral equation formula polarized continuum model (IEFPCM), the fluorescent behavior and recognizing mechanism of probe N'-((1-hydroxynaphthalen-2-yl)methylene)isoquinoline-3-carbohydrazide (NHMI) for Al³⁺/Mg²⁺ ion were investigated in more detail. Excited state intramolecular proton transfer (ESIPT) process in probe NHMI occurs in the stepwise pattern. The proton H₅ of enol structure (E1) firstly moves from O₄ to N₆ to form single proton-transfer (SPT2) structure, and then the proton H₂ of SPT2 transfers from N₁ to N₃ to form the stable double proton-transfer (DPT) structure. Subsequently, the transformation from DPT to its isomer (DPT1) induces the twisted intramolecular charge transfer (TICT) process. Two non-emissive TICT states (TICT1 and TICT2) were obtained, and TICT2 state quenches the fluorescence observed in the experiment. With the addition of aluminum (Al³⁺) or magnesium (Mg²⁺) ion, TICT process is prohibited by the coordination interaction between NHMI and Al³⁺/Mg²⁺, and the strong fluorescent signal is turned on. For probe NHMI, the twisted C-N single bond of acylhydrazone part leads to the TICT state. This sensing mechanism may inspire researchers to develop new probes from a different direction.

Regulation of Cardiac Cav1.2 Channels by Calmodulin

Cav1.2 Ca2+ channels, a type of voltage-gated L-type Ca2+ channel, are ubiquitously expressed, and the predominant Ca2+ channel type, in working cardiac myocytes. Cav1.2 channels are regulated by the direct interactions with calmodulin (CaM), a Ca2+-binding protein that causes Ca2+-dependent facilitation (CDF) and inactivation (CDI). Ca2+-free CaM (apoCaM) also contributes to the regulation of Cav1.2 channels. Furthermore, CaM indirectly affects channel activity by activating CaM-dependent enzymes, such as CaM-dependent protein kinase II and calcineurin (a CaM-dependent protein phosphatase). In this article, we review the recent progress in identifying the role of apoCaM in the channel ‘rundown’ phenomena and related repriming of channels, and CDF, as well as the role of Ca2+/CaM in CDI. In addition, the role of CaM in channel clustering is reviewed.

Synthesis and photophysical investigation of Schiff base as a Mg2+ and Zn2+ fluorescent chemosensor and its application

In view of the fluorescent switching properties and anti-fatigue properties of diarylethene, a diarylethene fluorescent chemosensor for the immediate detection of zinc ion (Zn²⁺ ) and magnesium ion (Mg²⁺ ) in acetonitrile was synthesized in this article. The structure of 1o was determined by performing spectroscopy and elemental analysis. The presence of Zn²⁺ or Mg²⁺ made the chemosensor 1o show an obvious "turn-on" fluorescent signal (bright yellow-green for Mg²⁺ and bright cyan for Zn²⁺ ). The fluorescent change caused by the 1:1 binding of 1o and Zn²⁺ or Mg²⁺ might be due to hindering the excited-state intramolecular proton transfer (ESIPT) process, which were bolstered by Benesi-Hildebrand analysis, Job's plot curves, proton nuclear magnetic resonance (¹ H-NMR) titration and mass spectrometry. The limits of detection were acquired from the standard curve plots for Mg²⁺ at 44.6 nM and for Zn²⁺ at 14 nM. Based on the fluorescent behaviors, a logic gate was constructed with the emission intensity at 528/518 nm as output signal, the ultraviolet-visible (UV-vis) lights, Mg²⁺ /Zn²⁺ and EDTA as input signals. Exogenous Zn²⁺ and Mg²⁺ fluorescent bioimaging were performed on Hela cells with 1o, indicating its potential application in biodiagnostic analysis. In particular, 1o was manufactured into test paper, and Zn²⁺ or Mg²⁺ can be conveniently, efficiently and qualitatively identified by the fluorescent color variation of the test strips.

Association between Elevated Magnesium Intake and Reduced Risk of Recurrent Falls and Frailty in Osteoarthritis: Data from the Osteoarthritis Initiative

OBJECTIVES

The objective of this 8-year follow-up study was to investigate the relationship between magnesium intake and frailty, as well as recurrent falls, in individuals diagnosed with Osteoarthritis (OA) or those at a heightened risk for developing the condition.

METHODS

This study utilized data from the Osteoarthritis Initiative (OAI) database and conducted a prospective cohort study with a 8-year follow-up period. Total magnesium intake from both food sources and supplements was assessed using a food frequency questionnaire (FFQ), while frailty and recurrent falls were evaluated through established criteria and self-report, respectively. To account for potential confounding factors, various covariates were considered, and statistical analyses, including generalized additive mixed models (GAMMs), were employed to examine the associations.

RESULTS

Among the 4,667 participants with OA, those with lower total magnesium intake were characterized by younger age, a higher proportion of African American individuals, higher body mass index (BMI), and lower dietary fiber intake (P<0.001). Notably, this group exhibited higher odds of experiencing recurrent falls and frailty (P = 0.034 and 0.006, respectively). Controlling for various factors, the GAMMs consistently revealed negative correlations between magnesium intake and the likelihood of frailty and recurrent falls, with each 1 mg/1000 kcal increase in magnesium intake associated with a 0.5% reduced frailty risk (p < 0.001) and a 0.2% decreased risk of recurrent falls (p = 0.001). Subgroup analyses suggested that increased total magnesium intake from both food sources and supplements may exert a more pronounced preventive effect on recurrent falls and frailty in men, older adults, individuals with normal BMI, and those with higher dietary fiber intake.

CONCLUSIONS

Elevated total magnesium intake from both food sources and supplements was found to be associated with a decreased risk of recurrent falls and frailty in individuals diagnosed with OA or those at risk of developing the condition. These findings imply that increased total magnesium intake might be beneficial in managing the risk of these outcomes, particularly within specific subgroups, including men, older adults, those with a normal BMI, and those with higher dietary fiber intake.

Models of the cardiac L-type calcium current: A quantitative review

The L-type calcium current (I CaL ) plays a critical role in cardiac electrophysiology, and models ofI CaL are vital tools to predict arrhythmogenicity of drugs and mutations. Five decades of measuring and modelingI CaL have resulted in several competing theories (encoded in mathematical equations). However, the introduction of new models has not typically been accompanied by a data-driven critical comparison with previous work, so that it is unclear which model is best suited for any particular application. In this review, we describe and compare 73 published mammalianI CaL models and use simulated experiments to show that there is a large variability in their predictions, which is not substantially diminished when grouping by species or other categories. We provide model code for 60 models, list major data sources, and discuss experimental and modeling work that will be required to reduce this huge list of competing theories and ultimately develop a community consensus model ofI CaL . This article is categorized under: Cardiovascular Diseases > Computational Models Cardiovascular Diseases > Molecular and Cellular Physiology.

Myocardial ATP depletion detected noninvasively predicts sudden cardiac death risk in heart failure patients

BACKGROUND

Sudden cardiac death (SCD) remains a worldwide public health problem in need of better noninvasive predictive tools. Current guidelines for primary preventive SCD therapies, such as implantable cardioverter defibrillators (ICDs), are based on left ventricular ejection fraction (LVEF), but these guidelines are imprecise: fewer than 5% of ICDs deliver lifesaving therapy per year. Impaired cardiac metabolism and ATP depletion cause arrhythmias in experimental models, but to our knowledge a link between arrhythmias and cardiac energetic abnormalities in people has not been explored, nor has the potential for metabolically predicting clinical SCD risk.

METHODS

We prospectively measured myocardial energy metabolism noninvasively with phosphorus magnetic resonance spectroscopy in patients with no history of significant arrhythmias prior to scheduled ICD implantation for primary prevention in the setting of reduced LVEF (≤35%).

RESULTS

By 2 different analyses, low myocardial ATP significantly predicted the composite of subsequent appropriate ICD firings for life-threatening arrhythmias and cardiac death over approximately 10 years. Life-threatening arrhythmia risk was approximately 3-fold higher in patients with low ATP and independent of established risk factors, including LVEF. In patients with normal ATP, rates of appropriate ICD firings were several-fold lower than reported rates of ICD complications and inappropriate firings.

CONCLUSION

To the best of our knowledge, these are the first data linking in vivo myocardial ATP depletion and subsequent significant arrhythmic events in people, suggesting an energetic component to clinical life-threatening ventricular arrhythmogenesis. The findings support investigation of metabolic strategies that limit ATP loss to treat or prevent life-threatening cardiac arrhythmias and herald noninvasive metabolic imaging as a complementary SCD risk stratification tool.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00181233.

FUNDING

This work was supported by the DW Reynolds Foundation, the NIH (grants HL61912, HL056882, HL103812, HL132181, HL140034), and Russell H. Morgan and Clarence Doodeman endowments at Johns Hopkins.

Synthesis and Characterization of a Mg2+-Selective Probe Based on Benzoyl Hydrazine Derivative and Its Application in Cell Imaging

A simple benzoyl hydrazine derivative P was successfully synthesized and characterized as Mg2+-selective fluorescent probe. The binding of P with Mg2+ caused an obvious fluorescence enhancement at 482 nm. The fluorescent, UV-vis spectra, 1H-NMR, and IR spectra confirmed the formation of P-Mg2+ complex, and the formation of a 1:1 stoichiometry complex was proved by Job's plot and mass spectrometry. The recognition mechanism of P to Mg2+ was owing to the photoinduced electron transfer effect (PET). The fluorescent response was linear in the range of 0.9-4.0 µM with the detection limit of 0.3 µM Mg2+ in water-ethanol solution (1:9, v:v, pH10.0, 20 mM HEPES). In addition, the results of cell imaging of Mg2+ in Hl-7701 cells was satisfying.

A novel diarylethene-based fluorescent "turn-on" sensor for the selective detection of Mg2

A new photochromic diarylethene derivative with a 4-methylphenol unit has been designed and synthesized. It displayed distinct photochromism and fluorescent ''turn on'' features to Mg2+ in acetonitrile solution. With the addition of Mg2+, there was an obvious increase of fluorescent emission intensity at 552 nm, accompanied by a clear change of fluorescent color from dark purple to green. Meantime, the 1 : 1 stoichiometry between the derivative and Mg2+ was verified by Job's plot and HRMS. Furthermore, the sensor was successfully applied in the detection of Mg2+ in practical samples. Moreover, based on the multiple-responsive fluorescence switching behaviors, it also could be used to construct a molecular logic circuit with UV/vis lights and Mg2+/EDTA as input signals and the emission at 552 nm as the output signal.

Fluorescent probes for the detection of magnesium ions (Mg2+): from design to application

Magnesium ions (Mg2+) play essential roles in various physiological and pathological processes, its abnormal homeostasis in cells is related to many diseases, such as diabetes, neuromuscular disorders, hypertension and other cardiovascular disorders. Investigation on the regulation of magnesium in cellular processes has attracted considerable interest in the past several decades. Among those reported strategies, fluorescent imaging technology has become a powerful and cost-effective tool for the real-time monitoring of magnesium distribution, uptake and trafficking, due to its superior features of high sensitivity and non-invasiveness, as well as excellent spatial and temporal fidelity. Herein, we critically summarize the progresses in the intracellular magnesium detection with fluorescent imaging probes. Our discussion focuses on the recent contributions concerning fluorescent imaging probes for mapping magnesium in biological processes. All the candidates are organized according to their acceptor structures. The sensing mechanisms of fluorescent probes are also highly taken into account. Challenges, trends and prospects of fluorescent imaging technology in magnesium detection are also set forth.

Calmodulin and ATP support activity of the Cav1.2 channel through dynamic interactions with the channel

Key points

Cav1.2 channels maintain activity through interactions with calmodulin (CaM). In this study, activities of the Cav1.2 channel (α1C) and of mutant‐derivatives, C‐terminal deleted (α1CΔ) and α1CΔ linked with CaM (α1CΔCaM), were compared in the inside‐out mode.

α1CΔ with CaM, but not without CaM, and α1CΔCaM were active, suggesting that CaM induced channel activity through a dynamic interaction with the channel, even without the distal C‐tail.

ATP induced α1C activity with CaM and enhanced activity of the mutant channels. Okadaic acid mimicked the effect of ATP on the wildtype but not mutant channels.

These results supported the hypothesis that CaM and ATP maintain activity of Cav1.2 channels through their dynamic interactions. ATP effects involve mechanisms both related and unrelated to channel phosphorylation.

CaM‐linked channels are useful tools for investigating Cav1.2 channels in the inside‐out mode; the fast run‐down is prevented by only ATP and the slow run‐down is nearly absent.

Abstract

Calmodulin (CaM) plays a critical role in regulation of Cav1.2 Ca²⁺ channels. CaM binds to the channel directly, maintaining channel activity and regulating it in a Ca²⁺‐dependent manner. To explore the molecular mechanisms involved, we compared the activity of the wildtype channel (α1C) and mutant derivatives, C‐terminal deleted (α1C∆) and α1C∆ linked to CaM (α1C∆CaM). These were co‐expressed with β2a and α2δ subunits in HEK293 cells. In the inside‐out mode, α1C and α1C∆ showed minimal open‐probabilities in a basic internal solution (run‐down), whereas α1C∆ with CaM and α1C∆CaM maintained detectable channel activity, confirming that CaM was necessary, but not sufficient, for channel activity. Previously, we reported that ATP was required to maintain channel activity of α1C. Unlike α1C, the mutant channels did not require ATP for activation in the early phase (3–5 min). However, α1C∆ with CaM + ATP and α1C∆CaM with ATP maintained activity, even in the late phase (after 7–9 min). These results suggested that CaM and ATP interacted dynamically with the proximal C‐terminal tail of the channel and, thereby, produced channel activity. In addition, okadaic acid, a protein phosphatase inhibitor, could substitute for the effects of ATP on α1C but not on the mutant channels. These results supported the hypothesis that CaM and ATP maintain activity of Cav1.2 channels, further indicating that ATP has dual effects. One maintains phosphorylation of the channel and the other becomes apparent when the distal carboxyl‐terminal tail is removed.

Cav1.2 channels maintain activity through interactions with calmodulin (CaM). In this study, activities of the Cav1.2 channel (α1C) and of mutant‐derivatives, C‐terminal deleted (α1CΔ) and α1CΔ linked with CaM (α1CΔCaM), were compared in the inside‐out mode.

α1CΔ with CaM, but not without CaM, and α1CΔCaM were active, suggesting that CaM induced channel activity through a dynamic interaction with the channel, even without the distal C‐tail.

ATP induced α1C activity with CaM and enhanced activity of the mutant channels. Okadaic acid mimicked the effect of ATP on the wildtype but not mutant channels.

These results supported the hypothesis that CaM and ATP maintain activity of Cav1.2 channels through their dynamic interactions. ATP effects involve mechanisms both related and unrelated to channel phosphorylation.

CaM‐linked channels are useful tools for investigating Cav1.2 channels in the inside‐out mode; the fast run‐down is prevented by only ATP and the slow run‐down is nearly absent.

A turn-on fluorescent chemosensor based on acylhydrazone for sensing of Mg2+ with a low detection limit

A novel highly selective chemosensor for Mg²⁺ ions based on the naphthalene group as the fluorophore has been designed and synthesized, which shows a fluorescence turn-on response from colorless to green for Mg²⁺ ions in DMSO–H₂O solutions.

A highly specific ‘turn-on’ fluorescent detection of Mg2+ through a xanthene based fluorescent molecular probe

Two acid hydrazones incorporating xanthene as a metal chelating centre (B-XAN and N-XAN) have been synthesized, characterized and evaluated for the optical sensing of Mg²⁺ and Al³⁺.

Highly sensitive fluorescent sensor for Mg2+ and Ca2+ based on a multi-addressable diarylethene

A new photochromic diarylethene bearing 8-aminoquinoline unit was designed and synthesized, and the multi-addressable behaviors were investigated. It was highly sensitive towards Mg²⁺ and Ca²⁺ with different fluorescence emission and color change.

PKA and phosphatases attached to the Ca(V)1.2 channel regulate channel activity in cell-free patches

Calmodulin (CaM) + ATP can reprime voltage-gated L-type Ca(2+) channels (Ca(V)1.2) in inside-out patches for activation, but this effect decreases time dependently. This suggests that the Ca(V)1.2 channel activity is regulated by additional cytoplasmic factors. To test this hypothesis, we examined the role of cAMP-dependent protein kinase A (PKA) and protein phosphatases in the regulation of Ca(V)1.2 channel activity in the inside-out mode in guinea pig ventricular myocytes. Ca(V)1.2 channel activity quickly disappeared after the patch was excised from the cell and recovered to only 9% of that in the cell-attached mode on application of CaM + ATP at 10 min after the inside out. However, immediate exposure of the excised patch to the catalytic subunit of PKA + ATP or the nonspecific phosphatase inhibitor okadaic acid significantly increased the Ca(V)1.2 channel activity recovery by CaM + ATP (114 and 96%, respectively) at 10 min. Interestingly, incubation of the excised patches with cAMP + ATP also increased CaM/ATP-induced Ca(V)1.2 channel activity recovery (108%), and this effect was blocked by the nonspecific protein kinase inhibitor K252a. The channel activity in the inside-out mode was not maintained by either catalytic subunit of PKA or cAMP + ATP in the absence of CaM, but was stably maintained in the presence of CaM for more than 40 min. These results suggest that PKA and phosphatase(s) attached on or near the Ca(V)1.2 channel regulate the basal channel activity, presumably through modulation of the dynamic CaM interaction with the channel.

The effect of acute vs chronic magnesium supplementation on exercise and recovery on resistance exercise, blood pressure and total peripheral resistance on normotensive adults

Background

Magnesium supplementation has previously shown reductions in blood pressure of up to 12 mmHg. A positive relationship between magnesium supplementation and performance gains in resistance exercise has also been seen. However, no previous studies have investigated loading strategies to optimise response. The aim of this study was to assess the effect of oral magnesium supplementation on resistance exercise and vascular response after intense exercise for an acute and chronic loading strategy on a 2-day repeat protocol.

Methods

The study was a randomised, double-blind, cross-over design, placebo controlled 2 day repeat measure protocol (n = 13). Intense exercise (40 km time trial) was followed by bench press at 80% 1RM to exhaustion, with blood pressure and total peripheral resistance (TPR) recorded. 300 mg/d elemental magnesium was supplemented for either a 1 (A) or 4 (Chr) week loading strategy. Food diaries were recorded.

Results

Dietary magnesium intake was above the Reference Nutrient Intake (RNI) for all groups. Bench press showed a significant increase of 17.7% (p = 0.031) for A on day 1. On day 2 A showed no decrease in performance whilst Chr showed a 32.1% decrease. On day 2 post-exercise systolic blood pressure (SBP) was significantly lower in both A (p = 0.0.47) and Chr (p = 0.016) groups. Diastolic blood pressure (DBP) showed significant decreases on day 2 solely for A (p = 0.047) with no changes in the Chr. TPR reduced for A on days 1 and 2 (p = 0.031) with Chr showing an increase on day 1 (p = 0.008) and no change on day 2.

Conclusion

There was no cumulative effect of Chr supplementation compared to A. A group showed improvement for bench press concurring with previous research which was not seen in Chr. On day 2 A showed a small non-significant increase but not a decrement as expected with Chr showing a decrease. DBP showed reductions in both Chr and A loading, agreeing with previous literature. This is suggestive of a different mechanism for BP reduction than for muscular strength. TPR showed greater reductions with A than Chr, which would not be expected as both interventions had reductions in BP, which is associated with TPR.

Solvent-dependent "turn-on" fluorescence chemosensor for Mg(2+) based on combination of C=N isomerization and inhibition of ESIPT mechanisms

A fluorescent chemosensor (L) for Mg(2+) has been synthesized and characterized, which exhibits turn-on fluorescence response for Mg(2+) only in alcohol solvent (methanol or ethanol) with high sensitivity and selectivity. But in both nonpolar and polar solvents (cyclohexane, DCM, DMSO or MeCN), L showed negligible fluorescent response for Mg(2+). In order to discover the unique phenomenon, optical measurements, liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) and a high performance liquid chromatography with a fluorescence detector (HPLC-FLD) of L and L with Mg(2+) ions in solvents were studied. In alcohol solvent, [L+alcohol molecule] was formed and the mechanism aspect of L concerning the remarkable fluorescence response for Mg(2+) has been discussed.

Nucleotides maintain the activity of Cav1.2 channels in guinea-pig ventricular myocytes

The activity of Cav1.2 Ca(2+) channels is maintained in the presence of calmodulin and ATP, even in cell-free patches, and thus a channel ATP-binding site has been suggested. In this study, we examined whether other nucleotides, such as GTP, UTP, CTP, ADP and AMP, could be substituted for ATP in guinea-pig ventricular myocytes. We found that all the nucleotides tested could re-prime the Ca(2+) channels in the presence of 1 μM calmodulin in the inside-out mode. The order of efficacy was ATP > GTP > UTP > ADP > CTP ≈ AMP. Thus, the presumed nucleotide-binding site in the channel seemed to favor a purine rather than pyrimidine base and a triphosphate rather than a di- or mono-phosphate group. Furthermore, a high concentration (10 mM) of GTP, UTP, CTP, ADP and AMP had inhibitory effects on the channel activity. These results provide information on the putative nucleotide-binding site(s) in Cav1.2 Ca(2+) channels.

A real-time fluorescent sensor specific to Mg2+: crystallographic evidence, DFT calculation and its use for quantitative determination of magnesium in drinking water

An “off-the-shelf” fluorescence “turn-on” Mg²⁺ chemosensor BCSA has been developed for real-time quantitative monitoring of magnesium in drinking water.

Adenosine triphosphate regulates the activity of guinea pig Cav1.2 channel by direct binding to the channel in a dose-dependent manner

The present study is to investigate the mechanism by which ATP regulates Cav1.2 channel activity. Ventricular tissue was obtained from adult guinea pig hearts using collagenase. Ca(2+) channel activity was monitored using the patch-clamp technique. Proteins were purified using wheat germ agglutinin-Sepharose, and the concentration was determined using the Coomassie brilliant blue technique. ATP binding to the Cav1.2 channel was examined using the photoaffinity method. EDA-ATP-biotin maintains Ca(2+) channel activity in inside-out membrane patches. ATP directly bound to the Cav1.2 channel in a dose-dependent manner, and at least two molecules of ATP bound to one molecule of the Cav1.2 channel. Low levels of calmodulin (CaM) increased ATP binding to the Cav1.2 channel, but higher levels of CaM decreased ATP binding to the Cav1.2 channel. In addition, Ca(2+) was another regulator for ATP binding to the Cav1.2 channel. Furthermore, ATP bound to GST-fusion peptides of NH2-terminal region (amino acids 6-140) and proximal COOH-terminal region (amino acids 1,509-1,789) of the main subunit (α1C) of the Cav1.2 channel. Our data suggest that ATP might regulate Cav1.2 channel activity by directly binding to the Cav1.2 channel in a dose-dependent manner. In addition, the ATP-binding effect to the Cav1.2 channel was both CaM- and Ca(2+) dependent.

A fluorescent probe for the detection of Mg(ii) and Cu(ii) and its application for imaging in living cells

A novel fluorescent probe 7-[4′-hydroxy-3′-(5′′-methyl-1H-benzo[d]imidazole-2-yl)styryl]nalidixic acid (HBIN) was synthesized that contains two independent fluorophores and acts as a very sensitive and selective probe for Mg²⁺ and Cu²⁺ ions.

A novel chromo- and fluorogenic dual sensor for Mg2+and Zn2+with cell imaging possibilities and DFT studies

A diformyl-p-cresol-8-aminoquinoline based probe exhibits dual colorimetric and fluorogenic properties on selective binding towards Mg²⁺and Zn²⁺. This probe could be made selective towards Mg²⁺over Zn²⁺in the presence of TPEN.

The SLC41 family of MgtE-like magnesium transporters

Magnesium is one of the most predominant intracellular divalent cations and is requisite to the regulation of a diverse array of cellular functions. Although accumulating data from multiple studies have begun to illuminate the critical role(s) played by Mg(2+) transporters in pathways involved in cell signaling, metabolism, growth and proliferation, there is still a lack of understanding of the underlying molecular mechanisms that govern those various functions. In this review, we focus on the recently described SLC41 family of magnesium transporters, two members of which have been shown to mediate Mg(2+) uptake and transport, and highlight what is known about their expression, localization, and function, as well as their roles and contributions to cellular Mg(2+) transport.

NHE inhibition does not improve Na(+) or Ca(2+) overload during reperfusion: using modeling to illuminate the mechanisms underlying a therapeutic failure

Reperfusion injury results from pathologies of cardiac myocyte physiology that develop when previously ischemic myocardium experiences a restoration of normal perfusion. Events in the development of reperfusion injury begin with the restoration of a proton gradient upon reperfusion, which then allows the sodium-proton exchanger (NHE) to increase flux, removing protons from the intracellular space while importing sodium. The resulting sodium overload drives increased reverse-mode sodium-calcium exchanger (NCX) activity, creating a secondary calcium overload that has pathologic consequences. One of the attempts to reduce reperfusion-related damage, NHE inhibition, has shown little clinical benefit, and only when NHE inhibitors are given prior to reperfusion. In an effort to further understand why NHE inhibitors have been largely unsuccessful, we employed a new mathematical cardiomyocyte model that we developed for the study of ischemia and reperfusion. Using this model, we simulated 20 minutes of ischemia and 10 minutes of reperfusion, while also simulating NHE inhibition by reducing NHE flux in our model by varying amounts and at different time points. In our simulations, when NHE inhibition is applied at the onset of reperfusion, increasing the degree of inhibition increases the peak sodium and calcium concentrations, as well as reducing intracellular pH recovery. When inhibition was instituted at earlier time points, some modest improvements were seen, largely due to reduced sodium concentrations prior to reperfusion. Analysis of all sodium flux pathways suggests that the sodium-potassium pump (NaK) plays the largest role in exacerbated sodium overload during reperfusion, and that reduced NaK flux is largely the result of impaired pH recovery. While NHE inhibition does indeed reduce sodium influx through that exchanger, the resulting prolongation of intracellular acidosis paradoxically increases sodium overload, largely mediated by impaired NaK function.

Myocardial ischemia, commonly observed when arteries supplying the heart become occluded, results when cardiac tissue receives inadequate blood perfusion. In order to minimize the amount of cardiac damage, ischemic tissue must be reperfused. However, reperfusion can result in deleterious effects that leave the heart muscle sicker than if the ischemia had been allowed to continue. Examples of these reperfusion injuries include lethal arrhythmias and an increased region of cell death. Some of the early events that result in reperfusion injury include changes in pH and an overload of sodium inside the cell. During reperfusion, the sodium-proton exchanger (NHE) removes protons from the cell in an effort to restore normal pH, in turn importing sodium ions. Many strategies have been attempted to prevent reperfusion injury, including inhibition of the NHE, with little clinical effect. Using a mathematical model that we developed to study ischemia and reperfusion in cardiac cells, we found that NHE inhibition produces more severe sodium overload, largely due to adverse consequences of the delayed pH recovery produced by NHE inhibition. These results suggest that NHE inhibition alone may not be a viable strategy, and that therapies which prolong intracellular acidosis may be problematic.

Calpastatin domain L is a partial agonist of the calmodulin-binding site for channel activation in Cav1.2 Ca2+ channels

Cav1.2 Ca(2+) channel activity diminishes in inside-out patches (run-down). Previously, we have found that with ATP, calpastatin domain L (CSL) and calmodulin (CaM) recover channel activity from the run-down in guinea pig cardiac myocytes. Because the potency of the CSL repriming effect was smaller than that of CaM, we hypothesized that CSL might act as a partial agonist of CaM in the channel-repriming effect. To examine this hypothesis, we investigated the effect of the competitions between CSL and CaM on channel activity and on binding in the channel. We found that CSL suppressed the channel-activating effect of CaM in a reversible and concentration-dependent manner. The channel-inactivating effect of CaM seen at high concentrations of CaM, however, did not seem to be affected by CSL. In the GST pull-down assay, CSL suppressed binding of CaM to GST fusion peptides derived from C-terminal regions in a competitive manner. The inhibition of CaM binding by CSL was observed with the IQ peptide but not the PreIQ peptide, which is the CaM-binding domain in the C terminus. The results are consistent with the hypothesis that CSL competes with CaM as a partial agonist for the site in the IQ domain in the C-terminal region of the Cav1.2 channel, which may be involved in activation of the channel.

Dynamic modulation of Ca2+ sparks by mitochondrial oscillations in isolated guinea pig cardiomyocytes under oxidative stress

Local control of Ca(2+)-induced Ca(2+) release (CICR) depends on the spatial organization of L-type Ca(2+) channels and ryanodine receptors (RyR) in the dyad. Analogously, Ca(2+) uptake by mitochondria is facilitated by their close proximity to the Ca(2+) release sites, a process required for stimulating oxidative phosphorylation during changes in work. Mitochondrial feedback on CICR is less well understood. Since mitochondria are a primary source of reactive oxygen species (ROS), they could potentially influence the cytosolic redox state, in turn altering RyR open probability. We have shown that self-sustained oscillations in mitochondrial inner membrane potential (ΔΨ(m)), NADH, ROS, and reduced glutathione (GSH) can be triggered by a laser flash in cardiomyocytes. Here, we employ this method to directly examine how acute changes in energy state dynamically influence resting Ca(2+) spark occurrence and properties. Two-photon laser scanning microscopy was used to monitor cytosolic Ca(2+) (or ROS), ΔΨ(m), and NADH (or GSH) simultaneously in isolated guinea pig cardiomyocytes. Resting Ca(2+) spark frequency increased with each ΔΨ(m) depolarization and decreased with ΔΨ(m) repolarization without affecting Ca(2+) spark amplitude or time-to-peak. Stabilization of mitochondrial energetics by pretreatment with the superoxide scavenger TMPyP, or by acute addition of 4'-chlorodiazepam, a mitochondrial benzodiazepine receptor antagonist that blocks the inner membrane anion channel, prevented or reversed, respectively, the increased spark frequency. Cyclosporine A did not block the ΔΨ(m) oscillations or prevent Ca(2+) spark modulation by ΔΨ(m). The results support the hypothesis that mitochondria exert an influential role on the redox environment of the Ca(2+) handling subsystem, with mechanistic implications for the pathophysiology of cardiac disease.

Regulation by glucose of oscillatory electrical activity and 5-HT/insulin release from single mouse pancreatic islets in absence of functional K(ATP) channels

The glucose sensitivity of bursting electrical activity and pulsatile insulin release from pancreatic islets was determined in absence of functional K(ATP) channels. Membrane potential, [Ca(2+)](i) and 5-HT/insulin release were measured by intracellular recording, fura-2 fluorescence and 5-HT amperometry, respectively. Single mouse islets, bathed in tolbutamide or glibenclamide and high extracellular Ca(2+) (Ca(2+)(o)), displayed bursting activity and concomitant fast [Ca(2+)](i) and 5-HT/insulin oscillations. Sulphonylurea block of K(ATP) channel current was unaffected by raising Ca(2+)(o). Raising glucose or alpha-ketoisocaproic acid (KIC) concentration from 3 to 30 mM increased spiking activity and burst plateau duration. Staurosporine did not impair glucose potentiation of electrical activity, ruling out the involvement of serine/threonine kinases. Glucose enhanced both [Ca(2+)](i) and 5-HT/insulin oscillatory activity, causing a approximately 3-fold increase in overall 5-HT release rate. Cells lacking bursting activity in high Ca(2+)(o) and low glucose (or KIC) developed a pattern of intensified spiking in response to 11 mM glucose. It is concluded that beta-cells exhibit graded oscillatory electrical and secretory responses to glucose in absence of functional K(ATP) channels. This suggests that, under physiological conditions, early glucose sensing may involve other channels besides the K(ATP) channel.

A coumarin-derived fluorescence probe selective for magnesium

Two different coumarin derivatives have been connected via an imine linkage to obtain a new fluorescence signaling system. This compound itself does not show any emission due to rapid isomerization around the C[double bond]N bond. However, in the presence of a Mg(II) ion, this isomerization is stopped because of bonding to the metal ion resulting in high-intensity (approximately 550-fold) emission. Other metal ions like Li(I), Ca(II), and Zn(II) show very little emission, while biologically relevant transition-metal ions do not show any emission. In this way, the Mg(II) ion can be detected in the presence of these ions.

Optical switches and triggers for the manipulation of ion channels and pores

Like fluorescence sensing techniques, methods to manipulate proteins with light have produced great advances in recent years. Ion channels have been one of the principal protein targets of photoswitched manipulation. In combination with fluorescence detection of cell signaling, this has enabled non-invasive, all-optical experiments on cell and tissue function, both in vitro and in vivo. Optical manipulation of channels has also provided insights into the mechanism of channel function. Optical control elements can be classified according to their molecular reversibility as non-reversible phototriggers where light breaks a chemical bond (e.g. caged ligands) and as photoswitches that reversibly photoisomerize. Synthetic photoswitches constitute nanoscale actuators that can alter channel function using three different strategies. These include (1) nanotoggles, which are tethered photoswitchable ligands that either activate channels (agonists) or inhibit them (blockers or antagonists), (2) nanokeys, which are untethered (freely diffusing) photoswitchable ligands, and (3) nanotweezers, which are photoswitchable crosslinkers. The properties of such photoswitches are discussed here, with a focus on tethered photoswitchable ligands. The recent literature on optical manipulation of ion channels is reviewed for the different channel families, with special emphasis on the understanding of ligand binding and gating processes, applications in nanobiotechnology, and with attention to future prospects in the field.

Ca2+ current of frog vestibular hair cells is modulated by intracellular ATP but not by long-lasting depolarisation

Some aspects of Ca(2+) channel modulation in hair cells isolated from semicircular canals of the frog (Rana esculenta) have been investigated using the whole-cell technique and intra and extracellular solutions designed to modify the basic properties of the Ca(2+) macrocurrent. With 1 mM ATP in the pipette solution, about 60% of the recorded cells displayed a Ca(2+) current constituted by a mix of an L and a drug-resistant (R2) component; the remaining 40% exhibited an additional drug-resistant fraction (R1), which inactivated in a Ca-dependent manner. If the pipette ATP was raised to 10 mM, cells exhibiting the R1 current fraction displayed an increase of both the R1 and L components by approximately 280 and approximately 70%, respectively, while cells initially lacking R1 showed a similar increase in the L component with R1 becoming apparent and raising up to a mean amplitude of approximately 44 pA. In both cell types the R2 current fraction was negligibly affect by ATP. The current run-up was unaffected by cyclic nucleotides, and was not triggered by 10 mM ATPgammaS, ADP, AMP or GTP. Long-lasting depolarisations (>5 s) produced a progressive, reversible decay in the inward current despite the presence of intracellular ATP. Ca(2+) channel blockade by Cd(2+) unmasked a slowly activating outward Cs(+) current flowing through a non-Ca(2+) channel type, which became progressively unblocked by prolonged depolarisation even though Cs(+) and TEA(+) were present on both sides of the channel. The outward current waveform could be erroneously ascribed to a Ca- and/or voltage dependence of the Ca(2+) macrocurrent.

Magnesium ion selective two-photon fluorescent probe based on a benzo[h]chromene derivative for in vivo imaging

A novel, two-photon probe for the detection of free Mg2+ ions in living cells and live tissues has been developed. The probe can be excited by 880 nm laser photons, emits strong two-photon excited fluorescence in response to Mg2+ ions, can be easily loaded into the cell and tissue, shows high photostability, and can measure the Mg2+ ion concentration without interference by Ca2+ ions in living cells. The intracellular dissociation constant (Kdi) for Mg2+ determined by the two-photon process is 2.5 mM, which is suitable for dynamic Mg2+ concentration measurement. In addition, the probe is capable of imaging endogenous stores of free Mg2+ at a few hundred micrometers depth in live tissues using two-photon microscopy (TPM).

Regulation of cation channels in cardiac and smooth muscle cells by intracellular magnesium

Magnesium regulates various ion channels in many tissues, including those of the cardiovascular system. General mechanisms by which intracellular Mg(2+) (Mg(i)(2+)) regulates channels are presented. These involve either a direct interaction with the channel, or an indirect modification of channel function via other proteins, such as enzymes or G proteins, or via membrane surface charges and phospholipids. To provide an insight into the role of Mg(i)(2+) in the cardiovascular system, effects of Mg(i)(2+) on major channels in cardiac and smooth muscle cells and the underlying mechanisms are then reviewed. Although Mg(i)(2+) concentrations are known to be stable, conditions under which they may change exist, such as following stimulation of beta-adrenergic receptors and of insulin receptors, or during pathophysiological conditions such as ischemia, heart failure or hypertension. Modifications of cardiovascular electrical or mechanical function, possibly resulting in arrhythmias or hypertension, may result from such changes of Mg(i)(2+) and their effects on cation channels.

Modeling regulation of cardiac KATP and L-type Ca2+ currents by ATP, ADP, and Mg2+

Changes in cytosolic free Mg(2+) and adenosine nucleotide phosphates affect cardiac excitability and contractility. To investigate how modulation by Mg(2+), ATP, and ADP of K(ATP) and L-type Ca(2+) channels influences excitation-contraction coupling, we incorporated equations for intracellular ATP and MgADP regulation of the K(ATP) current and MgATP regulation of the L-type Ca(2+) current in an ionic-metabolic model of the canine ventricular myocyte. The new model: 1), quantitatively reproduces a dose-response relationship for the effects of changes in ATP on K(ATP) current, 2), simulates effects of ADP in modulating ATP sensitivity of K(ATP) channel, 3), predicts activation of Ca(2+) current during rapid increase in MgATP, and 4), demonstrates that decreased ATP/ADP ratio with normal total Mg(2+) or increased free Mg(2+) with normal ATP and ADP activate K(ATP) current, shorten action potential, and alter ionic currents and intracellular Ca(2+) signals. The model predictions are in agreement with experimental data measured under normal and a variety of pathological conditions.

Calmodulin reverses rundown of L-type Ca(2+) channels in guinea pig ventricular myocytes

Calmodulin (CaM) is implicated in regulation of Ca(2+) channels as a Ca(2+) sensor. The effect of CaM on rundown of L-type Ca(2+) channels in inside-out patch form was investigated in guinea pig ventricular myocytes. Ca(2+) channel activity disappeared within 1-3 min and did not reappear when the patch was excised and exposed to an artificial intracellular solution. However, application of CaM (0.03, 0.3, 3 microM) + 3 mM ATP to the intracellular solution within 1 min after patch excision resulted in dose-dependent activation of channel activity. Channel activity averaged 11.2%, 94.7%, and 292.9%, respectively, of that in cell-attached mode. Channel activity in inside-out patch mode was induced by CaM + ATP at nanomolar Ca(2+) concentrations ([Ca(2+)]); however, increase to micromolar [Ca(2+)] rapidly inactivated the channel activity induced, revealing that the effect of CaM on the channel was Ca(2+) dependent. At the 2nd, 4th, 6th, 8th, and 10th minutes after patch excision, CaM (0.75 microM) + ATP induced Ca(2+) channel activity to 150%, 100%, 96.9%, 29.3%, and 16.6%, respectively, revealing a time-dependent action of CaM on the channel. CaM added with adenosine 5'-(beta,gamma-imido)triphosphate (AMP-PNP) also induced channel activity, although with much lower potency and shorter duration. Protein kinase inhibitors KN-62, CaM-dependent protein kinase (CaMK)II 281-309, autocamtide-related CaMKII inhibitor peptide, and K252a (each 1-10 microM) did not block the effect of CaM, indicating that the effect of CaM on the Ca(2+) channel was phosphorylation independent. Neither CaM nor ATP alone induced Ca(2+) channel activity, showing a cooperative effect of CaM and ATP on the Ca(2+) channel. These results suggest that CaM is a crucial regulatory factor of Ca(2+) channel basal activity.