Insulin modulation of intracellular free magnesium in heart: involvement of protein kinase C

Binding of calcium and magnesium to human cardiac troponin C

Cardiac muscle thin filaments are composed of actin, tropomyosin, and troponin that change conformation in response to Ca²⁺ binding, triggering muscle contraction. Human cardiac troponin C (cTnC) is the Ca²⁺-sensing component of the thin filament. It contains structural sites (III/IV) that bind both Ca²⁺ and Mg²⁺ and a regulatory site (II) that has been thought to bind only Ca²⁺. Binding of Ca²⁺ at this site initiates a series of conformational changes that culminate in force production. However, the mechanisms that underpin the regulation of binding at site II remain unclear. Here, we have quantified the interaction between site II and Ca²⁺/Mg²⁺ through isothermal titration calorimetry and thermodynamic integration simulations. Direct and competitive binding titrations with WT N-terminal cTnC and full-length cTnC indicate that physiologically relevant concentrations of both Ca²⁺/Mg²⁺ interacted with the same locus. Moreover, the D67A/D73A N-terminal cTnC construct in which two coordinating residues within site II were removed was found to have significantly reduced affinity for both cations. In addition, 1 mM Mg²⁺ caused a 1.4-fold lower affinity for Ca²⁺. These experiments strongly suggest that cytosolic-free Mg²⁺ occupies a significant population of the available site II. Interaction of Mg²⁺ with site II of cTnC likely has important functional consequences for the heart both at baseline as well as in diseased states that decrease or increase the availability of Mg²⁺, such as secondary hyperparathyroidism or ischemia, respectively.

Insulin Modulates the Na+/Mg2+ Exchanger SLC41A1 and Influences Mg2+ Efflux from Intracellular Stores in Transgenic HEK293 Cells

BACKGROUND

Magnesium deficiency is a common complication of diabetes with an unclear molecular background.

OBJECTIVE

We investigated the effect of the insulin (INS)-signaling pathway (ISP) on the regulation of Mg(2+) efflux (Mg(2+)E) conducted by solute carrier family 41, member A1 (SLC41A1; activated by protein kinase A) in transgenic human embryonic kidney (HEK) 293 cells.

METHODS

HEK293 cells overexpressing SLC41A1 were loaded with the Mg(2+) fluorescent indicator mag-fura-2 and Mg(2+). Measurements of Mg(2+)E were conducted in Mg(2+)-free buffer by using fast-filter fluorescence spectrometry. We examined the effects of INS, inhibitors of ISP or p38 mitogen-activated protein kinase (p38 MAPK), an activator of adenylate cyclase (ADC), and their combinations on SLC41A1-attributed Mg(2+)E.

RESULTS

The application of 400 μU/mL INS inhibited SLC41A1-mediated Mg(2+)E by up to 50.6% compared with INS-untreated cells (P < 0.001). Moreover, INS evoked the early onset of Mg(2+) release from intracellular stores. The application of 0.1 μM wortmannin or 10 μM zardaverine (both ISP inhibitors) restored SLC41A1 Mg(2+)E capacity in the presence of INS to the same levels in INS-untreated cells. The simultaneous application of 10 μM forskolin, an ADC activator, and INS resulted in a reduction of Mg(2+)E of up to 59% compared with untreated cells (P < 0.001), which was comparable to that in cells treated with INS alone. Inhibition of p38 MAPK with 10 μM SB 202190 (SB) in the absence of INS resulted in a decrease (P < 0.001) of SLC41A1-dependent Mg(2+)E (by up to 49%) compared with Mg(2+)E measured in untreated cells. Simultaneous exposure of cells to SB and INS had a stronger inhibitory effect on SLC41A1 activity than INS alone (P < 0.05).

CONCLUSIONS

INS affects intracellular Mg(2+) concentration in transgenic HEK293 cells by regulating SLC41A1 activity (via ISP) and by influencing the compartmentalization and cellular distribution of Mg(2+). In addition, p38 MAPK activates SLC41A1 independently of INS action.

Respiratory failure in diabetic ketoacidosis

Respiratory failure complicating the course of diabetic ketoacidosis (DKA) is a source of increased morbidity and mortality. Detection of respiratory failure in DKA requires focused clinical monitoring, careful interpretation of arterial blood gases, and investigation for conditions that can affect adversely the respiration. Conditions that compromise respiratory function caused by DKA can be detected at presentation but are usually more prevalent during treatment. These conditions include deficits of potassium, magnesium and phosphate and hydrostatic or non-hydrostatic pulmonary edema. Conditions not caused by DKA that can worsen respiratory function under the added stress of DKA include infections of the respiratory system, pre-existing respiratory or neuromuscular disease and miscellaneous other conditions. Prompt recognition and management of the conditions that can lead to respiratory failure in DKA may prevent respiratory failure and improve mortality from DKA.

Cellular magnesium homeostasis

Magnesium, the second most abundant cellular cation after potassium, is essential to regulate numerous cellular functions and enzymes, including ion channels, metabolic cycles, and signaling pathways, as attested by more than 1000 entries in the literature. Despite significant recent progress, however, our understanding of how cells regulate Mg(2+) homeostasis and transport still remains incomplete. For example, the occurrence of major fluxes of Mg(2+) in either direction across the plasma membrane of mammalian cells following metabolic or hormonal stimuli has been extensively documented. Yet, the mechanisms ultimately responsible for magnesium extrusion across the cell membrane have not been cloned. Even less is known about the regulation in cellular organelles. The present review is aimed at providing the reader with a comprehensive and up-to-date understanding of the mechanisms enacted by eukaryotic cells to regulate cellular Mg(2+) homeostasis and how these mechanisms are altered under specific pathological conditions.

Imipramine inhibition of TRPM-like plasmalemmal Mg2+ transport in vascular smooth muscle cells

Depression is associated with vascular disease, such as myocardial infarction and stroke. Pharmacological treatments may contribute to this association. On the other hand, Mg²⁺ deficiency is also known to be a risk factor for the same category of diseases. In the present study, we examined the effect of imipramine on Mg²⁺ homeostasis in vascular smooth muscle, especially via melastatin-type transient receptor potential (TRPM)-like Mg²⁺-permeable channels. The intracellular free Mg²⁺ concentration ([Mg²⁺]_i) was measured using ³¹P-nuclear magnetic resonance (NMR) in porcine carotid arteries that express both TRPM6 and TRPM7, the latter being predominant. pH_i and intracellular phosphorus compounds were simultaneously monitored. To rule out Na⁺-dependent Mg²⁺ transport, and to facilitate the activity of Mg²⁺-permeable channels, experiments were carried out in the absence of Na⁺ and Ca²⁺. Changing the extracellular Mg²⁺ concentration to 0 and 6 mM significantly decreased and increased [Mg²⁺]_i, respectively, in a time-dependent manner. Imipramine statistically significantly attenuated both of the bi-directional [Mg²⁺]_i changes under the Na⁺- and Ca²⁺-free conditions. This inhibitory effect was comparable in influx, and much more potent in efflux to that of 2-aminoethoxydiphenyl borate, a well-known blocker of TRPM7, a channel that plays a major role in cellular Mg²⁺ homeostasis. Neither [ATP]_i nor pH_i correlated with changes in [Mg²⁺]_i. The results indicate that imipramine suppresses Mg²⁺-permeable channels presumably through a direct effect on the channel domain. This inhibitory effect appears to contribute, at least partially, to the link between antidepressants and the risk of vascular diseases.

Glucose-transporter-mediated positive inotropic effects in human myocardium of diabetic and nondiabetic patients

Insulin causes inotropic effects via Ca(2+)-dependent and Ca(2+)-independent pathways. The latter one is potentially glucose dependent. We examined inotropic responses and signal transduction of insulin in human atrial myocardium of diabetic and nondiabetic patients to test for the role of glucose transporters. Experiments were performed in isolated atrial myocardium of 88 patients undergoing cardiac surgery and 28 ventricular muscle samples of explanted hearts. Influence of insulin (0.02 micromol/L) on isometric twitch force was examined with and without blocking glucose transporter (GLUT) 4 translocation (latrunculin), sodium-coupled glucose transporter (SGLT) 1 (phlorizin, T-1095A), or PI3-kinase (wortmannin). Experiments were performed in Tyrode solution containing glucose or pyruvate as energetic substrate. Messenger RNA expression of glucose transporters (GLUT1, GLUT4, SGLT1, SGLT2) was analyzed in atrial and ventricular myocardium of both diabetic and nondiabetic patients. Developed force increases after insulin (to 117.8% +/- 2.4% and 115.8% +/- 1.9%) in trabeculae from patients with and without diabetes. Inotropic effect was reduced after displacing glucose with pyruvate as well as after PI3-kinase inhibition (to 103% +/- 2%) or inhibition of glucose transporters GLUT4 (to 105% +/- 2%) and SGLT1 (phlorizin to 106% +/- 2%, T-1095A to 105% +/- 2%), without differences between the 2 groups. In glucose-free pyruvate-containing solution, only inhibition of PI3-kinase but not blocking glucose transporters resulted in further inhibitory effects. Messenger RNA expression did not show significant differences between patients with or without diabetes. Insulin exerts positive inotropic effects in human atrial myocardium. These effects are mediated via a PI3-kinase-sensitive and a glucose-transport-sensitive pathway. Differences in functional effects or messenger RNA expression of glucose transporters were not detectable between patients with and without diabetes.

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.

Heart slice NMR

Nuclear magnetic resonance (NMR) spectroscopy of the heart is normally carried out using whole heart preparations under coronary perfusion. In such preparations, either radical changes in ionic composition of the perfusate or applications of numerous drugs would affect coronary microcirculation. This report communicates the first (31)P NMR spectroscopy study using a heart slice preparation (left ventricular slices) superfused with extracellular medium. The ratio of phosphocreatine concentration to ATP concentration was approximately 2.1. Also, intracellular pH and Mg(2+) concentration ([Mg(2+)](i)), estimated from the chemical shifts of inorganic phosphate and ATP, were comparable with those under retrograde perfusion. [Mg(2+)](i) was significantly increased by the removal of extracellular Na(+), supporting the essential role of Na(+)-coupled Mg(2+) transport in Mg(2+) homeostasis of the heart. Heart slice preparation could also be used to evaluate the potency of cardiac drugs, regardless of their possible effects on coronary microcirculation.

Insulin Causes [Ca 2+ ] i -Dependent and [Ca 2+ ] i -Independent Positive Inotropic Effects in Failing Human Myocardium

Background— Insulin has been shown to exert positive inotropic effects in several in vitro and in vivo models, but signal transduction and substrate dependency remain unclear. We examined inotropic responses and signal transduction mechanisms of insulin in human myocardium.

Methods and Results— Experiments were performed in isolated trabeculae from end-stage failing hearts of 58 nondiabetic and 3 diabetic patients undergoing heart transplantation. The effect of insulin (0.3 and 3 IU/L) on isometric twitch force (37°C, 1 Hz) was tested in the presence of glucose or pyruvate as energetic substrate. Furthermore, intracellular Ca 2+ transients (aequorin method), sarcoplasmic reticulum (SR) Ca 2+ content (rapid cooling contractures), and myofilament Ca 2+ sensitivity (semiskinned fibers) were assessed. In addition, potential signaling pathways were tested by blocking glycolysis, PI-3-kinase, protein kinase C, diacylglycerol kinase, insulin-like growth factor-1 receptors, or transsarcolemmal Ca 2+ entry via the Na + /Ca 2+ exchanger. Insulin exerted concentration-dependent and partially substrate-dependent positive inotropic effects. The phosphatidylinositol-3-kinase inhibitor wortmannin and the Na 2+ /Ca 2+ exchanger reverse-mode inhibitor KB-R7943 completely or partially prevented the functional effects of insulin. In contrast, insulin-like growth factor-1 receptor blockade, protein kinase C inhibition, and diacylglycerol kinase blockade were without effect. The inotropic response was associated with increases in intracellular Ca 2+ transients, SR Ca 2+ content, and increased myofilament Ca 2+ sensitivity.

Conclusions— Insulin exerts Ca 2+ -dependent and -independent positive inotropic effects through a phosphatidylinositol-3-kinase–dependent pathway in failing human myocardium. The increased [Ca 2+ ] i originates at least in part from enhanced reverse-mode Na + /Ca 2+ exchange and consequently increased SR-Ca 2+ load. These nongenomic functional effects of insulin may be of clinical relevance, eg, during insulin-glucose-potassium infusions.

Ca2+-dependent modulation of intracellular Mg2+ concentration with amiloride and KB-R7943 in pig carotid artery

It has long been recognized that magnesium is associated with several important diseases, including diabetes, hypertension, cardiovascular, and cerebrovascular diseases. In the present study, we measured the intracellular free Mg2+ concentration ([Mg2+]i) using 31P nuclear magnetic resonance (NMR) in pig carotid artery smooth muscle. In normal solution, application of amiloride (1 mm) decreased [Mg2+]i by approximately 12% after 100 min. Subsequent washout tended to further decrease [Mg2+]i. In contrast, application of amiloride significantly increased [Mg2+]i (by approximately 13% after 100 min) under Ca2+-free conditions, where passive Mg2+ influx is facilitated. The treatments had little effect on intracellular ATP and pH (pHi). Essentially the same Ca2+-dependent changes in [Mg2+]i were produced with KB-R7943, a selective blocker of reverse mode Na+-Ca2+ exchange. Application of dimethyl amiloride (0.1 mM) in the presence of Ca2+ did not significantly change [Mg2+]i, although it inhibited Na+-H+ exchange at the same concentration. Removal of extracellular Na+ caused a marginal increase in [Mg2+]i after 100-200 min, as seen in intestinal smooth muscle in which Na+-Mg2+ exchange is known to be the primary mechanism of maintaining a low [Mg2+]i against electrochemical equilibrium. In Na+-free solution (containing Ca2+), neither amiloride nor KB-R7943 decreased [Mg2+]i, but they rather increased it. The results suggest that these inhibitory drugs for Na+-Ca2+ exchange directly modulate Na+-Mg2+ exchange in a Ca2+-dependent manner, and consequently produce the paradoxical decrease in [Mg2+]i in the presence of Ca2+.

A critical interplay between Ca2+ inhibition and activation by Mg2+ of AC5 revealed by mutants and chimeric constructs

Adenylyl cyclase type 5 (AC5) is sensitive to both high and low affinity inhibition by Ca(2+). This property provides a sensitive feedback mechanism of the Ca(2+) entry that is potentiated by cAMP in sources where AC5 is commonly expressed (e.g. myocardium). Remarkably little is known about the molecular mechanism whereby Ca(2+) inhibits AC5. Because previous studies had showed that Ca(2+) antagonized the activation of adenylyl cyclase brought about by Mg(2+), we have now evaluated the Mg(2+)-binding domain in the catalytic site as the potential site of the interaction, using a number of mutations of AC5 with impaired Mg(2+) activation. Mg(2+) activation exerted contrasting effects on the high and low affinity Ca(2+) inhibition. In both wild type and mutants, activation by Mg(2+) decreased the absolute amount of high affinity inhibition without affecting the K(i) value, whereas the K(i) value for low affinity inhibition was decreased. These effects were directly proportional to the sensitivity of the mutants to Mg(2+). Parallel changes were noted in the efficacies of Ca(2+), Sr(2+), and Ba(2+) in the mutant species, suggesting a simple mutation in a shared domain. Strikingly, forskolin, which activates by a mechanism different from Mg(2+), did not modify inhibition by Ca(2+). Deletion of the N terminus and the C1b domain of AC5 and a chimera formed with AC2 confirmed that the catalytic domain alone was responsible for high affinity inhibition. We therefore conclude that both low and high affinity inhibition by Ca(2+) are exerted on different conformations of the Mg(2+)-binding sites in the catalytic domain of AC5.

Protective role of magnesium in cardiovascular diseases: a review

A considerable number of experimental, epidemiological and clinical studies are now available which point to an important role of Mg2+ in the etiology of cardiovascular pathology. In human subjects, hypomagnesemia is often associated with an imbalance of electrolytes such as Na+, K+ and Ca2+. Abnormal dietary deficiency of Mg2+ as well as abnormalities in Mg2+ metabolism play important roles in different types of heart diseases such as ischemic heart disease, congestive heart failure, sudden cardiac death, atheroscelerosis, a number of cardiac arrhythmias and ventricular complications in diabetes mellitus. Mg2+ deficiency results in progressive vasoconstriction of the coronary vessels leading to a marked reduction in oxygen and nutrient delivery to the cardiac myocytes. Numerous experimental and clinical data have suggested that Mg2+ deficiency can induce elevation of intracellular Ca2+ concentrations, formation of oxygen radicals, proinflammatory agents and growth factors and changes in membrane perrmeability and transport processes in cardiac cells. The opposing effects of Mg2+ and Ca2+ on myocardial contractility may be due to the competition between Mg2+ and Ca2+ for the same binding sites on key myocardial contractile proteins such as troponin C, myosin and actin. Stimulants, for example, catecholamines can evoke marked Mg2+ efflux which appears to be associated with a concomitant increase in the force of contraction of the heart. It has been suggested that Mg2+ efflux may be linked to the Ca2+ signalling pathway. Depletion of Mg2+ by alcohol in cardiac cells causes an increase in intracellular Ca2+, leading to coronary artery vasospasm, arrhythmias, ischemic damage and cardiac failure. Hypomagnesemia is commonly associated with hypokalemia and occurs in patients with hypertension or myocardial infarction as well as in chronic alcoholism. The inability of the senescent myocardium to respond to ischemic stress could be due to several reasons. Mg2+ supplemented K+ cardioplegia modulates Ca2+ accumulation and is directly involved in the mechanisms leading to enhanced post ischemic functional recovery in the aged myocardium following ischemia. While many of these mechanisms remain controversial and in some cases speculative, the beneficial effects related to consequences of Mg2+ supplementation are apparent. Further research are needed for the incorporation of these findings toward the development of novel myocardial protective role of Mg2+ to reduce morbidity and mortality of patients suffering from a variety of cardiac diseases.

Selective enhanced phosphorylation of shrimp beta-tubulin by PKC-delta with PEP(taxol), a synthetic peptide encoding the taxol binding region

Beta-tubulin cDNA from the shrimp Penaeus japonicus was isolated by homology cloning. Expression of cDNA in Escherichia coli yielded a 55 kDa polypeptide, positive for monoclonal antibodies against mammalian beta-tubulin. Autoradiography demonstrated the bacterially expressed hepatopancreas beta-tubulin of P. japonicus is specifically phosphorylated by the delta isoenzyme of protein kinase C (PKC-delta) purified from the plasma membrane of the shrimp heart, in the presence of the receptor for activated PKC (RACK), but not in its absence. Purified shrimp heart PKC-delta is able to phosphorylate bacterially expressed shrimp beta-tubulin without the presence of Ca(++), but requires Mg(++). The kinase activity of purified PKC-delta on bacterially expressed beta-tubulin was enhanced by incubation with PEP(taxol), a synthetic peptide encoding the taxol-binding region of beta-tubulin. In other words, PEP(taxol) modulates the kinase activity of PKC-delta through RACK.

Age dependence of human gastrocnemius Mg2+: fitting 31P-NMR spectra using quantum mechanics-based prior knowledge

The age dependence of human gastrocnemius Mg2+ concentration is demonstrated. To quantitate Mg2+ concentration, an original and accurate fitting algorithm using quantum mechanics-based prior knowledge is detailed. In a group of 28 volunteers (14 females) in the age range 5-80 years, pH, PCr/ATP and Pi/ATP values in the gastrocnemius were 7.02 +/- 0.02 pH, 4.16 +/- 0.33 and 0.13 +/- 0.02, respectively and independent of age and sex. By contrast, intracellular Mg2+ concentration (mM) decreased linearly (p < 0.05) with age (Mg2+ = 0.7803 +/- 0.0247-0.0027 +/- 0.0005 * age). No difference was found between sexes. From these results, it follows that care must be taken when comparing muscle Mg2+ data from subjects of different age. The hypothesis can be put forward that during aging insufficient intake and/or increased depletion of Mg2+ (e.g., intestinal hypoabsorption or urinary leakage) may affect the musculoskeletal system.