Low [Mg(2+)](o) induces contraction of cerebral arteries: roles of tyrosine and mitogen-activated protein kinases

Beyond Ion Homeostasis: Hypomagnesemia, Transient Receptor Potential Melastatin Channel 7, Mitochondrial Function, and Inflammation

As the second most abundant intracellular divalent cation, magnesium (Mg2+) is essential for cell functions, such as ATP production, protein/DNA synthesis, protein activity, and mitochondrial function. Mg2+ plays a critical role in heart rhythm, muscle contraction, and blood pressure. A significant decline in Mg2+ intake has been reported in developed countries because of the increased consumption of processed food and filtered/deionized water, which can lead to hypomagnesemia (HypoMg). HypoMg is commonly observed in cardiovascular diseases, such as heart failure, hypertension, arrhythmias, and diabetic cardiomyopathy, and HypoMg is a predictor for cardiovascular and all-cause mortality. On the other hand, Mg2+ supplementation has shown significant therapeutic effects in cardiovascular diseases. Some of the effects of HypoMg have been ascribed to changes in Mg2+ participation in enzyme activity, ATP stabilization, enzyme kinetics, and alterations in Ca2+, Na+, and other cations. In this manuscript, we discuss new insights into the pathogenic mechanisms of HypoMg that surpass previously described effects. HypoMg causes mitochondrial dysfunction, oxidative stress, and inflammation. Many of these effects can be attributed to the HypoMg-induced upregulation of a Mg2+ transporter transient receptor potential melastatin 7 channel (TRMP7) that is also a kinase. An increase in kinase signaling mediated by HypoMg-induced TRPM7 transcriptional upregulation, independently of any change in Mg2+ transport function, likely seems responsible for many of the effects of HypoMg. Therefore, Mg2+ supplementation and TRPM7 kinase inhibition may work to treat the sequelae of HypoMg by preventing increased TRPM7 kinase activity rather than just altering ion homeostasis. Since many diseases are characterized by oxidative stress or inflammation, Mg2+ supplementation and TRPM7 kinase inhibition may have wider implications for other diseases by acting to reduce oxidative stress and inflammation.

A combination of genistein and magnesium enhances the vasodilatory effect via an eNOS pathway and BKCa current amplification

The phytoestrogen genistein (GST) and magnesium have been independently shown to regulate vascular tone; however, their individual vasodilatory effects are limited. The aim of this study was to examine the combined effects of GST plus magnesium on vascular tone in mesenteric arteries. The effects of pretreatment with GST (0–200 μmol/L), MgCl2 (0–4.8 mmol/L) and GST plus MgCl2 on 10 μmol/L phenylephrine (PE) precontracted mesenteric arteries in rats were assessed by measuring isometric force. BKCa currents were detected by the patch clamp method. GST caused concentration- and partial endothelium-dependent relaxation. Magnesium resulted in dual adjustment of vascular tone. Magnesium-free solution eliminated the vasodilatation of GST in both endothelium-intact and denuded rings. GST (50 μmol/L) plus magnesium (4.8 mmol/L) caused stronger relaxation in both endothelium-intact and denuded rings. Pretreatment with the nitric oxide synthase (NOS) inhibitor l-N-nitroarginine methyl ester (l-NAME, 100 μmol/L) significantly inhibited the effects of GST, high magnesium, and the combination of GST and magnesium. BKCa currents were amplified to a greater extent when GST (50 μmol/L) was combined with 4.8 versus 1.2 mmol/L Mg2+. Our data suggest that GST plus magnesium provides enhanced vasodilatory effects in rat mesenteric arteries compared with that observed when either is used separately, which was related to an eNOS pathway and BKCa current amplification.

Adenosine receptors and the heart: role in regulation of coronary blood flow and cardiac electrophysiology

Adenosine is an autacoid that plays a critical role in regulating cardiac function, including heart rate, contractility, and coronary flow. In this chapter, current knowledge of the functions and mechanisms of action of coronary flow regulation and electrophysiology will be discussed. Currently, there are four known adenosine receptor (AR) subtypes, namely A(1), A(2A), A(2B), and A(3). All four subtypes are known to regulate coronary flow. In general, A(2A)AR is the predominant receptor subtype responsible for coronary blood flow regulation, which dilates coronary arteries in both an endothelial-dependent and -independent manner. The roles of other ARs and their mechanisms of action will also be discussed. The increasing popularity of gene-modified models with targeted deletion or overexpression of a single AR subtype has helped to elucidate the roles of each receptor subtype. Combining pharmacologic tools with targeted gene deletion of individual AR subtypes has proven invaluable for discriminating the vascular effects unique to the activation of each AR subtype. Adenosine exerts its cardiac electrophysiologic effects mainly through the activation of A(1)AR. This receptor mediates direct as well as indirect effects of adenosine (i.e., anti-beta-adrenergic effects). In supraventricular tissues (atrial myocytes, sinuatrial node and atriovetricular node), adenosine exerts both direct and indirect effects, while it exerts only indirect effects in the ventricle. Adenosine exerts a negative chronotropic effect by suppressing the automaticity of cardiac pacemakers, and a negative dromotropic effect through inhibition of AV-nodal conduction. These effects of adenosine constitute the rationale for its use as a diagnostic and therapeutic agent. In recent years, efforts have been made to develop A(1)R-selective agonists as drug candidates that do not induce vasodilation, which is considered an undesirable effect in the clinical setting.

Phosphorylation of UT-A1 urea transporter at serines 486 and 499 is important for vasopressin-regulated activity and membrane accumulation

The UT-A1 urea transporter plays an important role in the urine concentrating mechanism. Vasopressin (or cAMP) increases urea permeability in perfused terminal inner medullary collecting ducts and increases the abundance of phosphorylated UT-A1, suggesting regulation by phosphorylation. We performed a phosphopeptide analysis that strongly suggested that a PKA consensus site(s) in the central loop region of UT-A1 was/were phosphorylated. Serine 486 was most strongly identified, with other potential sites at serine 499 and threonine 524. Phosphomutation constructs of each residue were made and transiently transfected into LLC-PK1 cells to assay for UT-A1 phosphorylation. The basal level of UT-A1 phosphorylation was unaltered by mutation of these sites. We injected oocytes, assayed [14C]urea flux, and determined that mutation of these sites did not alter basal urea transport activity. Next, we tested the effect of stimulating cAMP production with forskolin. Forskolin increased wild-type UT-A1 and T524A phosphorylation in LLC-PK1 cells and increased urea flux in oocytes. In contrast, the S486A and S499A mutants demonstrated loss of forskolin-stimulated UT-A1 phosphorylation and reduced urea flux. In LLC-PK1 cells, we assessed biotinylated UT-A1. Wild-type UT-A1, S486A, and S499A accumulated in the membrane in response to forskolin. However, in the S486A/S499A double mutant, forskolin-stimulated UT-A1 membrane accumulation and urea flux were totally blocked. We conclude that the phosphorylation of UT-A1 on both serines 486 and 499 is important for activity and that this phosphorylation may be involved in UT-A1 membrane accumulation.

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.

Role of Cav1.2 L-type Ca2+ channels in vascular tone: effects of nifedipine and Mg2+

Ca(2+) entry via L-type voltage-gated Ca(2+) channels (LVGCs) is a key factor in generating myogenic tone (MT), as dihydropyridines (DHPs) and other LVGC blockers, including Mg(2+), markedly reduce MT. Recent reports suggest, however, that elevated external Mg(2+) concentration and DHPs may also inhibit other Ca(2+)-entry pathways. Here, we explore the contribution of LVGCs to MT in intact, pressurized mesenteric small arteries using mutant mice (DHP(R/R)) expressing functional but DHP-insensitive Ca(v)1.2 channels. In wild-type (WT), but not DHP(R/R), mouse arteries, nifedipine (0.3-1.0 microM) markedly reduced MT and vasoconstriction induced by high external K(+) concentrations ([K(+)](o)), a measure of LVGC-mediated Ca(2+) entry. Blocking MT and high [K(+)](o)-induced vasoconstriction by <1 microM nifedipine in WT but not in DHP(R/R) arteries implies that Ca(2+) entry via Ca(v)1.2 LVGCs is obligatory for MT and that nifedipine inhibits MT exclusively by blocking LVGCs. We also examined the effects of Mg(2+) on MT and LVGCs. High external Mg(2+) concentration (10 mM) blocked MT, slowed the high [K(+)](o)-induced vasoconstrictions, and decreased their amplitude in WT and DHP(R/R) arteries. To verify that these effects of Mg(2+) are due to block of LVGCs, we characterized the effects of extracellular and intracellular Mg(2+) on LVGC currents in isolated mesenteric artery myocytes. DHP-sensitive LVGC currents are inhibited by both external and internal Mg(2+). The results indicate that Mg(2+) relaxes MT by inhibiting Ca(2+) influx through LVGCs. These data provide new information about the central role of Ca(v)1.2 LVGCs in generating and maintaining MT in mouse mesenteric small arteries.

Differential regulation of B/K protein expression in proximal and distal tubules of rat kidneys with ischemia-reperfusion injury

Brain/kidney (B/K) protein is a novel double C2-like-domain protein that is highly expressed in rat brain and kidney, but its cellular localization and functional role in the kidney are still undetermined. We examined the cellular localization of B/K protein in the rat kidney under normal and ischemic conditions. Ischemia-reperfusion (I/R) injury was induced by clamping both renal arteries for 45 min, and animals were killed at 1 and 6 h and 1, 2, 3, 5, 7, 14, and 28 days after the reperfusion. Kidney tissues were processed for immunohistochemistry and immunoblot analyses using rabbit anti-B/K polyclonal antibodies. In control kidneys, B/K protein was expressed primarily in distal tubules including the thick ascending limb, distal convoluted and connecting tubules, and collecting duct. Notably, B/K protein was also expressed in the straight portion (S3 segment), but not in the S1 or S2, of proximal tubules, and podocytes of the glomerulus. In rat kidneys with I/R injury, expression of B/K protein was differentially regulated according to the anatomic location. In distal tubules, overall expression of B/K protein was markedly decreased. On the other hand, I/R injury significantly increased B/K protein expression in the S3 segment of the outer medulla as well as in the rat proximal tubular epithelial cell line NRK-52E in vitro. Furthermore, B/K protein was strongly expressed in many exfoliated cells in the lumen and urine. These findings suggest that B/K protein is closely associated with cell death in proximal tubules, which are vulnerable to I/R injury in the kidney.

Src family kinase involvement in rat preglomerular microvascular contractile and [Ca2+]i responses to ANG II

Experiments were performed to investigate the potential role of Src family kinase(s) in the rat afferent arteriolar contractile response to ANG II. The in vitro blood-perfused juxtamedullary nephron technique was employed to monitor afferent arteriolar lumen diameter responses to 1-100 nM ANG II before and during Src family kinase inhibition (10 microM PP2). PP2 did not alter baseline diameter but attenuated ANG II-induced contractile responses by 33 +/- 6%. An inactive analog of PP2 (PP3) had no effect on ANG II-induced afferent arteriolar contraction. The effect of Src kinase inhibition on ANG II-induced intracellular free Ca(2+) concentration ([Ca(2+)](i)) responses was probed in fura 2-loaded preglomerular microvascular smooth muscle cells (PVSMCs) obtained from explants and studied after 3-5 days in culture. In untreated PVSMCs, ANG II evoked peak (Delta = 293 +/- 66 nM) and plateau (Delta = 23 +/- 8 nM) increases in [Ca(2+)](i). In PVSMCs pretreated with PP2, baseline [Ca(2+)](i) was unaltered, but both the peak (Delta = 140 +/- 22 nM) and plateau (Delta = 3 +/- 2 nM) phases of the ANG II response were significantly reduced compared with untreated cells. PP3 did not alter [Ca(2+)](i) responses to ANG II. Immunoprecipitation and Western blot analysis confirmed that 100 nM ANG II increased phosphorylation of c-Src (at Y(416)) in PVSMCs. The phosphorylation response was maximal 1 min after ANG II exposure and was prevented by PP2. We conclude that the preglomerular vasoconstriction evoked by ANG II involves rapid c-Src activation with subsequent effects that contribute to the [Ca(2+)](i) response to the peptide.

Mechanisms of hydroxyl radical-induced contraction of rat aorta

The present study was designed to investigate the effects of hydroxyl radicals (*OH), generated via the Fe2+-mediated Fenton reaction, on isolated rat aortic rings with and without endothelium. In the absence of any vasoactive agent, generation of *OH alone elicited an endothelium-independent contraction in rat aortic rings in a concentration-dependent manner. Hydroxyl radical-induced contractions of denuded rat aortic rings appeared, however, to be slightly stronger than those on intact rat aortic rings. The contractile responses to *OH were neither reversible nor reproducible in the same ring; even small concentrations of *OH radicals resulted in tachyphylaxis. Removal of extracellular calcium ions (Ca2+) or buffering intracellular Ca2+ with 10 microM acetyl methyl ester of bis(o-aminophenoxy) ethane-N,N,N',N',-tetraacetic acid (BAPTA-AM) significantly attenuated the contractile actions of *OH radicals. The presence of 1 microM staurosporine, 1 microM bisindolylmaleimide I, 1 microM Gö6976 [inhibitor of protein kinase C (PKC)], 2 microM PD-980592 (inhibitor of ERK), 10 microM genistein, and 1 microM wortmannin significantly inhibited the contractions induced by *OH. Proadifen (10 microM), on the other hand, significantly potentiated the hydroxyl radical-induced contractions. Exposure of primary cultured aortic smooth muscle cells to *OH produced significant, rapid rises of intracellular free Ca2+ ([Ca2+]i). Several, specific antagonists of possible endogenously formed vasoconstrictors did not inhibit or attenuate either hydroxyl radical-induced contractions or the elevation of [Ca2+]i. Our new results suggest that hydroxyl radical-triggered contractions on rat aortic rings are Ca2+-dependent. Several intracellular signal transduction systems seem to play some role in hydroxyl radical-induced vasoconstriction of rat aortic rings.

Low [Mg 2+ ] e Enhances Arterial Spontaneous Tone via Phosphatidylinositol 3-Kinase in DOCA-Salt Hypertension

Phosphatidylinositol 3-kinase (PI3K) has been implicated in low extracellular Mg 2+ concentration ( [Mg 2+ ] e )–induced aortic contraction, and Mg 2+ deficiency has been associated with hypertension. Moreover, arterial PI3K activity is increased in hypertensive deoxycorticosterone (DOCA)-salt rats. We hypothesized that low [Mg 2+ ] e activates PI3K, eliciting enhanced vascular contraction, PI3K activity, and norepinephrine (NE)-induced contraction. Spontaneous tone was monitored in endothelium-denuded aortic strips from sham and DOCA-salt rats exposed to low Mg 2+ (0.15 mmol/L), high Mg 2+ (4.8 mmol/L), or normal (1.17 mmol/L) physiologic salt solution (PSS) in isolated tissue baths. LY294002 (20 μmol/L), a PI3K inhibitor, or vehicle was added (30 minutes), followed by NE (10 −9 to 3 x10 −-5 mol/L). Low [Mg 2+ ] e significantly enhanced tone in aortas from DOCA-salt and sham rats compared with normal PSS (DOCA-salt low [Mg 2+ ] e , +51.5 +7.0 vs DOCA-salt normal PSS, +7.1 +1.4 % of initial phenylephrine [PE] contraction). LY294002 and incubation with high Mg 2+ PSS decreased tone in aortas from DOCA-salt rats (low [Mg 2+ ] e LY294002, −-87.5 +8.8; normal PSS LY294002, −81.7 +13.7; and high [Mg 2+ ] e , −31.2 +10.8 % of initial PE contraction). Low [Mg 2+ ] e leftward-shifted NE-induced aortic contractions in sham and thus matched the shift observed with DOCA (−log EC 50 mol/L: sham PSS, −7.7 +0.1; DOCA-salt PSS, −8.2 +0.1; sham low [Mg 2+ ] e , −8.2 +0.1; and DOCA-salt low [Mg 2+ ] e , −8.1 +0.1). Moreover, this shift was inhibited by LY294002. In conclusion, low [Mg 2+ ] e might activate PI3K, leading to enhanced tone and agonist-induced contraction observed in aortas from DOCA-salt hypertensive rats.

Immunolocalization of a microsomal prostaglandin E synthase in rabbit kidney

PGE2, the major cyclooxygenase (COX) metabolite of arachidonic acid, is an important paracrine regulator of numerous tubular and vascular functions in the kidney. To date, COX activity has been considered the key step in prostaglandin synthesis and is well characterized. However, much less is known about the recently cloned microsomal PGE2 synthase (mPGES), the terminal enzyme of PGE2 synthesis, which converts COX-derived PGH2 to the biologically important PGE2. Present studies provide the detailed localization of mPGES protein in the rabbit kidney using immunohistochemistry. In the cortex, strong mPGES labeling was found in the macula densa (MD) and principal cells of the connecting segment and cortical collecting tubule but not in intercalated cells. The medulla was abundant in mPGES-positive structures, with heavy labeling in the collecting duct system. In descending thin limbs and renal medullary interstitial cells, mPGES expression was less intense, and it was below the limits of detection in the vasa recta. Expression of MD mPGES, similarly to COX-2, was greatly increased in response to low-salt diet and angiotensin I-converting enzyme inhibition by captopril. These findings suggest autocrine regulation of renal salt and water transport by PGE2 in descending thin limb and collecting tubule and a paracrine effect of PGE2 on the glomerular and medullary vasculature. Similar to other organs, mPGES in the kidney is an inducible enzyme and may be similarly regulated and acts in concert with COX-2.

Cardiopulmonary bypass reduces peripheral microvascular contractile function by inhibition of mitogen-activated protein kinase activity

BACKGROUND

Mitogen-activated protein kinases (MAPK) have been implicated in pathophysiologic responses to cardiopulmonary bypass (CPB). MAPK are deactivated by phosphatases, such as MAPK phosphatase-1 (MKP-1). We hypothesized that MAPK mediate peripheral microvascular contractile dysfunction caused by CPB in humans.

METHODS

Skeletal muscle was harvested before and after CPB. Protein levels of MKP-1 and activated extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 were measured. MKP-1 gene expression was measured. Peripheral microvessel responses to vasopressors were studied by videomicroscopy. Contractile function also was measured after MAPK inhibition with PD98059 (ERK1/2) and SB203580 (p38). ERK1/2, p38, and MKP-1 were localized by immunohistochemistry and in situ hybridization.

RESULTS

ERK1/2 and p38 activity was decreased in peripheral tissue after CPB. MKP-1 was increased after CPB. Contractile responses of peripheral arterioles to phenylephrine and vasopressin were decreased after CPB. Microvessel reactivity also was reduced after treatment with PD98059 and SB203580. ERK1/2, p38, and MKP-1 localized to peripheral arterioles in tissue sections.

CONCLUSIONS

CPB reduces ERK1/2 and p38 activity in peripheral tissue, potentially by MKP-1. Contractile responses of peripheral arterioles to phenylephrine and vasopressin are dependent on ERK1/2 and p38 and are decreased after CPB. These results suggest that alterations in MAPK pathways in part regulate peripheral microvascular dysfunction after CPB in humans.

Inhibitors of Na+/Mg2+ exchange activity attenuate the development of hypertension in angiotensin II-induced hypertensive rats

OBJECTIVES

To investigate whether imipramine and quinidine, inhibitors of the Na /Mg exchanger, influence development of hypertension in rats infused with angiotensin (Ang) II.

METHODS

Sprague-Dawley rats were divided into six groups: (1) control (vehicle); (2) Ang II (150 ng/kg per min subcutaneously); (3) imipramine alone (5 mg/kg per day in drinking water); (4) quinidine alone (5 mg/kg per day in drinking water); (5) Ang II plus imipramine; (6) Ang II plus quinidine. Rats were studied for 3 weeks. To verify that Ang II directly influences Na -dependent Mg exchange, in-vitro studies were performed in vascular smooth muscle cells (VSMCs) derived from mesenteric arteries.

RESULTS

Ang II increased systolic blood pressure (SBP) in all groups. The magnitude of the increase was lower ( 0.01) in Ang II groups treated with imipramine (151 +/- 7.4 mmHg) or quinidine (163 +/- 4 mmHg) than in the Ang II only group (205 +/- 4 mmHg). Neither imipramine nor quinidine influenced SBP in vehicle-treated rats. Plasma concentrations of Mg and K were decreased in Ang II rats compared with controls (P < 0.05). Platelet intracellular free Mg concentration was reduced and platelet intracellular free Na concentration was increased in the Ang II group compared with control and treated groups (P < 0.01). These effects were normalized by imipramine and quinidine. Ang II stimulated Na -dependent Mg transport in VSMCs. These actions were abrogated by imipramine and quinidine and in Na -free conditions.

CONCLUSIONS

Our data demonstrate that inhibitors of Na -dependent Mg transport attenuate development of hypertension in rats infused with Ang II. These findings suggest a possible role for Na /Mg exchange activity in the pathogenesis of Ang II-dependent hypertension.

Effects of magnesium on the production of extracellular matrix metalloproteinases in cultured rat vascular smooth muscle cells

The precise correlation between magnesium and cardiovascular disease remains to be established. Matrix metalloproteinases (MMPs) are expressed in coronary arterial atherosclerotic lesions. MMP production in vascular smooth muscle cells (VSMCs) is stimulated by growth factors such as platelet-derived growth factor (PDGF). To assess the association between magnesium and MMPs, we examined the effects of different extracellular magnesium concentrations (0-3.0 mmol/l) on MMPs production in cultured rat VSMCs under basal and PDGF-stimulated conditions using gelatin zymography and western blotting. As magnesium is called a natural calcium antagonist, we further compared the effects of magnesium with some calcium antagonists. Magnesium reduced MMP-2 production dose-dependently at basal and PDGF-stimulated conditions in VSMCs. However, neither verapamil nor nifedipine influenced MMP-2 production under any conditions examined. The effect of magnesium on the production of MMP-2 was inhibited by two tyrosine kinase inhibitors-genistein and herbimycin A. The results of this study indicate that extracellularly added magnesium decreased MMPs secretion, which appears to be associated with protein tyrosine kinase.

Magnesium infusion improves endothelium-dependent vasodilation in the human forearm

The effect of intra-arterial magnesium infusion on endothelium-dependent vasodilation (EDV) in the forearm was studied in nine young healthy students (four men and five women). The EDV was assessed as forearm blood flow (FBF), measured by venous occlusion plethysmography, during infusion of methacholine (MCh). Endothelium-independent vasodilation (EIDV) was defined as FBF during infusion of sodium nitroprusside (SNP). During magnesium infusion in the brachial artery, 0.066 mmol/min, the concentration of ionized magnesium in venous plasma in the infused arm increased by 114%, from 0.59 (SD 0.04) to 1.26 (0.34) mmol/L (P = .0002). The FBF at baseline (ie, before administration of MCh or SNP) increased from 3.5 (1.1) to 7.3 (3.4) mL/min/100 mL tissue during magnesium infusion (P = .002). During low-dose MCh administration (2 microg/min), FBF increased by 24%, from 15.4 (5.5) to 19.1 (6.8) mL/min/100 mL tissue (P = .04), and during high-dose MCh administration (4 microg/min) FBF increased by 18%, from 20.3 (6.4) to 24.0 (7.2) mL/min/100 mL tissue (P = .04). The EIDV did not change significantly. Systemic blood pressure was not significantly altered by magnesium infusion. No change in FBF either at rest or during infusion of MCh or SNP was observed during the time-control protocol. In conclusion, this in vivo study showed that intraarterial magnesium infusion increased EDV in the infused human forearm, which is in accordance with findings in previous in vitro and animal experiments.