Magnesium-inhibited, TRPM6/7-like channel in cardiac myocytes: permeation of divalent cations and pH-mediated regulation

Magnesium transport in hypertension

Epidemiological, clinical and experimental evidence indicates an inverse association between Mg(2+) levels (serum and tissue) and blood pressure. Magnesium may influence blood pressure by modulating vascular tone and structure through its effects on numerous biochemical reactions that control vascular contraction/dilation, growth/apoptosis, differentiation and inflammation. Magnesium acts as a calcium channel antagonist, it stimulates production of vasodilator prostacyclins and nitric oxide and it alters vascular responses to vasoactive agonists. Mammalian cells regulate Mg(2+) concentration through specialized influx and efflux transport systems that have only recently been characterized. Magnesium efflux occurs via Na(2+)-dependent and Na(2+)-independent pathways. Mg(2+) influx is controlled by recently cloned transporters including Mrs2p, SLC41A1, SLC41A1, ACDP2, MagT1, TRPM6 and TRPM7. Alterations in some of these systems may contribute to hypomagnesemia and intracellular Mg(2+) deficiency in hypertension. In particular increased Mg(2+) efflux through altered regulation of the vascular Na(+)/Mg(2+) exchanger and decreased Mg(2+) influx due to defective vascular and renal TRPM6/7 expression/activity may be important. This review discusses the role of Mg(2+) in vascular biology and implications in hypertension and focuses on the putative transport systems that control vascular magnesium homeostasis. Much research is still needed to clarify the exact mechanisms of Mg(2+) regulation in the cardiovascular system and the implications of aberrant transcellular Mg(2+) transport in the pathogenesis of cardiovascular disease.

Involvement of transient receptor potential proteins in cardiac hypertrophy

Cardiac hypertrophy is an adaptive process that occurs in response to increased physical stress on the heart. Hypertrophy, which may be induced by hypertension among other factors, is characterized by an increase in left ventricular mass and an associated increase in force production capacity. However, as sustained cardiac hypertrophy may lead to heart failure and sudden death, an understanding of the molecular processes involved in both the onset and consequences of hypertrophy is of significant importance. Calcium is a key player in the process underlying the development of cardiac hypertrophy. Recently, several Transient Receptor Potential proteins (TRPs), including calcium-permeable and calcium-regulated ion channels, have been shown to be related to various aspects of cardiac hypertrophy. TRPs are implicated in the development of cardiac hypertrophy (TRPC1, TRPC3, TRPC6), the electrophysiological perturbations associated with hypertrophy (TRPM4) and the progression to heart failure (TRPC7). This review describes the major characteristics of cardiac hypertrophy and focuses on the roles of TRPs in the physiological processes underlying hypertrophy.

NGF-induced reduction of an outward-rectifying TRPM7-like current in rat CA1 hippocampal neurons

Transient receptor potential melastatin 7 (TRPM7) channels are widely expressed in the nervous system; however, their function and regulation are largely unknown. This study aimed to explore whether the current mediated by TRPM7-like channels in rat CA1 hippocampal neurons could be modulated by nerve growth factor (NGF). Using whole-cell patch clamp techniques, we identified an outward-rectifying TRPM7-like current in hippocampal neurons freshly isolated from postnatal (10-day-old) rats. The outward component of this current was reversibly reduced by NGF in dose- and time-dependent manners, and this effect was substantially blocked by K252a, an inhibitor of TrkA. In addition, NGF-induced reduction of the TRPM7-like current was abolished by U73122, a phopholipase C inhibitor. In light of the abundance of NGF in hippocampus that express both TrkA and TRPM7, these results suggest that the function of TRPM7-like channels in hippocampal neurons may be regulated by NGF.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

TRPM6: A Janus-like protein

TRPM6 and TRPM7 proteins share similar molecular structures and biophysical properties. Both proteins are Mg(2+)- and Ca(2+)-permeable cation channels with the typical topology of six transmembrane domains. In addition, TRPM6 and TRPM7 function as serine/threonine kinases with kinase domains at their C-terminal tails. At present, the role of the association of kinase and channel domains in TRPM6 and TRPM7 remains elusive. TRPM6 is mainly expressed in kidney and intestine, where it might be responsible for epithelial Mg2+ re/absorption. This hypothesis is strengthened by the identification of TRPM6 mutants in patients with a rare but severe hereditary disease called hypomagnesaemia with secondary hypocalcaemia. The aim of this review is to provide a brief but concise overview of the information currently available about TRPM6.

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.

Transient receptor potential channels in cardiovascular function and disease

Sustained elevation in the intracellular Ca2+ concentration via Ca2+ influx, which is activated by a variety of mechanisms, plays a central regulatory role for cardiovascular functions. Recent molecular biological research has disclosed an unexpectedly diverse array of Ca(2+-entry channel molecules involved in this Ca2+ influx. These include more than ten transient receptor potential (TRP) superfamily members such as TRPC1, TRPC3-6, TRPV1, TRPV2, TRPV4, TRPM4, TRPM7, and polycystin (TRPP2). Most of them appear to be multimodally activated or modulated and show relevant features to both acute hemodynamic control and long-term remodeling of the cardiovascular system, and many of them have been found to respond not only to receptor stimulation but also to various forms of stimuli. There is good evidence to implicate TRPC1 in neointimal hyperplasia after vascular injury via store-depletion-operated Ca2+ entry. TRPC6 likely contributes to receptor-operated and mechanosensitive Ca2+ mobilizations, being involved in vasoconstrictor and myogenic responses and pulmonary arterial proliferation and its associated disease (idiopathic pulmonary arterial hypertension). Considerable evidence has also been accumulated for unique involvement of TRPV1 in blood flow/pressure regulation via sensory vasoactive neuropeptide release. New lines of evidence suggest that TRPV2 may act as a Ca2+-overloading pathway associated with dystrophic cardiomyopathy, TRPV4 as a mediator of endothelium-dependent hyperpolarization, TRPM7 as a proproliferative vascular Mg2+ entry channel, and TRPP2 as a Ca2+-entry channel requisite for vascular integrity. This review attempts to provide an overview of the current knowledge on TRP proteins and discuss their possible roles in cardiovascular functions and diseases.

Tissue distribution profiles of the human TRPM cation channel family

Eight members of the TRP-melastatin (TRPM) subfamily have been identified, whose physiological functions and distribution are poorly characterized. Although tissue expression and distribution patterns have been reported for individual TRPM channels, comparisons between individual studies are not possible because of variations in analysis techniques and tissue selection. We report here a comparative analysis of the expression patterns of all of the human TRPM channels in selected peripheral tissues and the central nervous system (CNS) using two distinct but complimentary approaches: TaqMan and SYBR Green real-time quantitative reverse transcription polymerase chain reaction (RT-PCR). These techniques generated comparative distribution profiles and demonstrated tissue-specific co-expression of TRPM mRNA species, indicating significant potential for the formation of heteromeric channels. TRPM channels 2, 4, 5, 6, and 7 in contrast to 1, 3, and 8 are widely distributed in the CNS and periphery. The tissues demonstrating highest expression for individual family members were brain (TRPM1), brain and bone marrow (TRPM2), brain and pituitary (TRPM3), intestine and prostate (TRPM4), intestine, pancreas, and prostate (TRPM5), intestine and brain (TRPM6), heart, pituitary, bone, and adipose tissue (TRPM7), and prostate and liver (TRPM8). The data reported here will guide the elucidation of TRPM channel physiological functions.

An introduction to TRP channels

The aim of this review is to provide a basic framework for understanding the function of mammalian transient receptor potential (TRP) channels, particularly as they have been elucidated in heterologous expression systems. Mammalian TRP channel proteins form six-transmembrane (6-TM) cation-permeable channels that may be grouped into six subfamilies on the basis of amino acid sequence homology (TRPC, TRPV, TRPM, TRPA, TRPP, and TRPML). Selected functional properties of TRP channels from each subfamily are summarized in this review. Although a single defining characteristic of TRP channel function has not yet emerged, TRP channels may be generally described as calcium-permeable cation channels with polymodal activation properties. By integrating multiple concomitant stimuli and coupling their activity to downstream cellular signal amplification via calcium permeation and membrane depolarization, TRP channels appear well adapted to function in cellular sensation. Our review of recent literature implicating TRP channels in neuronal growth cone steering suggests that TRPs may function more widely in cellular guidance and chemotaxis. The TRP channel gene family and its nomenclature, the encoded proteins and alternatively spliced variants, and the rapidly expanding pharmacology of TRP channels are summarized in online supplemental material.

ATP and PIP2 dependence of the magnesium-inhibited, TRPM7-like cation channel in cardiac myocytes

The Mg(2+)-inhibited cation (MIC) current (I(MIC)) in cardiac myocytes biophysically resembles currents of heterologously expressed transient receptor potential (TRP) channels, particularly TRPM6 and TRPM7, known to be important in Mg(2+) homeostasis. To understand the regulation of MIC channels in cardiac cells, we used the whole cell voltage-clamp technique to investigate the role of intracellular ATP in pig, rat, and guinea pig isolated ventricular myocytes. I(MIC), studied in the presence or absence of extracellular divalent cations, was sustained for >or=50 min after patch rupture in ATP-dialyzed cells, whereas in ATP-depleted cells I(MIC) exhibited complete rundown. Equimolar substitution of internal ATP by its nonhydrolyzable analog adenosine 5'-(beta,gamma-imido)triphosphate failed to prevent rundown. In ATP-depleted cells, inhibition of lipid phosphatases by fluoride + vanadate + pyrophosphate prevented I(MIC) rundown. In contrast, under similar conditions neither the inhibition of protein phosphatases 1, 2A, 2B or of protein tyrosine phosphatase nor the activation of protein kinase A (forskolin, 20 microM) or protein kinase C (phorbol myristate acetate, 100 nM) could prevent rundown. In ATP-loaded cells, depletion of phosphatidylinositol 4,5-bisphosphate (PIP(2)) by prevention of its resynthesis (10 microM wortmannin or 15 microM phenylarsine oxide) induced rundown of I(MIC). Finally, loading ATP-depleted cells with exogenous PIP(2) (10 microM) prevented rundown. These results suggest that PIP(2), likely generated by ATP-utilizing lipid kinases, is necessary for maintaining cardiac MIC channel activity.

TRPM7 channel is regulated by magnesium nucleotides via its kinase domain

TRPM7 is a Ca(2+)- and Mg(2+)-permeable cation channel that also contains a protein kinase domain. While there is general consensus that the channel is inhibited by free intracellular Mg(2+), the functional roles of intracellular levels of Mg.ATP and the kinase domain in regulating TRPM7 channel activity have been discussed controversially. To obtain insight into these issues, we have determined the effect of purine and pyrimidine magnesium nucleotides on TRPM7 currents and investigated the possible involvement of the channel's kinase domain in mediating them. We report here that physiological Mg.ATP concentrations can inhibit TRPM7 channels and strongly enhance the channel blocking efficacy of free Mg(2+). Mg.ADP, but not AMP, had similar, albeit smaller effects, indicating a double protection against possible Mg(2+) and Ca(2+) overflow during variations of cell energy levels. Furthermore, nearly all Mg-nucleotides were able to inhibit TRPM7 activity to varying degrees with the following rank in potency: ATP > TTP > CTP > or = GTP > or = UTP > ITP approximately free Mg(2+) alone. These nucleotides also enhanced TRPM7 inhibition by free Mg(2+), suggesting the presence of two interacting binding sites that jointly regulate TRPM7 channel activity. Finally, the nucleotide-mediated inhibition was lost in phosphotransferase-deficient single-point mutants of TRPM7, while the Mg(2+)-dependent regulation was retained with reduced efficacy. Interestingly, truncated mutant channels with a complete deletion of the kinase domain regained Mg.NTP sensitivity; however, this inhibition did not discriminate between nucleotide species, suggesting that the COOH-terminal truncation exposes the previously inaccessible Mg(2+) binding site to Mg-nucleotide binding without imparting nucleotide specificity. We conclude that the nucleotide-dependent regulation of TRPM7 is mediated by the nucleotide binding site on the channel's endogenous kinase domain and interacts synergistically with a Mg(2+) binding site extrinsic to that domain.

Charge screening by internal pH and polyvalent cations as a mechanism for activation, inhibition, and rundown of TRPM7/MIC channels

The Mg²⁺-inhibited cation (MIC) current, believed to represent activity of TRPM7 channels, is found in lymphocytes and mast cells, cardiac and smooth muscle, and several other eukaryotic cell types. MIC current is activated during whole-cell dialysis with divalent-free internal solutions. Millimolar concentrations of intracellular Mg²⁺ (or other divalent metal cations) inhibit the channels in a voltage-independent manner. The nature of divalent inhibition and the mechanism of channel activation in an intact cell remain unknown. We show that the polyamines (spermine, spermidine, and putrescine) inhibit the MIC current, also in a voltage-independent manner, with a potency that parallels the number of charges. Neomycin and poly-lysine also potently inhibited MIC current in the absence of Mg²⁺. These same positively charged ions inhibited IRK1 current in parallel with MIC current, suggesting that they probably act by screening the head group phosphates on PIP₂ and other membrane phospholipids. In agreement with this hypothesis, internal protons also inhibited MIC current. By contrast, tetramethylammonium, tetraethylammonium, and hexamethonium produced voltage-dependent block but no inhibition. We show that inhibition by internal polyvalent cations can be relieved by alkalinizing the cytosol using externally applied ammonium or by increasing pH in inside-out patches. Furthermore, in perforated-patch and cell-attached recordings, when intracellular Mg²⁺ is not depleted, endogenous MIC or recombinant TRPM7 currents are activated by cytosolic alkalinization and inhibited by acidification; and they can be reactivated by PIP₂ following rundown in inside-out patches. We propose that MIC (TRPM7) channels are regulated by a charge screening mechanism and may function as sensors of intracellular pH.

Differential regulation of transient receptor potential melastatin 6 and 7 cation channels by ANG II in vascular smooth muscle cells from spontaneously hypertensive rats

Intracellular Mg2+ depletion has been implicated in vascular dysfunction in hypertension. We demonstrated that transient receptor potential melastatin 7 (TRPM7) cation channels mediate Mg2+ influx in VSMCs. Whether this plays a role in [Mg2+]i deficiency in hypertension is unclear. Here, we tested the hypothesis that downregulation of TRPM7 and its homologue TRPM6 is associated with reduced [Mg2+]i and that ANG II negatively regulates TRPM6/7 in vascular smooth muscle cells (VSMCs) from spontaneously hypertensive rats (SHR). Cultured VSMCs from Wistar Kyoto (WKY) and SHR were studied. mRNA and protein expression of TRPM6 and TRPM7 were assessed by RT-PCR and immunoblotting, respectively. Translocation of annexin-1, specific TRPM7 substrate, was measured as an index of TRPM7 activation. [Mg2+]i was determined using mag fura-2. VSMCs from WKY and SHR express TRPM6 and TRPM7. Basal TRPM6 expression was similar in WKY and SHR, but basal TRPM7 content was lower in VSMCs from SHR vs. WKY. This was associated with significantly reduced [Mg2+]i in SHR cells ( P < 0.01). ANG II time-dependently increased TRPM6 expression, with similar responses in WKY and SHR. ANG II significantly increased TRPM7 expression in WKY ( P < 0.05), but not in SHR. Annexin-1 translocation was reduced 1.5–2-fold in SHR vs. WKY. Our findings demonstrate that TRPM6 and TRPM7 are differentially regulated in VSMCs from SHR and WKY. Whereas TRPM6 is unaltered in SHR, expression of TRPM7 is blunted. This was associated with attenuated annexin-1 translocation and decreased VSMC [Mg2+]i in SHR. Downregulation of TRPM7, but not TRPM6, may play a role in altered Mg2+ homeostasis in VSMCs from SHR.

Would modulation of intracellular Ca2+ be antiarrhythmic?

Under several types of conditions, reversal of steps of excitation-contraction coupling (RECC) can give rise to nondriven electrical activity. In this review we explore those conditions for several cardiac cell types (SA, atrial, Purkinje, ventricular cells). We find that abnormal spontaneous Ca2+ release from intracellular Ca2+ stores, aberrant Ca2+ influx from sarcolemmal channels or abnormal Ca2+ surges in nonuniform muscle can be the initiators of the RECC. Often, with such increases in Ca2+, spontaneous Ca2+ waves occur and lead to membrane depolarizations. Because the change in membrane voltage is produced by Ca2+-dependent changes in ion channel function, we also review here what is known about the molecular interaction of Ca2+ and several Ca2+-dependent processes, including the intracellular Ca2+ release channels implicated in the genetic basis of some forms of human arrhythmias. Finally, we review what is known about the effectiveness of several agents in modifying such Ca2+-dependent arrhythmias.

Induction of contracture and extracellular Ca2+ influx in cardiac muscle by sanguinarine: a study on cardiotoxicity of sanguinarine

In this study, the toxic effect of sanguinarine (SANG) on heart was studied with isolated cardiac muscle strip isolated from Wistar rat. SANG induced positive inotropic action followed by contracture on the left ventricle and both atria strips. In addition, SANG dose-dependently inhibited spontaneous beat of the right atrium. SANG-induced contracture was completely suppressed by pretreatment with La3+ or in a Ca2+ free Tyrode solution containing 2.5 mM EGTA. Incubating isolated cardiomyocytes with SANG enhanced the 45Ca2+ influx, which could be inhibited by pretreatment with La3+. However, the SANG-induced 45Ca2+ influx could not be inhibited by pretreatment with other Ca2+ channel blockers, such as nifedipine, verapamil, diltiazem, nickel and manganese, and amiloride. Although antioxidants can inhibit the SANG-induced lipid peroxidation, they could not prevent the SANG-induced contracture. N-acetylcysteine and dithiothreitol, the sulfhydryl reducing agents, were shown to be effective in preventing the SANG-induced contracture. These data suggested that the SANG-induced contracture is caused by the influx of extracellular Ca2+ through a La3+-sensitive Ca2+ channel.

Potentiation of TRPM7 inward currents by protons

TRPM7 is unique in being both an ion channel and a protein kinase. It conducts a large outward current at +100 mV but a small inward current at voltages ranging from −100 to −40 mV under physiological ionic conditions. Here we show that the small inward current of TRPM7 was dramatically enhanced by a decrease in extracellular pH, with an ∼10-fold increase at pH 4.0 and 1–2-fold increase at pH 6.0. Several lines of evidence suggest that protons enhance TRPM7 inward currents by competing with Ca²⁺ and Mg²⁺ for binding sites, thereby releasing blockade of divalent cations on inward monovalent currents. First, extracellular protons significantly increased monovalent cation permeability. Second, higher proton concentrations were required to induce 50% of maximal increase in TRPM7 currents when the external Ca²⁺ and Mg²⁺ concentrations were increased. Third, the apparent affinity for Ca²⁺ and Mg²⁺ was significantly diminished at elevated external H⁺ concentrations. Fourth, the anomalous-mole fraction behavior of H⁺ permeation further suggests that protons compete with divalent cations for binding sites in the TRPM7 pore. Taken together, it appears that at physiological pH (7.4), Ca²⁺ and Mg²⁺ bind to TRPM7 and inhibit the monovalent cationic currents; whereas at high H⁺ concentrations, the affinity of TRPM7 for Ca²⁺ and Mg²⁺ is decreased, thereby allowing monovalent cations to pass through TRPM7. Furthermore, we showed that the endogenous TRPM7-like current, which is known as Mg²⁺-inhibitable cation current (MIC) or Mg nucleotide–regulated metal ion current (MagNuM) in rat basophilic leukemia (RBL) cells was also significantly potentiated by acidic pH, suggesting that MIC/MagNuM is encoded by TRPM7. The pH sensitivity represents a novel feature of TRPM7 and implies that TRPM7 may play a role under acidic pathological conditions.

Functional characterization of ACDP2 (ancient conserved domain protein), a divalent metal transporter

We have begun to identify and characterize genes that are differentially expressed with low magnesium. One of these sequences conformed to the ancient conserved domain protein, ACDP2. Real-time RT-PCR of mRNA isolated from distal epithelial cells cultured in low-magnesium media relative to normal media and in kidney cortex of mice maintained on low-magnesium diets compared with those animals consuming normal diets confirmed that the ACDP2 transcript is responsive to magnesium. Mouse ACDP2 was cloned from mouse distal convoluted tubule cells, expressed in Xenopus laevis oocytes, and studied with two-electrode voltage-clamp studies. When expressed in oocytes, ACDP2 mediates saturable Mg2+ uptake with a Michaelis constant of 0.56 +/- 0.05 mM. Transport of Mg2+ by ACDP2 is rheogenic, is voltage-dependent, and is not coupled to Na+ or Cl- ions. Expressed ACDP2 transports a range of divalent cations: Mg2+, Co2+, Mn2+, Sr2+, Ba2+, Cu2+, and Fe2+; accordingly, it is a divalent cation transporter with wide substrate selectivity. The cations Ca2+, Cd2+, Zn2+, and Ni2+ did not induce currents, and only Zn2+ effectively inhibited transport. The ACDP2 transcript is abundantly present in kidney, brain, and heart with lower amounts in liver, small intestine, and colon. Moreover, ACDP2 mRNA is upregulated with magnesium deficiency, particularly in the distal convoluted tubule cells, kidney, heart, and brain. These studies suggest that ACDP2 may provide a regulated transporter for Mg2+ and other divalent cations in epithelial cells.

The TRPM ion channel subfamily: molecular, biophysical and functional features

Significant progress in the molecular and functional characterization of a subfamily of genes that encode melastatin-related transient receptor potential (TRPM) cation channels has been made during the past few years. This subgroup of the TRP superfamily of ion channels contains eight mammalian members and has isoforms in most eukaryotic organisms. The individual members of the TRPM subfamily have specific expression patterns and ion selectivity, and their specific gating and regulatory mechanisms are tailored to integrate multiple signaling pathways. The diverse functional properties of these channels have a profound effect on the regulation of ion homoeostasis by mediating direct influx of Ca2+, controlling Mg2+ entry, and determining the potential of the cell membrane. TRPM channels are involved in several physiological and pathological conditions in electrically excitable and non-excitable cells, which make them exciting targets for drug discovery.

Transient Receptor Potential Melastatin 7 Ion Channels Regulate Magnesium Homeostasis in Vascular Smooth Muscle Cells

Magnesium modulates vascular smooth muscle cell (VSMC) function. However, molecular mechanisms regulating VSMC Mg 2+ remain unknown. Using biochemical, pharmacological, and genetic tools, the role of transient receptor potential membrane melastatin 7 (TRPM7) cation channel in VSMC Mg 2+ homeostasis was evaluated. Rat, mouse, and human VSMCs were studied. Reverse transcriptase polymerase chain reaction and immunoblotting demonstrated TRPM7 presence in VSMCs (membrane and cytosol). Angiotensin II (Ang II) and aldosterone increased TRPM7 expression. Gene silencing using small interfering RNA (siRNA) against TRPM7, downregulated TRPM7 (mRNA and protein). Basal [Mg 2+ ] i , measured by mag fura-2AM, was reduced in siRNA-transfected cells (0.39±0.01 mmol/L) versus controls (0.54±0.01 mmol/L; P <0.01). Extracellular Mg 2+ dose-dependently increased [Mg 2+ ] i in control cells (E max 0.70±0.02 mmol/L) and nonsilencing siRNA-transfected cells (E max 0.71±0.04 mmol/L), but not in siRNA-transfected cells (E max 0.5±0.01 mmol/L). The functional significance of TRPM7 was evaluated by assessing [Mg 2+ ] i and growth responses to Ang II in TRPM7 knockdown cells. Acute Ang II stimulation decreased [Mg 2+ ] i in control and TRPM7-deficient cells in a Na + -dependent manner. Chronic stimulation increased [Mg 2+ ] i in control, but not in siRNA-transfected VSMCs. Ang II–induced DNA and protein synthesis, measured by 3 [H]-thymidine and 3 [H]-leucine incorporation, respectively, were increased in control and nonsilencing cells, but not in TRPM7 knockdown VSMCs. Our data indicate that VSMCs possess membrane-associated, Ang II–, and aldosterone-regulated TRPM7 channels, which play a role in regulating basal [Mg 2+ ] i , transmembrane Mg 2+ transport and DNA and protein synthesis. These novel findings identify TRPM7 as a functionally important regulator of Mg 2+ homeostasis and growth in VSMCs.