Angiotensin II type I receptor modulates intracellular free Mg2+ in renally derived cells via Na+-dependent Ca2+-independent mechanisms

Dysregulated Erythroid Mg2+ Efflux in Type 2 Diabetes

Hyperglycemia is associated with decreased Mg²⁺ content in red blood cells (RBC), but mechanisms remain unclear. We characterized the regulation of Mg²⁺ efflux by glucose in ex vivo human RBC. We observed that hemoglobin A_1C (HbA_1C) values correlated with Na⁺-dependent Mg²⁺ efflux (Na⁺/Mg²⁺ exchange) and inversely correlated with cellular Mg content. Treatment of cells with 50 mM D-glucose, but not with sorbitol, lowered total cellular Mg (2.2 ± 0.1 to 2.0 ± 0.1 mM, p < 0.01) and enhanced Na⁺/Mg²⁺ exchange activity [0.60 ± 0.09 to 1.12 ± 0.09 mmol/10¹³ cell × h (flux units, FU), p < 0.05]. In contrast, incubation with selective Src family kinase inhibitors PP2 or SU6656 reduced glucose-stimulated exchange activation (p < 0.01). Na⁺/Mg²⁺ exchange activity was also higher in RBC from individuals with type 2 diabetes (T2D, 1.19 ± 0.13 FU) than from non-diabetic individuals (0.58 ± 0.05 FU, p < 0.01). Increased Na⁺/Mg²⁺ exchange activity in RBC from T2D subjects was associated with lower intracellular Mg content. Similarly increased exchange activity was evident in RBC from the diabetic db/db mouse model as compared to its non-diabetic control (p < 0.03). Extracellular exposure of intact RBC from T2D subjects to recombinant peptidyl-N-glycosidase F (PNGase F) reduced Na⁺/Mg²⁺ exchange activity from 0.98 ± 0.14 to 0.59 ± 0.13 FU (p < 0.05) and increased baseline intracellular Mg content (1.8 ± 0.1 mM) to normal values (2.1 ± 0.1 mM, p < 0.05). These data suggest that the reduced RBC Mg content of T2D RBC reflects enhanced RBC Na⁺/Mg²⁺ exchange subject to regulation by Src family kinases and by the N-glycosylation state of one or more membrane proteins. The data extend our understanding of dysregulated RBC Mg²⁺ homeostasis in T2D.

Overexpression of Na+/Mg2+ exchanger SLC41A1 attenuates pro-survival signaling

The Na⁺/Mg²⁺ exchanger SLC41A1 (A1), a key component of intracellular Mg homeostasis (IMH), is the major cellular Mg²⁺ efflux system, and its overexpression decreases [Mg²⁺]_{intracellular}. IMH plays an important role in the regulation of many cellular processes, including cellular signaling. However, whether the overexpression of A1 and the consequent drop of [Mg²⁺]_i impact on intracellular signaling is unknown. To examine the latter, we utilized dynamic mass redistribution (DMR) assay, PathScan^® RTK signaling antibody (PRSA) array, confirmatory Western blot (WB) analyses of phosphorylation of kinases selected by PRSA, and mag-fura 2-assisted fast filter spectrometry (FFS). We demonstrate here that the overexpression of A1 quantitatively and qualitatively changes the DMR signal evoked by the application of PAR-1-selective activating peptide and/or by changing [Mg²⁺]_{extracellular} in HEK293 cells. PRSA profiling of the phosphorylation of important signaling nodes followed by confirmatory WB has revealed that, in HEK293 cells, A1 overexpression significantly attenuates the phosphorylation of Akt/PKB on Thr³⁰⁸ and/or Ser⁴⁷³ and of Erk1/2 on Thr²⁰²/Tyr²⁰⁴ in the presence of 0 or 1 mM (physiological) Mg²⁺ in the bath solution. The latter is also true for SH-SY5Y and HeLa cells. Overexpression of A1 in HEK293 cells significantly lowers [Mg²⁺]_i in the presence of [Mg²⁺]_e = 0 or 1 mM. This correlates with the observed attenuation of prosurvival Akt/PKB - Erk1/2 signaling in these cells. Thus, A1 expression status and [Mg²⁺]_e (and consequently also [Mg²⁺]_i) modulate the complex physiological fingerprint of the cell and influence the activity of kinases involved in anti-apoptotic and, hence, pro-survival events in cells.

Angiotensin II type 1 receptor antagonists in animal models of vascular, cardiac, metabolic and renal disease

We have reviewed the effects of angiotensin II type 1 receptor antagonists (ARBs) in various animal models of hypertension, atherosclerosis, cardiac function, hypertrophy and fibrosis, glucose and lipid metabolism, and renal function and morphology. Those of azilsartan and telmisartan have been included comprehensively whereas those of other ARBs have been included systematically but without intention of completeness. ARBs as a class lower blood pressure in established hypertension and prevent hypertension development in all applicable animal models except those with a markedly suppressed renin-angiotensin system; blood pressure lowering even persists for a considerable time after discontinuation of treatment. This translates into a reduced mortality, particularly in models exhibiting marked hypertension. The retrieved data on vascular, cardiac and renal function and morphology as well as on glucose and lipid metabolism are discussed to address three main questions: 1. Can ARB effects on blood vessels, heart, kidney and metabolic function be explained by blood pressure lowering alone or are they additionally directly related to blockade of the renin-angiotensin system? 2. Are they shared by other inhibitors of the renin-angiotensin system, e.g. angiotensin converting enzyme inhibitors? 3. Are some effects specific for one or more compounds within the ARB class? Taken together these data profile ARBs as a drug class with unique properties that have beneficial effects far beyond those on blood pressure reduction and, in some cases distinct from those of angiotensin converting enzyme inhibitors. The clinical relevance of angiotensin receptor-independent effects of some ARBs remains to be determined.

Additional antihypertensive effect of magnesium supplementation with an angiotensin II receptor blocker in hypomagnesemic rats

BACKGROUND/AIMS

Magnesium (Mg) is an essential element for vascular function and blood pressure regulation. Several studies have demonstrated that Mg concentration is inversely associated with blood pressure, and that Mg supplementation attenuates hypertension. The purpose of this study was to evaluate the effect of dietary Mg supplementation on the blood pressure effects of an angiotensin II receptor blocker (ARB) in hypomagnesemic rats.

METHODS

Fifty male Sprague-Dawley rats were randomly divided into Mg-deficient (n = 30), normal diet plus Mg (n = 10), and control groups (n = 10). Mg-free, high-Mg, and normal-Mg diets were respectively fed to the rats. After 14 weeks, 10 of the 30 Mg-deficient rats were treated with Mg, 10 Mg-deficient rats received an ARB, and 10 Mg-deficient rats received an ARB plus Mg for 4 weeks.

RESULTS

Systolic blood pressure was significantly higher in the Mg-deficient rats than in the control rats at week 14. Hypomagnesemic rats exhibited decreased systolic blood pressure after treatment with Mg, and systolic blood pressure showed a greater decrease after ARB treatment. Treatment with the ARB/Mg combination resulted in the greatest decrease in systolic blood pressure. Mg deficiency did not affect the serum angiotensin II level, but did increase the serum aldosterone concentration. Concomitant Mg/ARB supplementation significantly decreased the elevated serum aldosterone level in hypomagnesemic rats. Kidney tissues of the hypomagnesemic rats revealed mild to moderate inflammatory infiltrates. Mg and/or ARB treatment did not reverse the inflammatory reaction in the kidneys of hypomagnesemic rats.

CONCLUSIONS

Concurrent dietary Mg supplementation can enhance ARB-induced blood pressure reduction in rats with hypomagnesemic hypertension.

Mechanism of [Ca2+]i rise induced by angiotensin 1-7 in MDCK renal tubular cells

The effect of angiotensin 1-7 (Ang 1-7) on cytosolic Ca(2+) concentrations ([Ca(2+)](i)) in MDCK renal tubular cells was explored. The Ca(2+)-sensitive fluorescent dye fura-2 was applied to measure [Ca(2+)](i). Ang 1-7 at concentrations of 10-50 µM induced a [Ca(2+)](i) rise in a concentration-dependent manner. The response was reduced partly by removing Ca(2+). Ang 1-7 evoked store operated Ca(2+) entry that was inhibited by La(3+) and aristolochic acid. In the absence of extracellular Ca(2+), incubation with the endoplasmic reticulum Ca(2+) pump inhibitor thapsigargin prevented Ang 1-7 from releasing more Ca(2+). Inhibition of phospholipase C with U73122 abolished Ang 1-7-induced [Ca(2+)](i) rise. Ang 1-7-induced [Ca(2+)](i) rise was abolished by the angiotensin type 1 receptor antagonist losartan, but was not affected by the angiotensin type 2 receptor antagonist PD 123,319. In sum, in MDCK cells, Ang 1-7 stimulated angiotensin type 1 receptors leading to a [Ca(2+)](i) rise that was composed of phospholipase C-dependent Ca(2+) release from the endoplasmic reticulum and Ca(2+) entry via phospholipase A2-sensitive store-operated Ca(2+) channels.

[Expression and regulation of renal sodium-cotransporters and -antiporters, and related-transport proteins]

The authors' researches have been focused on pathogenic, physiological and biochemical mechanisms in hypertension and diabetes. Studies on hypertension were performed using salt-sensitive hypertensive Dahl rats as compared with the corresponding normotensive rats. Especially, implication with mobilization of electrolytes such as sodium, potassium, calcium and magnesium in hypertension gave rise to provocative to the author. Furthermore, complications of diabetes with hypertension were themes for the authors' researches. Thus, sodium-dependent glucose transport has been studied on sodium-dependent glucose transporters such as SGLT1 and SGLT2 using cell lines of porcelain renal cell, LLC-PK(1), and murine renal cell, NRK-52E. Relationship between magnesium mobilization and NO in hypertension has been explored using renal epithelial cell-lines and salt-sensitive hypertensive Dahl rats in the latter half of the author's research life.

Loading rat heart myocytes with Mg2+ using low-[Na+] solutions

The objective of our study was to investigate how Mg2+ enters mammalian cardiac cells. During this work, we found evidence for a previously undescribed route for Mg2+ entry, and now provide a preliminary account of its properties. Changes in Mg2+ influx into rat ventricular myocytes were deduced from changes in intracellular ionized Mg2+ concentration ([fMg2+]i) measured from the fluorescence of mag-fura-2 loaded into isolated cells. Superfusion of myocytes at 37 degrees C with Ca2+-free solutions with both reduced [Na+] and raised [Mg2+] caused myocytes to load with Mg2+. Uptake was seen with solutions containing 5 mm Mg2+ and 95 mm Na+, and increased linearly with increasing extracellular [Mg2+] or decreasing extracellular [Na+]. It was very sensitive to temperature (Q(10) > 9, 25--37 degrees C), was observed even in myocytes with very low Na+ contents, and stopped abruptly when external [Na+] was returned to normal. Uptake was greatly reduced by imipramine or KB-R7943 if these were added when [fMg2+]i was close to the physiological level, but was unaffected if they were applied when [fMg2+]i was above 2 mm. Uptake was also reduced by depolarizing the membrane potential by increasing extracellular [K+] or voltage clamp to 0 mV. We suggest that initial Mg2+ uptake may involve several transporters, including reversed Na+-Mg2+ antiport and, depending on the exact conditions, reversed Na+-Ca2+ antiport. The ensuing rise of [fMg2+]i, in conjunction with reduced [Na+], may then activate a new Mg2+ transporter that is highly sensitive to temperature, is insensitive to imipramine or KB-R7943, but is inactivated by depolarization.

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.

Activation of Na+-independent Mg2+ efflux by 20-hydroxyeicosatetraenoic acid in rat renal epithelial cells

Renal epithelial cells may have Mg(2+) transport pathways that regulate intracellular free Mg(2+) concentration ([Mg(2+)](i)) and reabsorption into the body. In mag-fura 2 fluorescent measurement, extracellular Mg(2+) removal induced a Na(+)-independent [Mg(2+)](i) decrease. The [Mg(2+)](i) decrease was suppressed by methyl arachidonyl fluorophosphonate, a cytosolic and Ca(2+)-independent phospholipase A(2) (iPLA(2)) inhibitor, and bromoenol lactone, an iPLA(2) inhibitor, but it was not suppressed by a secretory phospholipase A(2) inhibitor. On the contrary, the [Mg(2+)](i) decrease was enhanced by the addition of exogenous arachidonic acid (AA). Next, we examined the effect of AA metabolite inhibitors on the [Mg(2+)](i) decrease. 17-octadecynoic acid inhibited the [Mg(2+)](i) decrease, but indomethacin and nordihydroguaiaretic acid did not. In the 17-octadecynoic acid-treated cells, 20-hydroxy-(5Z,8Z,11Z,14Z)-eicosatetraenoic acid (20-HETE) recovered the [Mg(2+)](i) decrease. Nicardipine inhibited both the basal and the 20-HETE-enhanced [Mg(2+)](i) decrease. These results suggest that 20-HETE is a key mediator in the activation of Na(+)-independent Mg(2+) efflux.

Identification and characterization of a novel mammalian Mg2+ transporter with channel-like properties

BACKGROUND

Intracellular magnesium is abundant, highly regulated and plays an important role in biochemical functions. Despite the extensive evidence for unique mammalian Mg2+ transporters, few proteins have been biochemically identified to date that fulfill this role. We have shown that epithelial magnesium conservation is controlled, in part, by differential gene expression leading to regulation of Mg2+ transport. We used this knowledge to identify a novel gene that is regulated by magnesium.

RESULTS

Oligonucleotide microarray analysis was used to identify a novel human gene that encodes a protein involved with Mg2+-evoked transport. We have designated this magnesium transporter (MagT1) protein. MagT1 is a novel protein with no amino acid sequence identity to other known transporters. The corresponding cDNA comprises an open reading frame of 1005 base pairs encoding a protein of 335 amino acids. It possesses five putative transmembrane (TM) regions with a cleavage site, a N-glycosylation site, and a number of phosphorylation sites. Based on Northern analysis of mouse tissues, a 2.4 kilobase transcript is present in many tissues. When expressed in Xenopus laevis oocytes, MagT1 mediates saturable Mg2+ uptake with a Michaelis constant of 0.23 mM. Transport of Mg2+ by MagT1 is rheogenic, voltage-dependent, does not display any time-dependent inactivation. Transport is very specific to Mg2+ as other divalent cations did not evoke currents. Large external concentrations of some cations inhibited Mg2+ transport (Ni2+, Zn2+, Mn2+) in MagT1-expressing oocytes. Ca2+and Fe2+ were without effect. Real-time reverse transcription polymerase chain reaction and Western blot analysis using a specific antibody demonstrated that MagT1 mRNA and protein is increased by about 2.1-fold and 32%, respectively, in kidney epithelial cells cultured in low magnesium media relative to normal media and in kidney cortex of mice maintained on low magnesium diets compared to those animals consuming normal diets. Accordingly, it is apparent that an increase in mRNA levels is translated into higher protein expression.

CONCLUSION

These studies suggest that MagT1 may provide a selective and regulated pathway for Mg2+ transport in epithelial cells.

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.

Magnesium supplementation prevents angiotensin II-induced myocardial damage and CTGF overexpression

OBJECTIVES AND DESIGN

Magnesium deficiency promotes vasoconstriction and myocardial damage. Recent studies provide evidence that Ang II mobilizes intracellular Mg through AT1 receptor-mediated pathways. We tested the hypothesis of whether magnesium supplementation prevents Ang II-induced myocardial damage and induction of the profibrotic connective tissue growth factor (CTGF).

METHODS

Four-week-old double transgenic rats harboring human renin and angiotensinogen genes (dTGR) were given dietary magnesium supplementation (0.6%) for 3 weeks. Control dTGR and normotensive Sprague-Dawley (SD) rats received normal diet (Mg 0.2%). Histopathological, immunohistochemical and mRNA analysis were used to detect the treatment-related effects of dietary magnesium in dTGR.

RESULTS

Magnesium (Mg) supplementation decreased blood pressure, ameliorated cardiac hypertrophy, protected against the development of Ang II-induced myocardial damage and increased serum ionized Mg2+ concentration (all variables P < 0.05). There was no difference in serum ionized Mg2+ concentration between dTGR and SD rats. Myocardial connective tissue growth factor (CTGF) mRNA and protein expressions were increased by 300% in dTGR (P < 0.05), especially in areas with myocardial infarctions and vascular inflammation. Magnesium supplementation prevented Ang II-induced myocardial CTGF overexpression (P < 0.05). Magnesium supplementation also improved the therapeutic effects of the calcineurin inhibitor tacrolimus, which produced marked hypomagnesemia when given as monotherapy.

CONCLUSION

Our findings suggest a salutary effect for magnesium supplementation in the treatment of Ang II-induced myocardial complications.

Differential effect of imipramine and related compounds on Mg2+ efflux from rat erythrocytes

The effect of imipramine on Mg2+ efflux in NaCl medium (Na+/Mg2+ antiport), on Mg2+ efflux in choline.Cl medium (choline/Mg2+ antiport) and on Mg2+ efflux in sucrose medium (Cl- -coupled Mg2+ efflux) was investigated in rat erythrocytes. In non-Mg2+-loaded rat erythrocytes, imipramine stimulated Na+/Mg2+ antiport but inhibited choline/Mg2+ antiport and Cl- -coupled Mg2+ efflux. The same effect could be obtained by several other compounds structurally related to imipramine. These drugs contain a cyclic hydrophobic ring structure to which a four-membered secondary or tertiary amine side chain is attached. At a physiological pH, the amine side chain expresses a cationic choline-like structure. The inhibitory effect on choline/Mg2+ antiport is lost when the amine side chain is modified or abandoned, pointing to competition of the choline-like side chain with choline or another cation at the unspecific choline antiporter or at the Cl- -coupled Mg2+ efflux. Other related drugs either stimulated Na+/Mg2+ antiport and choline/Mg2+ antiport, or they were ineffective. For stimulation of Na+/Mg2+ antiport and choline/Mg2+ antiport, there is no specific common structural motif of the drugs tested. The effects of imipramine on Na+/Mg2+ antiport and choline/Mg2+ antiport are not mediated by PKCalpha but are caused by a direct reaction of imipramine with these transporters. By increasing the intracellular Mg2+ concentration, the stimulation of Na+/Mg2+ antiport at a physiological intracellular Mg2+ concentration changed to an inhibition of Na+/Mg2+ antiport. This effect can be explained by the hypothesis that Mg2+ loading induced an allosteric transition of the Mg2+/Mg2+ exchanger with low Na+/Mg2+ antiport capacity to the Na+/Mg2+ antiporter with high Na+/Mg2+ antiport capacity. Both forms of the Mg2+ exchanger may be differently affected by imipramine.

TRPM7: Channeling the Future of Cellular Magnesium Homeostasis?

A recent paper by Schmitz and colleagues provides persuasive evidence that the ion channel transient receptor potential melastatin 7 (TRPM7) may be the long-sought regulator of magnesium (Mg) homeostasis in mammalian cells. This finding is not unexpected, because TRPM channels contain a kinase domain that allows them to participate in signal transduction pathways and regulatory networks. However, these studies introduce an exciting new twist into our understanding of Mg homeostasis; TRPM7 facilitates Mg entry into the cell, whereas other putative Mg transporters apparently operate in the opposite direction. By combining electrophysiological, biochemical, and genetic approaches, Schmitz and colleagues characterized most of the key features that demonstrate a well-defined and biologically plausible regulator of Mg homeostasis. TRPM7 genetics are well in hand, its regulation by intracellular free Mg2+ unravels the mechanisms of regulatory feedback loops, and its kinase domain modulates its sensitivity to free Mg2+. These findings are discussed in light of the indirect and descriptive information we had about Mg regulation before this rigorous characterization of TRPM7 brought it to the center of the Mg stage. Although the molecular events downstream of TRPM7 phosphorylation and dephosphorylation await in-depth elucidation, these results open up exciting perspectives in Mg research and may provide a much-needed tool with which to reexamine the role of Mg in cell proliferation and other important pathophysiologic events. Likewise, these findings will offer guidelines for research on disease states that are characterized by Mg imbalance.

Arachidonic acid-activated Na+-dependent Mg2+ efflux in rat renal epithelial cells

Arachidonic acid (AA), a metabolite of membrane phospholipids, and its metabolites are increased in Mg2+ deficiency. We examined whether the extracellular Mg2+ concentration affects AA production and whether AA regulates a putative Na+-dependent Mg2+ efflux pathway in renal epithelial NRK-52E cells. We used the cells cultured in 5 mM Mg2+-containing medium for 2 days because they enable us to detect Na+-stimulated Mg2+ efflux that was not observed in normal culture medium. Removal of extracellular Mg2+ increased AA release both in the absence and presence of extracellular Na+. This was inhibited by methyl arachidonyl fluorophosphonate (MAFP, 10 microM), an inhibitor of cytosolic phospholipase A) (cPLA2) and Ca2+-independent phospholipase A2 (iPLA2), and bromoenol lactone (BEL, 10 microM), an inhibitor of iPLA2. However, LY-311727 (10 microM), a secretory phospholipase A2 (sPLA2) inhibitor, had no inhibitory effect. Reverse transcriptase-polymerase chain reaction (RT-PCR) showed that NRK-52E cells express cPLA2 and iPLA2 mRNAs, but not sPLA2. In the mag-fura 2 fluorescence measurements, extracellular Mg2+ removal caused slight decrease in the intracellular free Mg2+ concentration ([Mg2+]i) in the Na+-free condition. The addition of Na+ caused a rapid decrease in [Mg2+]i, indicating the presence of a Na+-dependent Mg2+ efflux pathway. The Na+-dependent [Mg2+]i decrease was suppressed by MAFP and BEL. On the other hand, AA metabolite inhibitors, nordihydroguaiaretic acid (NDGA) (50 microM), indomethacin (10 microM) and 17-octadecynoic acid (ODYA) (10 microM), enhanced the Na+-dependent [Mg2+]i decrease. Furthermore, the addition of exogenous AA (30 microM) enhanced the Na+-dependent [Mg2+]i decrease, which was significantly inhibited by imipramine (0.1 mM), a putative Na+/Mg2+-exchanger inhibitor. These results suggest that extracellular Mg2+ removal elevates AA release mediated mainly by iPLA2 and that AA upregulates the Na+-dependent Mg2+ efflux in NRK-52E cells.

Mesaconitine-induced relaxation in rat aorta: role of Na+/Ca2+ exchangers in endothelial cells

Previously, we reported that mesaconitine, an aconite alkaloid, increased intracellular Ca(2+) concentration ([Ca(2+)](i)) level in endothelium and caused relaxation in rat aorta via nitric oxide production. In the present study, we investigated the mechanisms of increase in the [Ca(2+)](i) level induced by mesaconitine in rat aorta and in human umbilical vein endothelial cells (HUVECs). Treatment with the low Na(+) buffer delayed the 30 microM mesaconitine-, but not 10 microM acetylcholine-, induced relaxation in rat aorta. Treatments with an inhibitor of Na(+)/Ca(2+) exchangers (20 microM 3',4'-dichlorobenzamil) and a reversed mode (Ca(2+) influx) inhibitor of the exchangers (30 microM 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate, KBR7943) showed similar effects. In HUVECs, 30 microM mesaconitine increased the [Ca(2+)](i) level in the presence of extracellular CaCl(2) and NaCl, and the response was inhibited by KBR7943. Mesaconitine increased intracellular Na(+) concentration level in HUVECs. The [Ca(2+)](i) response by mesaconitine was inhibited by 100 microM D-tubocurarine (an inhibitor of nicotinic acetylcholine receptors), but was not inhibited in the glucose-free buffer and by inhibitors of Na(+)/H(+) exchangers. These findings suggest that mesaconitine stimulated Ca(2+) influx via the Na(+)/Ca(2+) exchangers in endothelial cells and caused relaxation in the aorta. The possibility of D-tubocurarine-sensitive Na(+) channels as target(s) of mesaconitine is discussed.

Na+/Mg2+ exchange is functionally coupled to the insulin receptor

Regulation of cellular Mg(2+) levels by insulin has been shown in various tissues. However, the mechanisms for hormonal regulation of cellular Mg(2+) have not been well described. We studied the effect of insulin on Na(+)/Mg(2+) exchange in normal human cells, measuring Na(+)/Mg(2+) exchange activity as net total Mg(2+) efflux driven by an inward Na(+) gradient in Mg(2+)-loaded red blood cells (RBCs). Na(+)/Mg(2+) exchange was increased significantly by the addition of 2.4 nmol/L of insulin to the flux medium (from 0.60 +/- 0.06 mmol/L cell x h to 0.75 +/- 0.08 mmol/L cell x h [P = 0.0098, n = 44]). A dose-response curve for the effects of insulin on the exchanger activity gave an estimated EC(50) for insulin of 0.95 +/- 0.31 nmol/L and a V(max) of 0.86 +/- 0.12 mmol/L cell x h (n = 7). Kinetics of the Na(+)/Mg(2+) exchange were characterized by measuring its activity as a function of Mg(2+) and Na(+) concentrations. The K(0.5) for cellular Mg(2+) was not affected by incubation with insulin. However, the K(0.5) for extracellular Na(+) decreased from 69.9 +/- 6.3 to 40.3 +/- 8.4 mol/L (n = 5, P = 0.03) in the presence of insulin. We also studied the effect of wortmannin (WT), a PI 3-kinase inhibitor, on activity of the exchanger. WT significantly blocked the insulin-stimulated Na(+)/Mg(2+) activity (n = 6, P = 0.048), with an IC(50) of 0.5 nmol/L. LY294002, another PI 3-kinase inhibitor, likewise blocked the insulin-stimulated activity of the exchanger. Therefore, insulin regulates cellular Mg(2+) metabolism in part via an increase in the affinity for Na(+) of the Na(+)/Mg(2+) exchange and PI 3-kinase activation, suggesting another role for the PI 3-kinase pathway in insulin-mediated cellular events.

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.

An exchanger-like protein underlies the large Mg2+ current in Paramecium

There are very few molecules known to transport Mg(2+) in eukaryotes. The membrane of Paramecium tetraurelia passes a large Mg(2+)-selective current and exhibits a corresponding backward swimming behavior. Both are missing in a group of mutants called eccentric. By sorting an indexed WT genomic library through microinjection into the macronucleus, we have isolated a DNA fragment that complements the eccentric mutations. The Mg(2+) currents and behavior are restored fully in the transformed cells. Surprisingly, the conceptually translated protein is not homologous to any known ion channel but instead has some similarity to K(+)-dependent Na(+)Ca(2+) exchangers. Exchangers are either electrically silent or only pass very small and slow currents compared with ion-channel currents. In light of recent ion-channel crystal structures and considering the need to have narrow ion-selective filters, we speculate on how an exchanger might evolve to show channel-like activities in special circumstances. The significance of finding the molecular basis of a Mg(2+)-specific pathway is also discussed.

Differential regulation of renal Na,K-ATPase by splice variants of the gamma subunit

Sodium and potassium-exchanging adenosine triphosphatase (Na,K-ATPase) in the kidney is associated with the gamma subunit (gamma, FXYD2), a single-span membrane protein that modulates ATPase properties. Rat and human gamma occur in two splice variants, gamma(a) and gamma(b), with different N termini. Here we investigated their structural heterogeneity and functional effects on Na,K-ATPase properties. Both forms were post-translationally modified during in vitro translation with microsomes, indicating that there are four possible forms of gamma. Site-directed mutagenesis revealed Thr(2) and Ser(5) as potential sites for post-translational modification. Similar modification can occur in cells, with consequences for Na,K-ATPase properties. We showed previously that stable transfection of gamma(a) into NRK-52E cells resulted in reduction of apparent affinities for Na(+) and K(+). Individual clones differed in gamma post-translational modification, however, and the effect on Na(+) affinity was absent in clones with full modification. Here, transfection of gamma(b) also resulted in clones with or without post-translational modification. Both groups showed a reduction in Na(+) affinity, but modification was required for the effect on K(+) affinity. There were minor increases in ATP affinity. The physiological importance of the reduction in Na(+) affinity was shown by the slower growth of gamma(a), gamma(b), and gamma(b') transfectants in culture. The differential influence of the four structural variants of gamma on affinities of the Na,K-ATPase for Na(+) and K(+), together with our previous finding of different distributions of gamma(a) and gamma(b) along the rat nephron, suggests a highly specific mode of regulation of sodium pump properties in kidney.