Isoproterenol-induced Mg2+ uptake in liver

Transient receptor potential melastatin 6 and 7 channels, magnesium transport, and vascular biology: implications in hypertension

Magnesium, an essential intracellular cation, is critically involved in many biochemical reactions involved in the regulation of vascular tone and integrity. Decreased magnesium concentration has been implicated in altered vascular reactivity, endothelial dysfunction, vascular inflammation, and structural remodeling, processes important in vascular changes and target organ damage associated with hypertension. Until recently, very little was known about mechanisms regulating cellular magnesium homeostasis, and processes controlling transmembrane magnesium transport had been demonstrated only at the functional level. Two cation channels of the transient receptor potential melastatin (TRPM) cation channel family have now been identified as magnesium transporters, TRPM6 and TRPM7. These unique proteins, termed chanzymes because they possess a channel and a kinase domain, are differentially expressed, with TRPM6 being found primarily in epithelial cells and TRPM7 occurring ubiquitously. Vascular TRPM7 is modulated by vasoactive agents, pressure, stretch, and osmotic changes and may be a novel mechanotransducer. In addition to its magnesium transporter function, TRPM7 has been implicated as a signaling kinase involved in vascular smooth muscle cell growth, apoptosis, adhesion, contraction, cytoskeletal organization, and migration, important processes involved in vascular remodeling associated with hypertension and other vascular diseases. Emerging evidence suggests that vascular TRPM7 function may be altered in hypertension. This review discusses the importance of magnesium in vascular biology and implications in hypertension and highlights the transport systems, particularly TRPM6 and TRPM7, which may play a role in the control of vascular magnesium homeostasis. Since the recent identification and characterization of Mg2+-selective transporters, there has been enormous interest in the field. However, there is still a paucity of information, and much research is needed to clarify the exact mechanisms of magnesium regulation in the cardiovascular system and the implications of aberrant transmembrane magnesium transport in the pathogenesis of hypertension and other vascular diseases.

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.

Effect of Repeated Doses of Ethanol on Hepatic Mg2+Homeostasis and Mobilization

The acute administration of a first dose of ethanol (EtOH) to rat liver cells reduces the amount of Mg(2+) extruded by a second dose of EtOH or the subsequent addition of adrenergic agonists. In contrast, the Mg(2+) extrusion normally elicited by the alpha(1)-adrenergic or beta-adrenergic agonist does not impair the Mg(2+) mobilization induced by the subsequent addition of EtOH. Inhibition of EtOH metabolism by 4-methylpyrazole abolishes almost completely the Mg(2+) extrusion induced by the first dose of EtOH, and partially enlarges that elicited by the second dose of alcohol or the subsequent adrenergic stimulation. Ethanol-treated liver cells stimulated by the adrenergic agonist show a reduced level of membrane-bound Galphas as well as a reduced cellular cAMP content. Analysis of cellular Mg(2+) distribution indicates that EtOH administration decreases the Mg(2+) content of the cytoplasm, mitochondria, and endoplasmic reticulum to a comparable extent. These data indicate that acute EtOH administration directly impairs cellular Mg(2+) homeostasis and also prevents a further Mg(2+) mobilization by additional doses of alcohol or alpha(1)-adrenoceptor and beta-adrenoceptor agonist by decreasing cytosolic and intraorganelle Mg(2+) content and by affecting G-protein membrane distribution/signaling.

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.

Physiological and pathophysiological role of magnesium in the cardiovascular system: implications in hypertension

Attention is growing for a potential role of magnesium in the pathoetiology of cardiovascular disease. Magnesium modulates mechanical, electrical and structural functions of cardiac and vascular cells, and small changes in extracellular magnesium levels and/or intracellular free magnesium concentration may have significant effects on cardiac excitability and on vascular tone, contractility and reactivity. Thus, magnesium may be important in the physiological regulation of blood pressure whereas alterations in cellular magnesium metabolism could contribute to the pathogenesis of blood pressure elevation. Although most epidemiological and experimental studies support a pathological role for magnesium in the etiology and development of hypertension, data from clinical studies have been less convincing. Furthermore, the therapeutic value of magnesium in the management of essential hypertension is unclear. The present review discusses the molecular, biochemical, physiological and pharmacological roles of magnesium in the regulation of vascular function and blood pressure and introduces novel concepts relating to magnesium as a second messenger in intracellular signaling in cardiovascular cells. In addition, alterations in magnesium regulation in experimental and clinical hypertension and the potential antihypertensive therapeutic effects of magnesium are addressed.

Angiotensin II and vasopressin modulate intracellular free magnesium in vascular smooth muscle cells through Na+-dependent protein kinase C pathways

Vasoactive peptides mobilize cytosolic free Mg2+ in vascular smooth muscle cells. It is unknown whether angiotensin II and arginine vasopressin, potent vasoconstrictor agents, influence intracellular Mg2+. The effects of angiotensin II and vasopressin on intracellular free Mg2+ concentrations ([Mg2+]i) were therefore investigated in primary cultured unpassaged vascular smooth muscle cells (VSMC) from mesenteric arteries of Wistar Kyoto rats, and in an established cell line of rat thoracic aorta cells (A10 cells). Underlying mechanisms of agonist-stimulated [Mg2+]i changes were assessed in A10 cells by pharmacologically manipulating phospholipase C, protein kinase C, and the Na+/H+ exchanger. In addition, the dependence of [Mg2+]i on intracellular Ca2+ was determined. [Mg2+]i was measured in single cells by fluorescent digital imaging using mag-fura-2/AM. Basal [Mg2+]i levels in Wistar Kyoto rat and A10 cells were 0.62 +/- 0.02 mmol/liter and 0.58 +/- 0.01 mmol/liter, respectively. Angiotensin II and vasopressin induced a dose-dependent biphasic [Mg2+]i response where [Mg2+]i increased rapidly and transiently to a peak level and then declined to subbasal levels, which were sustained. Preexposure of cells to neomycin, a nonspecific phospholipase C inhibitor, U-73122, a selective phospholipase C inhibitor, calphostin C, a selective protein kinase C inhibitor, and 5-(N, N-hexamethylene)amiloride, a selective Na+/H+ exchange blocker, attenuated angiotensin II- and vasopressin-induced [Mg2+]i responses in a concentration-dependent manner. Removal of extracellular Na+ completely inhibited agonist-elicited [Mg2+]i transients. To determine whether intracellular free Ca2+ concentration ([Ca2+]i) influences agonist-induced [Mg2+]i changes, thapsigargin, a selective sarcoplasmic reticular Ca2+-ATPase inhibitor, was used to deplete intracellular Ca2+ stores. In thapsigargin-pretreated cells, angiotensin II-elicited [Ca2+]i responses were significantly attenuated, whereas agonist-induced [Mg2+]i responses were unchanged. These data demonstrate that in primary cultured VSMC and in an established VSMC line, angiotensin II and vasopressin modulate [Mg2+]i through receptor-mediated pathways, which are [Ca2+]i-independent but which involve phospholipase C, protein kinase C, and the Na+/H+ exchanger. These pathways are linked to a Na+-dependent Mg2+ transporter, which facilitates transmembrane Mg2+ transport.

Potentiation of cAMP responses by metabotropic glutamate receptors depresses excitatory synaptic transmission by a kinase-independent mechanism

Coactivation of metabotropic glutamate receptors (mGluRs) and beta-adrenergic receptors causes a synergistic increase in cAMP formation in the rat hippocampus. Increases in cAMP are known to have many actions in the hippocampus via activation of cAMP-dependent protein kinase. We now report that coactivation of mGluRs and beta-adrenergic receptors induces an acute depression of EPSCs at the Schaffer collateral-CA1 synapse. Interestingly, this depression of EPSCs is dependent upon increases in cAMP levels but independent of protein kinase activity. A series of studies suggests that cAMP-mediated depression of EPSCs is dependent on metabolism of cAMP and release of adenosine or 5'-AMP into the extracellular space with resultant activation of presynaptic adenosine receptors. These studies suggest that cAMP can have local hormone-like effects in the hippocampal formation which are independent of cAMP-dependent protein kinase.