Dietary magnesium does not affect blood pressure in spontaneously hypertensive rats

Magnesium in hypertension: mechanisms and clinical implications

Hypertension is associated with increased risk of cardiovascular disease and death. Evidence suggests that Mg2+ depletion contributes to hypertension. It is estimated that 25% or more of the United States population experiences chronic, latent Mg2+ depletion. This review explores mechanisms by which Mg2+ influences blood pressure, modifying risk of hypertension and complicating its treatment. Mechanisms addressed include effects upon i) sympathetic tone, via the modulation of N-methyl-D-aspartate (NMDA) receptor and N-type Ca2+ channel activity, influencing catecholamine release from sympathetic nerve endings; ii) vascular tone, via alteration of L-type Ca2+ and endothelial nitric oxide synthase (eNOS) activity and prostacyclin release; iii) renal K+ handling, influencing systemic K+ balance and potentially indirectly influencing blood pressure; iv) aldosterone secretion from the adrenal cortex; and v) modulation of pro-hypertensive inflammatory processes in dendritic cells and macrophages, including activation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome and stimulation of isolevuglandin (IsoLG) production. Discovery of these mechanisms has furthered our understanding of the pathogenesis of hypertension, with implications for treatment and has highlighted the role of Mg2+ balance in hypertension and cardiovascular disease.

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.

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.

Time-course of the change in blood pressure level in magnesium-deficient Wistar rats

To investigate the relationships between maternal, umbilical cord and piglet fatty acid status, multiparous sows (six per diet) were fed on diets containing supplements (30 g/kg) of either soyabean oil or tuna oil for the last 21 d of pregnancy. The proportions of most fatty acids differed between diets: in particular, the tuna-oil-containing diet supplied more 22:6n-3 and less 18:2n-6 fatty acids than the soyabean-oil-containing diet. Maternal plasma fatty acid concentrations (mg/l) were greater than those in umbilical plasma and 20:4n-6 and 22:6n-3 fatty acids were present in higher proportions (g/100 g fatty acids) in umbilical than maternal plasma. Feeding tuna oil increased the proportionate amounts (g/100 g fatty acids) of total n-3 fatty acids (particularly 22:6n-3) in umbilical cord, plasma and piglet tissues compared with feeding soyabean oil: in contrast, the proportion of 20:4n-6 was decreased by feeding tuna oil. Changes in piglet fatty acid proportions as a result of oil feeding were not influenced by piglet weight. While proportions of the long-chain n-3 and n-6 polyunsaturated fatty acids in piglet liver, spleen and reproductive tract (ovaries plus uterus of the female, testes of the male) correlated well with those of umbilical plasma, those in brain and retina were poorly correlated. Therefore umbilical plasma cannot be used to predict the fatty acid status of piglet brain.

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.