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McGuigan JAS et al.  
Magnesium Homeostasis in Heart: A Critical Reappraisal

Journal of Clinical and Basic Cardiology 2002; 5 (1): 5-22

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Fig. 1: Ionen - Magnesium Fig. 2: Magnesiumefflux - Atrium Fig. 3A-B: Magnesiummakroelektrode - Kalibrierung Fig. 4: Herzzellen - Magnesiumefflux Fig. 5A-B: Magnesium - Konzentration Fig. 6A-B: Magnesium - Konzentration

Keywords: HerzmagnesiumMagnesiumregulationMembrantransportzytosolischer Buffercytosolic bufferingheartmagnesiummagnesium regulationmembrane transport

Ionised magnesium concentration in heart cytosol ((Mg2+)i) at ~1 mmol/l is far from equilibrium, making an efflux mechanism essential and various possibilities are first considered. Then the methods available to study such mechanisms are described. New methods include 1) the study of net Mg2+ flux from cell suspensions using novel Mg2+-macroelectrodes 2) the self referencing probe to measure net flux from single myocytes and 3) molecular methods in eukaryotes The mechanism of influx is unclear. Efflux is via a Na+/Mg2+ antiport, but the stoichiometry and characteristics await clarification. Little is known about other extrusion mechanism. The parameters of electromotive force and ion concentration, which predicate the most probable mechanisms are considered. In regulation of (Mg2+)i, limited diffusion in the submembrane "fuzzy" space, cytosolic buffering and uptake/release by mitochondria and SR play a role. Binding of Mg2+ to ATP has been determined but the other aspects of (Mg2+)i regulation are unclear. Amongst hormones, catecholamines cause a net loss, and carbachol and insulin a net gain, with little change in (Mg2+)i; but the physiological functions of these changes remain to be determined. Recovery from such changes awaits study. This is a dismal picture, but two major advances make a reinvestigation of Mg2+ homeostasis in heart opportune. The first is conceptual in that both total and ionised concentrations must be measured and a clear distinction made between cytosol, mitochondria and SR. The second is developments in methodology. Combination of these two makes advances in our understanding of Mg2+ regulation in heart likely.
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