Hormonal control of Mg2+ transport in the heart

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Abstract
MAGNESIUM is abundant in the mammalian body and the second most abundant cation in cells1,2. Because the concentration of intracellular free Mg2+ is relatively high (0.2–1 mM)1,3, Mg2+ is unlikely to act as a second messenger, like Ca2+, by rapidly changing its cytosolic concentration. But changes in Mg2+ do have profound effects on cellular metabolism, structure and bio-energetics3–7. Key enzymes or metabolic pathways1,6–8, mitochon-drial ion transport6,9–11, Ca2+ channel activities in the plasma membrane and intracellular oganelles3,12,13, ATP-requiring reactions, and structural properties of cells4,6,14 and nucleic acids1,3,7 are modified by changes in Mg2+ concentration. Yet, although some information is available from giant cells3,15 and bacteria3,16,17, little is known about the regulation of intracellular Mg2+ in mammalian cells. Here we report a new transport mechanism for Mg2+ across the sarcolemma of cardiac cells in both intact hearts and dissociated myocytes. We show that noradrenaline, through β-adrenergic stimulation and increase of cyclic AMP, stimulates a large efflux of Mg2+ from cardiac cells. This transport is of major dimensions and can move up to 20% of total cellular Mg2+ within a few minutes.