Hormonal control of Mg²⁺ transport in the heart

Abstract

MAGNESIUM is abundant in the mammalian body and the second most abundant cation in cells^1,2. 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-energetics^3–7. Key enzymes or metabolic pathways^1,6–8, mitochon-drial ion transport^6,9–11, Ca²⁺ channel activities in the plasma membrane and intracellular oganelles^3,12,13, ATP-requiring reactions, and structural properties of cells^4,6,14 and nucleic acids^1,3,7 are modified by changes in Mg²⁺ concentration. Yet, although some information is available from giant cells^3,15 and bacteria^3,16,17, 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.