Brief Reviews: Magnesium in Heart Muscle

• In physiological studies of the major inorganic constituents of heart muscle cells, magnesium (Mg) has until recently been the neglected stepchild. There are several reasons for this neglect. Chemical analysis for Mg was for many years difficult and tedious. The only available radioactive isotope of Mg, ~Mg, is expensive and has a short half-life. Moreover, Mg acts on relatively inaccessible intracellular processes some of which have been identified and measured only in tissue fractions or in purified proteins extracted from cells; the intracellular processes in which Mg is implicated are multiple and themselves poorly understood. The ionized Mg concentration ([Mg]) in the cardiac cell cannot at present be directly determined, and the rate at which Mg is transported into and out of myocardial cells is so much slower than the rates for potassium, sodium, calcium, and chloride that isotopic or electrophysiological measurements of ion transport in the usual in vitro preparations of heart muscle become insentitive or inconvenient. Nevertheless, there are compelling reasons for reexamining the role of Mg in heart muscle at this time. First, recent experiments on skeletal muscle indicate that [Mg + ] may be a critical modulator of the tension with which the contractile apparatus of striated muscle responds to the prevailing ionized calcium concentration ([Ca + ]) (1-5); at the same time, the Mg complex with adenosine triphosphate (MgATP) is the substrate for the enzymatic reactions that underlie the sliding filament mechanism for myofibrillar contraction and relaxation. Second, work with subcellular model systems indicates that Mg participates in many of the most vital oxidative, synthetic, and transport processes of the myocardial cell. Finally, advances in the microchemical and radioactive measurement of Mg as well as in the