REGULATION OF INTRACELLULAR FREE MG2+ AND CONTRACTION IN SINGLE ADULTMAMMALIAN CARDIAC MYOCYTES

Studies in isolated cardiac myocytes have increased our understanding of intracellular Ca2+ regulation. Because less is known about Mg2+ reg ulation, adult rat ventricular myocytes were loaded with the Mg2+-sens itive fluorescent probe mag-indo 1, and changes in intracellular Mg2concentration ([Mg2+](i)) and cell length were examined under a variet y of conditions. The fluorescent signal was calibrated intracellularly and found to differ slightly from that for the probe in solution. Rou ghly 40% of the signal was intramitochondrial; the remainder was local ized in the cytosol. Basal [Mg2+](i) averaged 1.02 +/- 0.03 mM (n = 53 cells). No change in [Mg2+](i) was observed during a single electrica lly stimulated contraction, and only a minor increase was seen during rapid electrical stimulation, which was expected to raise intracellula r Ca2+ concentration ([Ca2+](i)) to similar to 1 mu M. An acid shift i n intracellular pH of similar to 1 pH unit was accompanied by a small change in [Mg2+](i) (0.34 +/- 0.03 mM, n = 6, P < 0.05). No change in [Mg2+](i) was observed when cells were superfused with 15 mM Mg2+, des pite marked changes in contraction. [Mg2+](i) more than doubled when c ells were depleted of ATP by exposure to hypoxia or metabolic inhibito rs. The increase in [Mg2+](i) was abrupt and occurred at the time of t he failure of contraction, plateauing as rigor contracture developed. Reoxygenation was accompanied by a gradual fall in [Mg2+](i) in cells that recovered mechanical function, and in a subset of cells that unde rwent hypercontracture. Studies in cell suspensions confirmed that rap id cellular energy depletion was accompanied by increases in [Mg2+](i) and parallel decreases in ATP. Thus [Mg2+](i) was largely insensitive to changes in [Ca2+](i) or pH(i) and extracellular [Mg2+] but was rap idly altered by changes in energy state in a manner that was related t o specific changes in cell morphology and contractile function.