Regulation of intracellular free Mg²⁺ and contraction in single adult mammalian cardiac myocytes

Studies in isolated cardiac myocytes have increased our understanding of intracellular Ca²⁺ regulation. Because less is known about Mg²⁺ regulation, adult rat ventricular myocytes were loaded with the Mg²⁺- sensitive fluorescent probe mag-indo 1, and changes in intracellular Mg²⁺ concentration ([Mg²⁺](i)) and cell length were examined under a variety of conditions. The fluorescent signal was calibrated intracellularly and found to differ slightly from that for the probe in solution. Roughly 40% of the signal was intramitochondrial; the remainder was localized 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 electrically stimulated contraction, and only a minor increase was seen during rapid electrical stimulation, which was expected to raise intracellular Ca²⁺ concentration ([Ca²⁺](i)) to ~1 μM. An acid shift in intracellular pH of ~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⁺, despite marked changes in contraction. [Mg²⁺](i) more than doubled when cells were depleted of ATP by exposure to hypoxia or metabolic inhibitors. The increase in [Mg²⁺](i) was abrupt and occurred at the time of the 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 underwent hypercontracture. Studies in cell suspensions confirmed that rapid 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 rapidly altered by changes in energy state in a manner that was related to specific changes in cell morphology and contractile function.