title: Regulation of intracellular free Mg2+ and contraction in single adult mammalian cardiac myocytes.
abstract: 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 Mg(2+)-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 approximately 1 microM. An acid shift in intracellular pH of approximately 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 pHi 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.
authors.affiliation: Laboratory of Cardiovascular Science, National Institute on Aging, National Institutes of Health, Baltimore 21224.
authors.forename: H S
authors.initials: HS
authors.lastname: Silverman
authors.forename: F
authors.initials: F
authors.lastname: Di Lisa
authors.forename: R C
authors.initials: RC
authors.lastname: Hui
authors.forename: H
authors.initials: H
authors.lastname: Miyata
authors.forename: S J
authors.initials: SJ
authors.lastname: Sollott
authors.forename: R G
authors.initials: RG
authors.lastname: Hanford
authors.forename: E G
authors.initials: EG
authors.lastname: Lakatta
authors.forename: M D
authors.initials: MD
authors.lastname: Stern
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002469:Cell Separation; D004734:Energy Metabolism; D005110:Extracellular Space / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D000860:Hypoxia / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism*; D009200:Myocardial Contraction / Q000502:physiology*; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009994:Osmolar Concentration; D010100:Oxygen / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar
chemical_list: D000255:Adenosine Triphosphate; D008274:Magnesium; D010100:Oxygen; D002118:Calcium