title: Use of cytosolic metabolite patterns to estimate free magnesium in normoxic myocardium.
abstract: Cytosolic free magnesium (Mgf) is considered relatively constant. To test this concept, Mgf was estimated during hyperkalemic ventricular akinesis, normal and maximum adrenergic stimulation, and sulfate loading of the normoxic perfused guinea-pig heart. The Mgf estimates utilized a new sliding scale derived from the Mg(2+)-dependence of glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase (GAPDH/PGK). The pseudo constant K'GAPDH.K'PGK was measured as ([creatine phosphate][3-phosphoglycerate][lactate]KLDH)/([creatine][Pi] [glyceraldehyde 3-phosphate][pyruvate]KCK), which varied with magnesium due to KCK (CK, LDH = creatine kinase, lactate dehydrogenase). However, the correct magnesium dependencies of the true constants KGAPDH.KPGK and KCK were taken from the literature. The [Mg2+] at which pseudo K'GAPDH.K'PGK equalled true KGAPDH.KPGK was the best estimate of Mgf.Mgf fell to approximately 0.13 mM in hyperkalemic arrest from a control of approximately 0.6 mM, rising to approximately 0.85 mM only during maximum adrenergic stress. Mgf increased further to approximately 1.3 mM during sulfate loading which induced ATP catabolism. Mgf and ATP were reciprocally related. Thus; (1) myocardial free [Mg2+] judged from GADPH/PGK mass-action relations changed appreciably only under extreme physiological states; (2) ATP was a major chelator of Mg2+ in perfused myocardium, i.e., acute ATP pool size reduction may be associated with increments in Mgf.
authors.affiliation: Department of Physiology, Texas College of Osteopathic Medicine, Fort Worth 76107-2699.
authors.forename: R T
authors.initials: RT
authors.lastname: Mallet
authors.forename: Y H
authors.initials: YH
authors.lastname: Kang
authors.forename: N
authors.initials: N
authors.lastname: Mukohara
authors.forename: R
authors.initials: R
authors.lastname: Bünger
mesh_terms: D000154:Aconitate Hydratase / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D003600:Cytosol / Q000378:metabolism; D005260:Female; D006019:Glycolysis; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism*; D008297:Male; D009200:Myocardial Contraction; D009206:Myocardium / Q000378:metabolism*; D013431:Sulfates / Q000378:metabolism
chemical_list: D013431:Sulfates; D000255:Adenosine Triphosphate; D000154:Aconitate Hydratase; D008274:Magnesium