Use of cytosolic metabolite patterns to estimate free magnesium in normoxic myocardium

Abstract

Cytosolic free magnesium (Mg f) is considered relatively constant. To test this concept, Mg f was estimated during hyperkalemic ventricular akinesis, normal and maximum adrenergic stimulation, and sulfate loading of the normoxic perfused guinea-pig heart. The Mg f 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] K LDH/([creatine][P i[glyceraldehyde 3-phosphate][pyruvate]K CK), which varied with magnesium due to K CK (CK, LDH = creatine kinase, lactate dehydrogenase). However, the correct magnesium dependencies of the true constants K GAPDH· K PGK and K CK were taken from the literature. The [Mg 2+] at which pseudo K GAPDH′· K PGK′ equalled true K GAPDH·K PGK was the best estimate of Mg f. Mg f fell to ≈0.13 mM in hyperkalemic arrest from a control of ≈0.6 mM, rising to ≈0.85 mM only during maximum adrenergic stress. Mg f increased further to ≈1.3 mM during sulfate loading which induced ATP catabolism. Mg f and ATP were reciprocally related. Thus; (1) myocardial free [Mg 2+] judged from GAPDH/PGK mass-action relations changed appreciably only under extreme physiological states; (2) ATP was a major chelator of Mg 2+ in perfused myocardium, i.e., acute ATP pool size reduction may be associated with increments in Mg f.