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 Mg2+-dependence of glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase (GAPDH/PGK). The pseudo constant KGAPDH′·KPGK′ 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 KGAPDH′·KPGK′ equalled true KGAPDH·KPGK was the best estimate of Mgf. Mgf 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. Mgf increased further to ≈1.3 mM during sulfate loading which induced ATP catabolism. Mgf and ATP were reciprocally related. Thus; (1) myocardial free [Mg2+] judged from GAPDH/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.
- ATP phosphorylation potential
- Creatine kinase
- Cytosolic free magnesium
- Glyceraldehyde 3-phosphate dehydrogenase
- Guinea-pig myocardium
- Phosphoglycerate kinase