Free magnesium concentration in isolated rabbit hearts subjected to high dose isoproterenol infusion: a 31P NMR study

The hypothesis of magnesium deficiency in isoproterenol (ISO) induced myocardial injury has been investigated by P-31 nuclear magnetic resonance spectroscopy. High energy phosphate concentrations, pH(i), and intracellular free magnesium concentration ([Mg2+](i)) were measured in isolated rabbit hearts perfused at constant flow and subjected to 10(-6) M isoproterenol during 30 min. Recent calibrations were used for [Mg2+](i) measurements, and uncertainties on [Mg2+](i) estimated values were calculated. During isoproterenol infusion, pH(i), [PCr], and [ATP] decreased, while [P-i] increased. When it was stopped, [PCr] completely repleted, whereas only a partial restoration was observed for pH(i) and [P-i]. A rise of end-diastolic pressure and perfusion pressure expressed a contracture, concomitant with a lack of [ATP] recovery, which remained at 59 +/- 13% of the rest value. These results establish that 10(-6) M isoproterenol caused severe myocardial injury. [Mg2+](i) increased from 0.70 mM at rest to 0.88 mM at the end of the isoproterenol period. Considering the estimated uncertainties on the [Mg2+](i) values, this increase was not significant. After isoproterenol infusion, [Mg2+](i) progressively decreased to reach 0.72 mM at 45 min recovery. It is concluded that isoproterenol myocardial toxicity may not be related to [Mg2+](i) deficiency.