Magnesium net fluxes and distribution in rabbit myocardium in irreversible contracture

Distribution of magnesium (Mg) in heart muscle was studied by measuring fluxes of Mg and transmembrane potentials as a function of perfusate [Mg2+] after a massive increase in permeability of the sarcolemma was induced in the Langendorff prepared heart from the Nembutal-anesthetized rabbit. After onset of 0 mM [Ca2+] perfusion which produced excitation-contraction (E-C) uncoupling and mechanical arrest, action potentials recorded from subepicardial cells showed an increase in duration and decrease in amplitude, which progressed until no transmembrane potentials could be observed. Restoration of physiological salt solution perfusion after 15 min of [Ca2+]-free perfusion caused an irreversible contracture that was associated with 1) efflux of potassium (K) and myoglobin, 2) perfusate [Mg2+]-dependent flux of Mg, and 3) transmembrane potentials of 0 mV. The magnitude of net efflux of K and myoglobin during contracture was unaffected by perfusate [Mg2+]. During the first 2 min of contracture, net efflux of Mg (mumoles per gram wet muscle +/- SE) was 1.37 +/- 0.09 and 0.48 +/- 0.19 during 0 mM and 2.5 mM [Mg2+] perfusion, respectively; but a net influx of 0.56 +/- 0.23 occurred during 5 mM [Mg2+] perfusion. Total sarcoplasmic [Mg] may correspond to perfusate [Mg2+] of 3.6 mM, which was found by interpolation to prevent any net flux of Mg during contracture. 3.6 mM may, therefore, represent the upper limit of the intracellular free-ionized Mg concentration in rabbit heart.