Magnesium and the calcium paradox: The occurrence of “spasmodic contractions” during Ca²⁺−Mg²⁺-free perfusion of isolated rat heart

Summary

The effects of omission of Mg²⁺ during Ca²⁺-free perfusion (3 min) of either spontaneously beating and electrically stimulated rat hearts were studied. Ca²⁺-free perfusion per se induced cardiac arrest and coronary vasodilation, and increased intrinsic pulse rate of the heart. Upon reperfusion with Ca²⁺, cardiac function was lost and parallelled by a sudden and massive release of cellular constituents (“calcium paradox”). Mg²⁺-free perfusion evoked effects opposite from Ca²⁺-free, with exception of heart rate which was increased.

During Ca²⁺−Mg²⁺-free perfusion the electrocardiogram became irregular within 30 s, and this effect was followed by transient “spasmodic contractions”. The effects of normal reperfusion were indistinguishable from those observed after Ca²⁺-free perfusion in the presence of Mg²⁺. Addition of Mn²⁺ or La³⁺ to the Ca²⁺−Mg²⁺-free perfusion medium completely inhibited the induction of electrical irregularities and spasmodic contractions.

The typical effects of Ca²⁺−Mg²⁺-free perfusion are discussed in terms of Mg−Ca and Mg−K interactions at the sarcolemmal surface. It was concluded that our results may contribute indirectly to an explanation of the protective effect of high Mg²⁺-concentrations during Ca²⁺-free as well as ischemic perfusion of mammalian hearts.