Magnesium and the calcium paradox: The occurrence of "spasmodic contractions" during Ca super(2+)-Mg super(2+)-Mg super(2+)-free perfusion of isolated rat heart

The effects of omission of Mg super(2+) during Ca super(2+)-free perfusion (3 min) of either spontaneously beating and electrically stimulated rat hearts were studied. Ca super(2+)-free perfusion per se induced cardiac arrest and coronary vasodilation, and increased intrinsic pulse rate of the heart. Upon reperfusion with Ca super(2+), cardiac function was lost and parallelled by a sudden and massive release of cellular constituents. Mg super(2+)-free perfusion evoked effects opposite from Ca super(2+)-free with exception of heart rate which was increased. The typical effects of Ca super(2+)-Mg super(2+)-free perfusion are discussed in terms of Mg-Ca and Mg-K interactions at the sarcolemmal surface. It was concluded that the results may contribute indirectly to an explanation of the protective effect of high Mg super(2+)-concentrations during Ca super(2+)-free as well as ischemic perfusion of mammalian hearts.