Effects of Mg2+ on Ca2+ waves and Ca2+ transients of rat ventricular myocytes

It has been shown that the occurrence of the transient inward current, which is responsible for triggered activity, was often associated with propagating regions of increased intracellular Ca2+ concentration ([Ca2+]i), i.e., the "Ca2+ wave." To investigate the mechanism of antiarrhythmic action of Mg2+, we have studied effects of high concentrations of Mg2+ on Ca2+ waves in isolated rat ventricular myocytes. [Ca2+]i was estimated using the Ca(2+)-indicating probe indo 1. Ca2+ waves in myocytes, stimulated at 0.2 Hz, were induced by perfusion of isoproterenol (10(-7) M). High Mg2+ concentration suppressed Ca2+ waves in a concentration-dependent manner (36% at 4 mM, 70% at 8 mM, and 82% at 12 mM). The Ca2+ channel blocker verapamil also suppressed Ca2+ waves in a similar way. In contrast with marked depression of Ca2+ transients by verapamil, Ca2+ transients were not affected by high Mg2+ concentration (8 mM). High Mg2+ concentration also reduced frequencies of Ca2+ waves in the absence of electrical stimulation, whereas verapamil failed to reduce frequencies of Ca2+ waves. Reduction in frequency of Ca2+ waves by high Mg2+ concentration was associated with slowing of propagation velocity of Ca2+ waves. To examine whether suppressive effects of high Mg2+ concentration on Ca2+ waves were related to an increase in intracellular Mg2+ concentration ([Mg2+]i), the effect of high-Mg2+ solution on [Mg2+]i was examined in myocytes loaded with mag-fura 2. An increase in extracellular Mg2+ concentration from 1 to 12 mM increased [Mg2+]i from 1.06 +/- 0.16 to 1.87 +/- 0.22 mM (P < 0.01) in 30 min. To examine the effect of high Mg2+ concentration on amount of releasable Ca2+ in the sarcoplasmic reticulum, the effect of high Mg2+ concentration on the Ca2+ transient induced by a rapid application of caffeine was examined. High-Mg2+ solution increased the peak of the caffeine-induced Ca2+ transient. These results suggest that the inhibitory effect of Mg2+ on Ca2+ waves was not due to inhibition of the sarcolemmal Ca2+ channel but could be due to a decreased propensity for the sarcoplasmic reticulum to divest itself of excess Ca2+.