High extracellular magnesium prolongs the effective refractory period and A-V conduction time, while low magnesium increases ventricular vulnerability in isolated rabbit hearts.
Electrophysiological interactions of Mg and K ions were studied in isolated rabbit heart preparations. Threefold increase in extracellular Mg concentration, in the presence of either high (7·5 mM), normal (4·5 mM), or low (1·5 mM) K concentration, tended to increase the action potential amplitude, membrane resting potential, maximal rate of depolarization, and action potential duration of ventricular muscle fibres, and prolonged the A-V conduction time. These effects were more prominent at higher K concentrations. Reduction of Mg concentration to one third produced opposite results. Prolongation of the effective refractory, by high Mg was greater than that of the action potential duration. This resulted in a better propagation of the earliest premature response. Low Mg shortened the effective refractory, but tended to prolong the relative refractory period, and increased ventricular vulnerability. Possible ionic mechanisms underlying these Mg effects were discussed in comparison with Ca action.
Watanabe et al. (Sat,) studied this question.