EFFECTS OF MAGNESIUM ON EARLY AFTERDEPOLARIZATIONS AND CESIUM-INDUCED ACTIVITY ON THE FERRET PURKINJE-FIBERS

The effects of magnesium (Mg) on afterdepolarisations and cesium-induced activity were studied in 15 preparations of ferret Purkinje fibres. When the perfusion medium was enriched with Mg to concentrations of 5 mM, the duration of the action potentials stopped increasing and the diastolic membrane potential was stabilised despite persistant exposure to cesium. A suppressor effect of Mg on the high and low voltage early afterdepolarisations was observed which was significant at 2.2 mM (p < 0.03) and even more so at 5 mM (p = 3. 10-5), related to a lower probability of early afterdepolarisations. A suppressor effect of Mg on high and low voltage induced activities was also recorded, significant at 5 mM, due to a reduction in the probability of apparition of a first induced action potential (p = 2. 10-4) and a reduction in the number of induced action potentials (p = 8. 10-4). The rapid onset of action (a few minutes) of Mg suggests an extracellular mode of action. The suppressor effect of Mg is probably related to inhibition of an unmasked current by the cesium, a calcium antagonist effect and/or non-specific membrane stabilising effect. The suppressor effect of Mg on early afterdepolarisations and induced activity could, at least partially, explain its in vitro antiarrhythmic action on torsades de pointe.