Effects of lidocaine and verapamil on early afterdepolarizations in isolated rabbit sinoatrial node

The effects of local anaesthetic anti-arrhythmic agents (lidocaine) and Ca antagonists (Verapamil) have been examined on the early afterdepolarizations (EADs) in isolated rabbit sinoatrial (SA) node. In a nominally calcium free and magnesium free solution, strontium (0.5-4.5 mM) produced an EAD in small pieces isolated from the SA node. The additional presence of 0.02-0.6 mM lidocaine did not abolish the strontium (0.5 mM)-induced EAD. 0.6 mM lidocaine produced an increase in EAD amplitude and then abolished a prolonged action potential (AP) associated with repetitive EADs. On the other hand, the addition of 4 micro M verapamil abolished the strontium (0.5 mM)-induced EAD but did not abolish the AP.It is concluded that under conditions when the AP is not abolished, EAD blockade by lidocaine is less effective than that by verapamil.