2.7 Arrhythmia induction protocol
Stimuli were generated by an isolated constant voltage/current stimulator (VCS3001, MappingLab Ltd., UK). They were delivered with a platinum electrode onto the epicardial apex at an amplitude 1.5 × the diastolic voltage threshold and a 2 ms pulse width. The optical mapping was performed in conjunction with an “alternans mapping” protocol. The heart was consecutively paced at basic cycle lengths (BCL) from 210 with 10 ms decrement until 170 ms. The heart was paced 50 times with 2 ms duration pulses and the optical mapping recording was performed at the last 2 second in each episode. This protocol was performed both before and after drugs were delivered.