Optical Potential Mapping of the Excitation Process in the Field stimulated Guinea pig Papillary Muscle using short Pulses

We have performed optical potential mapping in guinea pig papillary muscles during the application of short electrical pulses (1 ms). Our results showed, that pulses near threshold, which caused membrane depolarizations in the monitored area, can lead to action potential upstrokes which show pronounced differences in the occurrence times (taken at 50 % of action potential amplitude) in the range of 4 ms in an area of 0.5 times 0.5 mm². The pulse induced membrane voltages within the same area, taken at pulse offset, covered the range from 21 to 42 mV. In general, as expected, increasing membrane voltages caused shorter upstroke delays, but this was not always the case resulting in a weak correlation of a linear fit (r=0.57). These findings indicate that the excitation process is based on both, the membrane properties and the tissue structure. Weak pulses near threshold may be applied to trigger action potentials with low, pulse induced disturbances; however, spatially distributed delays within the area of early depolarizations may have to be considered