Rate dependence of ischaemic myocardial depolarisation: evidence for a novel membrane current.

Depolarisation of ischaemic myocardial cells is at least partly due to loss of cellular potassium. Whereas most manifestations of ischaemia vary with heart rate potassium loss, however, reportedly does not. Cellular depolarisation was therefore correlated with extracellular potassium activity during serial coronary artery occlusions in Langendorff perfused canine hearts. Occlusions in sinus rhythm (92(11) beats X min-1) were alternated with rapidly paced occlusions (180 beats X min-1). For each occlusion cellular depolarisation was estimated from TQ depression and compared with the simultaneous increase in potassium electrode potential, delta EK. Although potassium accumulation accounted for most of the estimated depolarisation at slow heart rates, a potassium independent mechanism predominated during rapid pacing. The potassium independent mechanism was especially important in the first minute of ischaemia when pacing increased depolarisation by 324%, with little increase in delta EK. It appears that ischaemia induces a rate sensitive depolarising membrane current, which worsens conduction and promotes arrhythmias.