Counterpoint: M cells do not have a functional role in the ventricular myocardium of the intact heart

Abstract

h a m t p t 1 t r s A heart is not a wedge Transmural heterogeneities in repolarization between the subendocardial, subepicardial myocardium, and a so-called layer of M cells are thought to be implied in the genesis of the T-wave and in arrhythmogenesis. We address these issues herein. The work of Antzelevitch et al on the arterially perfused anine wedge preparation has given rise to the concept that here is a considerable transmural gradient in repolarization hat is responsible for the T-wave in the electrocardiogram ECG). In the wedge preparation, the peak of the T-wave oincides with the end of epicardial repolarization, and the nd of the T-wave with the end of repolarization of the idmural M cells. The Tpeak-end interval was proposed as an index of transmural repolarization. The fact that in the pseudo ECG of the wedge preparation there is concordance between the QRS complex and the T-wave, whereas in the intact dog there is discordance in most leads, suggests that a wedge preparation is not a suitable model for studying repolarization patterns in the intact heart. There are numerous studies on intact hearts in which ntramural measurements have shown the absence of relvant transmural gradients in repolarization despite the resence of gradients in action potential duration (see pthof et al for references). Figure 1 shows strengthnterval curves for cathodal stimuli determined at succesive intramural electrodes in a study that was performed ome 50 years ago. In the middle layers, recovery of xcitability occurs earlier than at epicardial and endocarial layers, and the sum of activation time (AT) and efractory period, which is equivalent to repolarization ime (RT), is longest in the epicardium, contrary to what odern textbooks describe.