Upstroke velocity of the cardiac action potential (AP) is an important index of tissue excitability. Previous studies have demonstrated that in epicardial tissue preparations, surface AP upstroke velocity is greater when electrical propagation is in the transverse direction relative to the longitudinal. However, the electrophysiological basis for these differences is debated. Furthermore, no previous studies have examined AP upstroke velocity in the transmural axis during the normal endo-epicardial activation sequence. In this study we report transmural AP characteristics from layers up to 600μm below the epicardial surface and compare the data with a computational model of cardiac electrical propagation.
Hearts from male New Zealand white rabbits were Langendorff-perfused at 37°C and paced at a cycle length of 300ms. Preparations were loaded with the ratiometric dye di-4-ANEPPS and optical APs were recorded using both widefield epifluorescence and two-photon (2P) microscopy. BDM (10mM) and blebbistatin (10μM) were used to minimise motion artefacts.
During normal endo-epicardial activation (right-atrial (RA) pacing), mean 10-90% AP rise times for 2P recordings prolonged steadily with increasing tissue depth (3.4 ± 0.2 ms vs. 6.8 ± 0.9 ms; 50 vs. 500μm from surface, P < 0.05, n=11) while epifluorescence recordings demonstrated consistently longer rise times (7.5 ± 0.4 ms, n=11). This prolongation of AP rise time was not observed beyond 500μm below the surface. Surface microelectrode recordings revealed significant differences in rise times between RA, transverse and longitudinal activation (3.6 ± 0.6, 6.1 ± 0.6 and 8.7 ± 1.2 ms; RA vs. transverse vs. longitudinal, n=5), consistent with previous studies. In contrast to RA pacing, AP rise times at depth did not significantly change from those recorded 50μm below surface with either transverse or longitudinal conduction, suggesting the effect is not an optical artefact. Comparison of these data with the output from an augmented monodomain cardiac tissue model revealed that the inclusion of an extracellular resistance compartment along the fibre axis and a lower resistance component at the tissue surface as described in the literature reproduced the increasing rise time with depth seen experimentally (0.9 vs. 2.0 ms; 100μm deep vs. 500μm deep), and resulted in the appearance of differences in AP rise time between transverse and longitudinal epicardial activation. Collectively these data suggest that AP upstroke characteristics are influenced by the anisotropic extracellular resistances within the ventricle and the low resistance pathway at the surface.