Membrane polarization at the excitation threshold induced by external electric fields in cardiomyocytes of rats at different developmental stages

External electric fields ( E ), used for cardiac pacing and defibrillation/cardioversion, induce a spatially variable change in cardiomyocyte transmembrane potential (Δ V m ) that depends on cell geometry and E orientation. This study investigates E -induced Δ V m in cardiomyocytes from rats at different ages, which show marked size/geometry variation. Using a tridimensional numerical electromagnetic model recently proposed (NM3D), it was possible: (a) to evaluate the suitability of the simpler, prolate spheroid analytical model (PSAM) to calculate amplitude and location of Δ V m maximum (Δ V max ) for E = 1 V.cm −1 ; and (b) to estimate the Δ V max required for excitation (Δ V T ) from experimentally determined threshold E values ( E T ). Ventricular myocytes were isolated from neonatal, weaning, adult, and aging Wistar rats. NM3D was constructed as the extruded 2D microscopy cell image, while measured minor and major cell dimensions were used for PSAM. Acceptable Δ V m estimates can be obtained with PSAM from paralelepidal cells for small θ . E T , but not Δ V T , was higher for neonate cells. Δ V T was significantly greater in the cell from older animals, which indicate lower responsiveness to E associated with aging, rather than with altered cell geometry/dimensions. Δ V T might be used as a non-invasive indicator of cell excitability as it is little affected by cell geometry/size. Graphical Abstract