Differential response to acute ischemia between isolated contracting hearts and hearts perfused with excitation-contraction uncouplers

Acute myocardial ischemia triggers electrophysiological changes, including altered cardiac action potential and conduction slowing. Optical mapping is widely used to study these changes, but most experiments use excitation-contraction uncouplers to suppress contractile motion and motion artifacts. We hypothesized that contraction suppression with these uncouplers masks ischemic effects, leading to misleading results. We compared Langendorff-perfused, noncontracting hearts treated with blebbistatin to contracting hearts under acute ischemic conditions. Optical mapping with emission ratiometry and motion-tracking postprocessing minimized motion artifacts, whereas ischemia was induced by ligating the obtuse marginal branch of the left circumflex coronary artery. Contracting hearts exhibited faster and more pronounced reductions in action potential duration (APD) and action potential triangulation (2 min vs. 14 min), along with an increased incidence of spatially discordant alternans (SDA). They also displayed steeper APD restitution slopes, whereas these slopes were flattened in noncontracting hearts. These differences may stem from reduced metabolic demands and the absence of mechanoelectric feedback in noncontracting hearts. In contrast, contracting hearts, with higher metabolic activity and mechanical feedback, experienced more severe ischemic changes. These findings highlight the limitations of using blebbistatin-treated, noncontracting hearts in electrophysiological research, as critical ischemic effects may be underestimated. This study underscores the need to integrate mechanical and electrical dynamics in preclinical models to accurately replicate ischemic conditions, enhancing the translational relevance of experimental cardiac research.<b>NEW & NOTEWORTHY</b> This study highlights key differences in acute ischemic responses between contracting and blebbistatin-treated noncontracting rabbit hearts. Contracting hearts showed faster, more severe action potential duration reductions, increased spatially discordant alternans, and steeper restitution slopes, emphasizing the role of mechanoelectric feedback and higher metabolic demands. These findings challenge reliance on noncontracting models in electrophysiological research, underscoring the need for models integrating mechanical and electrical dynamics to improve the translational relevance of ischemic studies.