Intro: Sarcolemmal ATP-sensitive K+ channels (KATP) open in response to low [ATP]/[ADP] to link cardiac energetics and action potential duration (APD). The effect of workload and oxygenation on KATP activation in excised working hearts is important for arrhythmia mechanisms, yet is unknown. Using novel motion-corrected ratiometric optical mapping, we hypothesized that, due to KATP activation, APD shortening in LV working (LVW) hearts during hypoxia is more severe than in unloaded Langendorff perfused hearts (LANG). Methods: Epicardial APDs were measured from LVW and LANG rabbit hearts (n=11) using di-4-ANEPPS excitation ratiometry and a motion-tracking algorithm. Circulating perfusate was gradually deoxygenated by bubbling with N2 gas. Perfusate %O2 was measured. In a subset of studies, 10µM glibenclamide (GLIB) was added to identify the level of APD shortening attributed to KATP. Results: APD dropped more rapidly in LVW than LANG hearts during gradual deoxygenation. Between 75 to 50 %O2, LVW APD dropped at a rate of 1.33±0.84 %O2/msec while LANG APD was constant. LANG APD dropped most rapidly at 50 %O2. GLIB diminished APD shortening in LVW hearts to a rate of 0.61±0.11 %O2/msec until 45 %O2, when APD dropped rapidly. In LVW hearts with GLIB, the APD vs. %O2 curve closely mirrored the LANG curve. Conclusion: APD shortens severely in LVW hearts during deoxygenation. High workload precipitates a mismatch of O2 supply:demand sooner, and to a greater extent, than in unloaded hearts. GLIB blocks KATP to decouple energetics and electrical activity to align the deoxygenation curves of loaded and unloaded hearts
