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Background: Cardiac alternans are proarrhythmic and mechanistically link cardiac mechanical dysfunction and sudden cardiac death. Beat-to-beat alternans occur when beats with large Ca^2 + transients and long action potential duration (APD) alternate with the converse. APD alternans are typically driven by Ca^2 + alternans and sarcoplasmic reticulum (SR) Ca^2 + release alternans. But the effect of intercellular communication via gap junctions (GJ) on alternans in the intact heart remains unknown. Objective: We assessed the effects of cell-to-cell coupling on local alternans in intact Langendorff-perfused mouse hearts, measuring single myocyte [Ca^2 +] alternans synchronization among neighboring cells, and effects of β-adrenergic receptor (β-AR) activation and reduced GJ coupling. Methods and results: Mouse hearts (C57BL/6) were retrogradely perfused and loaded with Fluo8-AM to record cardiac myocyte [Ca …