Cardiac myocyte alternans in intact heart: Influence of cell-cell coupling and β-adrenergic stimulation

Background: Cardiac alternans are proarrhythmic and mechanistically link cardiac mechanical dysfunction and sudden cardiac death. Beat-to-beat alternans occur when beats with large Ca2 + transients and long action potential duration (APD) alternate with the converse. APD alternans are typically driven by Ca2 + alternans and sarcoplasmic reticulum (SR) Ca2 + 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 [Ca2 +] 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 [Ca2 +] in situ with confocal microscopy. Single cell resolution allowed analysis of alternans within the intact organ during alternans induction. Carbenoxolone (25 μM), a GJ inhibitor, significantly increased the occurrence and amplitude of alternans in single cells within the intact heart. Alternans were concordant between neighboring cells throughout the field of view, except transiently during onset. β-AR stimulation only reduced Ca2 + alternans in tissue that had reduced GJ coupling, matching effects seen in isolated myocytes. Conclusions: Ca2 + alternans among neighboring myocytes is predominantly concordant, likely because of electrical coupling between cells. Consistent with this, partial GJ uncoupling increased propensity and amplitude of Ca2 + alternans, and made them more sensitive to reversal by β-AR activation, as in isolated myocytes. Electrical coupling between myocytes may thus limit the alternans initiation, but also allow alternans to be more stable once established.