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(2+)] 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 Ca(2+) alternans in tissue that had reduced GJ coupling, matching effects seen in isolated myocytes.

Conclusions

Ca(2+) 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 Ca(2+) 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.

There are over 1 million hospitalizations for heart failure (HF) annually in the United States alone, and a similar number has been reported in Europe. Recent clinical trials investigating novel therapies in patients with hospitalized HF (HHF) have been negative, and the post-discharge event rate remains unacceptably high. The lack of success with HHF trials stem from problems with understanding the study drug, matching the drug to the appropriate HF subgroup, and study execution. Related to the concept of study execution is the importance of including appropriate study sites in HHF trials. Often overlooked issues include consideration of the geographic region and the number of patients enrolled at each study center. Marked differences in baseline patient co-morbidities, serum biomarkers, treatment utilization and outcomes have been demonstrated across geographic regions. Furthermore, patients from sites with low recruitment may have worse outcomes compared to sites with higher enrollment patterns. Consequently, sites with poor trial enrollment may influence key patient end points and likely do not justify the costs of site training and maintenance. Accordingly, there is an unmet need to develop strategies to identify the right study sites that have acceptable patient quantity and quality. Potential approaches include, but are not limited to, establishing a pre-trial registry, developing site performance metrics, identifying a local regionally involved leader and bolstering recruitment incentives. This manuscript summarizes the roundtable discussion hosted by the Food and Drug Administration between members of academia, the National Institutes of Health, industry partners, contract research organizations and academic research organizations on the importance of selecting optimal sites for successful trials in HHF.