[Heart rate: when macrophages hit the note].

Macrophages regulate cardiac homeostasis under pathological and physiological conditions. Recent studies have elegantly substantiated the presence of specific subset of macrophages residing within the distal atrioventricular node in mice and humans. These macrophages directly couple with cardiomyocytes via connexin-43-containing gap junctions and increase atrioventricular conduction by accelerating cardiomyocyte repolarization. Conditional deletion of connexin-43 in macrophages or congenital lack of macrophages delay nodal conduction and foster progressive atrioventricular block. Exhaustive understanding of the role of tissue-resident macrophages in normal and aberrant cardiac conduction could initiate the development of therapeutic strategies focused on the modulation of macrophage functions in heart arrhythmia.