Al-Othman, S.; Wu, Y.; Fontanaud, P.; Puttur, F.; Conesa, D.; Zhu, C.; Moore, S.; Tikhomirov, R.; Francis, A.; Nair, S.; Chowdhury, R. A.; Husain, M.; Boyle, J. J.; Oceandy, D.; Niederer, S. A.; Walton, R.; Howell, G.; Roberts, L.; Boyett, M. R.; Colman, M. A.; Mangoni, M. E.; D'Souza, A.

Resident cardiac macrophages have been proposed to facilitate atrioventricular (AV) node conduction through connexin 43 (Cx43) gap-junction coupling to nodal myocytes. We tested this mechanism using biophysical modelling, high resolution imaging of mouse and human AV conduction tissue, and pharmacological macrophage depletion. In silico, coupling macrophage membrane phenotypes to HCN4+ AV node myocytes imposed an electrotonic load that suppressed pacemaking and promoted conduction slowing, including stable 2:1 block in strand simulations. Anatomically, HCN4 defined components of the mouse AV conduction axis were essentially devoid of Cx43 and overlap between CD68+ macrophages and Cx43 was negligible in both mouse AV node and human penetrating bundle. Finally, near-complete macrophage depletion with CSF1R inhibition (PLX5622) did not alter AV conduction in vivo (PR interval) or ex vivo (Wenckebach cycle length and AV nodal effective refractory period). Together, these data argue against a physiologically relevant role for Cx43 mediated macrophage myocyte electrical coupling in baseline AV node function.