GJA1 and atrial fibrillation: Our data show (1) that profibrotic markers are upregulated in patients with HFrEF, independent of the presence of AF; (2) that protein expression and phosphorylation of connexin-43 are reduced in HFrEF only; (3) that classical indices of AF-related atrial electrical remodeling only occur in cAF and HFrEF-cAF; and (4) that potentially proarrhythmic atrial Ca2+-handling abnormalities are a typical finding in both HFrEF and HFrEF-cAF patients.