Key pointsNatriuretic peptides are powerful regulators of the cardiovascular system, but their direct electrophysiological effects in the heart are poorly understood.We have used optical mapping techniques to determine the effects of B‐type and C‐type natriuretic peptide (BNP and CNP) as well as their receptors (NPR‐A, NPR‐B and NPR‐C) on electrical conduction in the sinoatrial node and atrial myocardium.In basal conditions, BNP and CNP speed conduction within the sinoatrial node (SAN) and into the atrial myocardium in association with shifts in the initial exit site by activating NPR‐A and NPR‐B.In the presence of isoproterenol, BNP and CNP have the opposite effect whereby conduction in the SAN and atria is slowed. These inhibitory effects are due to the activation of NPR‐C as well as NPR‐A/B.We conclude that natriuretic peptides have potent effects on electrical conduction in the heart. These effects are complex and can involve multiple natriuretic peptide receptors.AbstractNatriuretic peptides, including B‐type and C‐type natriuretic peptide (BNP and CNP), are powerful regulators of the cardiovascular system; however, their electrophysiological effects in the heart, particularly in the sinoatrial node (SAN), are incompletely understood. We have used high‐resolution optical mapping to measure the effects of BNP and CNP, and the roles of natriuretic peptide receptors (NPR‐A, NPR‐B and NPR‐C), on electrical conduction within the SAN and atrial myocardium. In basal conditions BNP and CNP (50–500 nm) increased conduction velocity (CV) within the SAN by ∼30% at the high dose and shifted the initial exit site superiorly. These effects sped conduction from the SAN to the surrounding atrial myocardium and were mediated by the NPR‐A and NPR‐B receptors. In the presence of isoproterenol (1 μm) the NPR‐C receptor made a major contribution to the effects of BNP and CNP in the heart. In these conditions BNP, CNP and the NPR‐C agonist cANF each decreased SAN CV and shifted the initial exit site inferiorly. The effects of cANF (30% reduction) were larger than BNP or CNP (∼15% reduction), indicating that BNP and CNP activate multiple natriuretic peptide receptors. In support of this, the inhibitory effects of BNP were absent in NPR‐C knockout mice, where BNP instead elicited a further increase (∼25%) in CV. Measurements in externally paced atrial preparations demonstrate that the effects of natriuretic peptides on CV are partially independent of changes in cycle length. These data provide detailed novel insight into the complex effects of natriuretic peptides and their receptors on electrical conduction in the heart.