Key points Natriuretic peptides (NPs) elicit their effects via multiple NP receptors (including NPR‐A, NPR‐B and NPR‐C, with NPR‐C being relatively poorly understood). We have studied the effects of NPR‐C ablation on cardiac structure, function and arrhythmogenesis using NPR‐C knockout (NPR‐C−/−) mice. NPR‐C−/− mice are characterized by sinoatrial node (SAN) dysfunction and a profound increase in susceptibility to atrial fibrillation. Increased susceptibility to arrhythmias in NPR‐C−/− mice was associated with slowed electrical conduction in the SAN as well as the right and left atria due to enhanced collagen expression and deposition in the atria (structural remodelling), but without changes in action potential morphology (electrical remodelling) in isolated SAN or atrial myocytes. This study demonstrates a critical protective role for NPR‐C in the heart. Abstract Natriuretic peptides (NPs) are critical regulators of the cardiovascular system that are currently viewed as possible therapeutic targets for the treatment of heart disease. Recent work demonstrates potent NP effects on cardiac electrophysiology, including in the sinoatrial node (SAN) and atria. NPs elicit their effects via three NP receptors (NPR‐A, NPR‐B and NPR‐C). Among these receptors, NPR‐C is poorly understood. Accordingly, the goal of this study was to determine the effects of NPR‐C ablation on cardiac structure and arrhythmogenesis. Cardiac structure and function were assessed in wild‐type (NPR‐C+/+) and NPR‐C knockout (NPR‐C−/−) mice using echocardiography, intracardiac programmed stimulation, patch clamping, high‐resolution optical mapping, quantitative polymerase chain reaction and histology. These studies demonstrate that NPR‐C−/− mice display SAN dysfunction, as indicated by a prolongation (30%) of corrected SAN recovery time, as well as an increased susceptibility to atrial fibrillation (6% in NPR‐C+/+ vs. 47% in NPR‐C−/−). There were no differences in SAN or atrial action potential morphology in NPR‐C−/− mice; however, increased atrial arrhythmogenesis in NPR‐C−/− mice was associated with reductions in SAN (20%) and atrial (15%) conduction velocity, as well as increases in expression and deposition of collagen in the atrial myocardium. No differences were seen in ventricular arrhythmogenesis or fibrosis in NPR‐C−/− mice. This study demonstrates that loss of NPR‐C results in SAN dysfunction and increased susceptibility to atrial arrhythmias in association with structural remodelling and fibrosis in the atrial myocardium. These findings indicate a critical protective role for NPR‐C in the heart.

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Sinoatrial node (SAN) dysfunction increases with age, although not all older adults are affected in the same way. This is because people age at different rates and individuals of the same chronological age vary in health status from very fit to very frail. Our objective was to determine the impacts of age and frailty on heart rate (HR) and SAN function using a new model of frailty in ageing mice. Frailty, which was quantified in young and aged mice using a frailty index (FI), was greater in aged vs. young mice. Intracardiac electrophysiology demonstrated that HR was reduced whereas SAN recovery time (SNRT) was prolonged in aged mice; however, both parameters showed heteroscedasticity suggesting differences in health status among mice of similar chronological age. Consistent with this, HR and corrected SNRT were correlated with, and graded by, FI score. Optical mapping of the SAN demonstrated that conduction velocity (CV) was reduced in aged hearts in association with reductions in diastolic depolarization (DD) slope and action potential (AP) duration. In agreement with in vivo results, SAN CV, DD slope and AP durations all correlated with FI score. Finally, SAN dysfunction in aged mice was associated with increased interstitial fibrosis and alterations in expression of matrix metalloproteinases, which also correlated with frailty. These findings demonstrate that age-related SAN dysfunction occurs in association with electrical and structural remodelling and that frailty is a critical determinant of health status of similarly aged animals that correlates with changes in HR and SAN function.

Background: Atrial fibrillation (AF) commonly occurs in hypertension and in association with elevated Ang II (angiotensin II) levels. The specific mechanisms underlying Ang II–mediated AF are unclear, and interventions to prevent the effects of Ang II are lacking. NPs (natriuretic peptides), which elicit their effects through specific NP receptors, including NPR-C (natriuretic peptide receptor-C), are cardioprotective hormones that affect cardiac structure and function. Methods: This study used wild-type and NPR-C knockout (NPR-C −/ − ) mice to investigate the effects of Ang II (3 mg/kg per day for 3 weeks) on AF susceptibility and atrial function using in vivo electrophysiology, high-resolution optical mapping, patch clamping, and molecular biology. In some experiments, wild-type mice were cotreated with Ang II and the NPR-C agonist cANF (0.07–0.14 mg/kg per day) for 3 weeks. Results: In wild-type mice, Ang II increased susceptibility to AF in association with a prolongation of P-wave duration, increased atrial refractory period, and slowed atrial conduction. These effects were exacerbated in Ang II–treated NPR-C −/− mice. Ang II prolonged action potential duration and reduced action potential upstroke velocity (V max ). These effects were greater in left atrial myocytes from Ang II–treated NPR-C −/− mice. Ang II also increased fibrosis in both atria in wild-type mice, whereas Ang II–treated NPR-C −/− mice exhibited substantially higher fibrosis throughout the atria. Fibrotic responses were associated with changes in expression of profibrotic genes, including TGFβ and TIMP1 . Cotreating wild-type mice with Ang II and the NPR-C agonist cANF dose dependently reduced AF inducibility by preventing some of the Ang II–induced changes in atrial myocyte electrophysiology and preventing fibrosis throughout the atria. Conclusions: NPR-C may represent a new target for the prevention of Ang II–induced AF via protective effects on atrial electrical and structural remodeling.

Atrial fibrillation (AF) is prevalent in aging populations; however not all individuals age at the same rate. Instead, individuals of the same chronological age can vary in health status from fit to frail. Our objective was to determine the impacts of age and frailty on atrial function and arrhythmogenesis in mice using a frailty index (FI). Aged mice were more frail and demonstrated longer lasting AF compared to young mice. Consistent with this, aged mice showed longer P wave duration and PR intervals; however, both parameters showed substantial variability suggesting differences in health status among mice of similar chronological age. In agreement with this, P wave duration and PR interval were highly correlated with FI score. High resolution optical mapping of the atria demonstrated reduced conduction velocity and action potential duration in aged hearts that were also graded by FI score. Furthermore, aged mice had increased interstitial fibrosis along with changes in regulators of extracellular matrix remodelling, which also correlated with frailty. These experiments demonstrate that aging results in changes in atrial structure and function that create a substrate for atrial arrhythmias. Importantly, these changes were heterogeneous due to differences in health status, which could be identified using an FI.

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Aims Heart rate (HR) is a critical indicator of cardiac performance that is determined by sinoatrial node (SAN) function and regulation. Natriuretic peptides, including C-type NP (CNP) have been shown to modulate ion channel function in the SAN when applied exogenously. CNP is the only NP that acts as a ligand for natriuretic peptide receptor-B (NPR-B). Despite these properties, the ability of CNP and NPR-B to regulate HR and intrinsic SAN automaticity in vivo, and the mechanisms by which it does so, are incompletely understood. Thus, the objective of this study was to determine the role of NPR-B signaling in regulating HR and SAN function. Methods and Results We have used NPR-B deficient mice (NPR-B+/-) to study HR regulation and SAN function using telemetry in conscious mice, intracardiac electrophysiology in anesthetized mice, high resolution optical mapping in isolated SAN preparations, patch-clamping in isolated SAN myocytes, and molecular biology in isolated SAN tissue. These studies demonstrate that NPR-B+/- mice exhibit slow HR, increased corrected SAN recovery time, and slowed SAN conduction. Spontaneous AP firing frequency in isolated SAN myocytes was impaired in NPR-B+/- mice due to reductions in the hyperpolarization activated current (If) and L-type Ca2+ current (ICa,L). If and ICa,L were reduced due to lower cGMP levels and increased hydrolysis of cAMP by phosphodiesterase 3 (PDE3) in the SAN. Inhibiting PDE3 or restoring cGMP signaling via application of 8-Br-cGMP abolished the reductions in cAMP, AP firing, If, and ICa,L, and normalized SAN conduction, in the SAN in NPR-B+/- mice. NPR-B+/- mice did not exhibit changes in SAN fibrosis and showed no evidence of cardiac hypertrophy or changes in ventricular function. Conclusions NPR-B plays an essential physiological role in maintaining normal HR and SAN function by modulating ion channel function in SAN myocytes via a cGMP/PDE3/cAMP signaling mechanism.