Loss of Natriuretic Peptide Receptor C Enhances Sinoatrial Node Dysfunction in Aging and Frail Mice.

Heart rate (HR) is controlled by the sinoatrial node (SAN). SAN dysfunction is highly prevalent in aging; however, not all individuals age at the same rate. Rather, health status during aging is affected by frailty. Natriuretic peptides regulate SAN function in part by activating natriuretic peptide receptor C (NPR-C). The impacts of NPR-C on HR and SAN function in aging and as a function of frailty are unknown. Frailty was measured in aging wild-type and NPR-C knockout (NPR-C-/-) mice using a mouse clinical frailty index (FI). HR and SAN structure and function were investigated using intracardiac electrophysiology in anesthetized mice, high-resolution optical mapping in intact atrial preparations, histology, and molecular biology. NPR-C-/- mice rapidly became frail leading to shortened life span. HR was reduced and SAN recovery time was increased in older versus younger mice, and these changes were exacerbated in NPR-C-/- mice; however, there was substantial variability among age groups and genotypes. HR and SAN recovery time were correlated with FI score and fell along a continuum regardless of age or genotype. Optical mapping demonstrates impairments in SAN function that were also correlated with FI score. SAN fibrosis was increased in aged and NPR-C-/- mice and was graded by FI score. Loss of NPR-C results in accelerated aging and rapid decline in health status in association with impairments in HR and SAN function. Frailty assessment was effective and better able to distinguish aging-dependent changes in SAN function in the setting of shortened life span due to loss of NPR-C.

Jansen HJ ，Moghtadaei M ，Rafferty SA ，Belke DD ，Rose RA ... -  《-》

The impacts of age and frailty on heart rate and sinoatrial node function.

Sinoatrial node (SAN) function declines with age; however, not all individuals age at the same rate and health status can vary from fit to frail. Frailty was quantified in young and aged mice using a non-invasive frailty index so that the impacts of age and frailty on heart rate and SAN function could be assessed. SAN function was impaired in aged mice due to alterations in electrical conduction, changes in SAN action potential morphology and fibrosis in the SAN. Changes in SAN function, electrical conduction, action potential morphology and fibrosis were correlated with, and graded by, frailty. This study shows that mice of the same chronological age have quantifiable differences in health status that impact heart rate and SAN function and that these differences in health status can be identified using our frailty index. 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.

Moghtadaei M ，Jansen HJ ，Mackasey M ，Rafferty SA ，Bogachev O ，Sapp JL ，Howlett SE ，Rose RA ... -  《-》

Natriuretic peptide receptor B maintains heart rate and sinoatrial node function via cyclic GMP-mediated signalling.

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 signalling in regulating HR and SAN function. We have used NPR-B deficient mice (NPR-B+/-) to study HR regulation and SAN function using telemetry in conscious mice, intracardiac electrophysiology in anaesthetized 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 signalling 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. 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 signalling mechanism.

Dorey TW ，Mackasey M ，Jansen HJ ，McRae MD ，Bohne LJ ，Liu Y ，Belke DD ，Atkinson L ，Rose RA ... -  《-》

Natriuretic peptides regulate heart rate and sinoatrial node function by activating multiple natriuretic peptide receptors.

Natriuretic peptides, including BNP and CNP, elicit their effects via two guanylyl cyclase-linked receptors denoted NPR-A and NPR-B as well as a third receptor, NPR-C. The relative contributions of these receptors to the overall effects of NPs on heart rate (HR) and sinoatrial node (SAN) function are very poorly understood. The effects of BNP and CNP (10-500 nM) on HR and SAN myocyte spontaneous action potential (AP) firing were studied using wildtype mice and mice lacking functional NPR-C receptors (NPR-C(-/-)). In basal conditions and 10 nM doses of the β-adrenergic receptor (β-AR) agonist isoproterenol (ISO) BNP and CNP increased HR and AP firing in SAN myocytes. The NPR-C selective agonist cANF (10-500 nM) had no effects in basal conditions, but decreased HR and SAN AP frequency in the presence of ISO. These effects of cANF were completely absent in NPR-C(-/-) mice. Strikingly, in the presence of 1 μM doses of ISO, BNP and CNP switched to causing decreases in HR and SAN AP frequency. These decreases were not as large as those elicited by cANF and were absent in NPR-C(-/-) hearts, where BNP instead elicited a further increase in HR. Inhibition of NPR-A with A71915, in the presence of 1 μM ISO, enabled BNP to signal exclusively through NPR-C and to decrease HR as effectively as cANF. Together these data demonstrate that BNP and CNP affect HR and SAN function by activating multiple receptor subtypes. NPR-A/B mediate increases in HR and SAN function, but these effects are opposed by NPR-C, which plays an increasingly important signaling role in the presence of β-AR stimulation.

Azer J ，Hua R ，Vella K ，Rose RA ... -  《-》

Impaired sinoatrial node function and increased susceptibility to atrial fibrillation in mice lacking natriuretic peptide receptor C.

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.

Egom EE ，Vella K ，Hua R ，Jansen HJ ，Moghtadaei M ，Polina I ，Bogachev O ，Hurnik R ，Mackasey M ，Rafferty S ，Ray G ，Rose RA ... -  《-》