Natriuretic Peptide Receptor B Protects Against Atrial Fibrillation by Controlling Atrial cAMP Via Phosphodiesterase 2.

BACKGROUND: ß-AR (ß-adrenergic receptor) stimulation regulates atrial electrophysiology and Ca2+ homeostasis via cAMP-dependent mechanisms; however, enhanced ß-AR signaling can promote atrial fibrillation (AF). CNP (C-type natriuretic peptide) can also regulate atrial electrophysiology through the activation of NPR-B (natriuretic peptide receptor B) and cGMP-dependent signaling. Nevertheless, the role of NPR-B in regulating atrial electrophysiology, Ca2+ homeostasis, and atrial arrhythmogenesis is incompletely understood. METHODS: Studies were performed using atrial samples from human patients with AF or sinus rhythm and in wild-type and NPR-B-deficient (NPR-B+/-) mice. Studies were conducted in anesthetized mice by intracardiac electrophysiology, in isolated mouse atrial preparations using high-resolution optical mapping, in isolated mouse and human atrial myocytes using patch-clamping and Ca2+ imaging, and in mouse and human atrial tissues using molecular biology. RESULTS: Atrial NPR-B protein levels were reduced in patients with AF, and NPR-B+/- mice were more susceptible to AF. Atrial cGMP levels and PDE2 (phosphodiesterase 2) activity were reduced in NPR-B+/- mice leading to larger increases in atrial cAMP in the presence of the ß-AR agonist isoproterenol. NPR-B+/- mice displayed larger increases in action potential duration and L-type Ca2+ current in the presence of isoproterenol. This resulted in the occurrence of spontaneous sarcoplasmic reticulum Ca2+ release events and delayed afterdepolarizations in NPR-B+/- atrial myocytes. Phosphorylation of the RyR2 (ryanodine receptor) and phospholamban was increased in NPR-B+/- atria in the presence of isoproterenol compared with the wildtypes. C-type natriuretic peptide inhibited isoproterenol-stimulated L-type Ca2+ current through PDE2 in mouse and human atrial myocytes. CONCLUSIONS: NPR-B protects against AF by preventing enhanced atrial responses to ß-adrenergic receptor agonists.

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

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 signalling 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 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. 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 signalling mechanism.

Variability in opioid prescribing practices among cardiac surgeons and trainees.

BACKGROUND AND AIM: The opioid epidemic has become a major public health crisis in recent years. Discharge opioid prescription following cardiac surgery has been associated with opioid use disorder; however, ideal practices remain unclear. Our aim was to examine current practices in discharge opioid prescription among cardiac surgeons and trainees. METHODS: A survey instrument with open- and closed-ended questions, developed through a 3-round Delphi method, was circulated to cardiac surgeons and trainees via the Canadian Society of Cardiac Surgeons. Survey questions focused on routine prescription practices including type, dosage and duration. Respondents were also asked about their perceptions of current education and guidelines surrounding opioid medication. RESULTS: Eighty-one percent of respondents reported prescribing opioids at discharge following routine sternotomy-based procedures, however, there remained significant variability in the type and dose of medication prescribed. The median (interquartile range) number of pills prescribed was 30 (20-30) with a median total dose of 135 (113-200) Morphine Milligram Equivalents. Informal teaching was the most commonly reported primary influence on prescribing habits and a lack of formal education regarding opioid prescription was associated with a higher number of pills prescribed. A majority of respondents (91%) felt that there would be value in establishing practice guidelines for opioid prescription following cardiac surgery. CONCLUSIONS: Significant variability exists with respect to routine opioid prescription at discharge following cardiac surgery. Education has come predominantly from informal sources and there is a desire for guidelines. Standardization in this area may have a role in combatting the opioid epidemic.

Surgically positioned paravertebral catheters and postoperative analgesia after open abdominal aortic aneurysm repair.

OBJECTIVE: To compare postoperative morphine equivalent intake after open abdominal aortic aneurysm (AAA) repair among analgesic modalities: systemic analgesia (SA) only with no regional anesthesia, surgically positioned paravertebral catheter (PVC), and thoracic epidural analgesia (TEA). METHODS: This retrospective cohort study included patients undergoing elective open AAA at the Queen Elizabeth II Health Science Center, Halifax, Nova Scotia. Demographics, morphine equivalents, methods of analgesia administration, and outcomes data were collected on all patients from 2005 to 2016. Total morphine equivalent (MEQ) on postoperative days (PODs) 1, 2, and 3 were compared among patients with SA, PVC, and TEA. A multivariable zero-inflated log-linear regression was used to determine the association between analgesic modality and MEQ. Multivariable logistic regression models were used to determine associations between analgesic modality and postoperative pain, rates of discharge from intensive care within 1 day and opioid-related adverse events. RESULTS: The study cohort included 355 patients: 177 retroperitoneal and 178 transperitoneal repairs; 173 patients underwent SA, 117 PVC, and 65 TEA. On POD1, median MEQs were 984 (interquartile range [IQR], 342-1525) for SA, 89 (33-246) for PVC, and 49 (0-90) for TEA. On POD2, the median MEQs were 105 (IQR, 57-210) for SA, 45 (15-99) for PVC, and 30 (0-64) for TEA. On POD3, the median MEQs were 45 (IQR, 15-120) for SA, 30 (0-60) for PVC, and 10 (0-45) for TEA. On multivariable log-linear regression, compared with SA, PVC and TEA were associated with increased odds of receiving no opioids on POD1 (odds ratio [OR], 66.85; 95% confidence interval [CI], 17.49-255.57; and OR, 214.68; 95% CI, 60.20-766.38; respectively), POD 2 (OR, 6.97; 95% CI, 3.61-13.46; and OR, 28.73; 95% CI, 15.68-52.62; respectively), and POD 3 (OR, 3.93; 95% CI, 2.72-5.67; and OR, 4.68; 95% CI, 3.20-6.86; respectively). If patients did receive opioids, compared with SA, PVC and TEA were associated with decreased consumption on POD1 (RR, 0.22; 95% CI, 0.18-0.27; and RR, 0.16; 95% CI, 0.12-0.20; respectively), POD2 (RR, 0.50; 95% CI, 0.42-0.58; and RR, 0.46; 95% CI, 0.37-0.56; respectively), and POD3 (RR, 0.78; 95% CI, 0.66-0.93; and RR, 0.76; 95% CI, 0.63-0.93; respectively). Compared with SA, PVC was associated with earlier discharge from intensive care (OR, 2.75; 95% CI, 1.17-6.45) and TEA was not (OR, 1.12; 95% CI, 0.56-2.2). Compared with TEA, PVC was not associated with increased rate of opioid-related adverse events (OR, 0.44; 95% CI, 0.08-2.44). CONCLUSIONS: PVC and TEA are associated with decreased MEQ compared with SA. PVC is associated with earlier discharge from intensive care compared with SA and similar rates of opioid-related adverse events compared with TEA. Paravertebral analgesia appears to be a safe and effective analgesic modality in patients undergoing retroperitoneal approach for abdominal aneurysm repair.

Inhibition of serine and proline racemases by substrate-product analogues.

d-Amino acids can play important roles as specific biosynthetic building blocks required by organisms or act as regulatory molecules. Consequently, amino acid racemases that catalyze the formation of d-amino acids are potential therapeutic targets. Serine racemase catalyzes the reversible formation of d-serine (a modulator of neurotransmission) from l-serine, while proline racemase (an essential enzymatic and mitogenic protein in trypanosomes) catalyzes the reversible conversion of l-proline to d-proline. We show the substrate-product analogue α-(hydroxymethyl)serine is a modest, linear mixed-type inhibitor of serine racemase from Schizosaccharomyces pombe (Ki=167±21mM, Ki'=661±81mM, cf. Km=19±2mM). The bicyclic substrate-product analogue of proline, 7-azabicyclo[2.2.1]heptan-7-ium-1-carboxylate is a weak inhibitor of proline racemase from Clostridium sticklandii, giving only 29% inhibition at 142.5mM. However, the more flexible bicyclic substrate-product analogue tetrahydro-1H-pyrrolizine-7a(5H)-carboxylate is a noncompetitive inhibitor of proline racemase from C. sticklandii (Ki=111±15mM, cf. Km=5.7±0.5mM). These results suggest that substrate-product analogue inhibitors of racemases may only be effective when the active site is capacious and/or plastic, or when the inhibitor is sufficiently flexible.