Richard Ang

Brainstem Hypoxia Contributes to the Development of Hypertension in the Spontaneously Hypertensive Rat

Systemic arterial hypertension has been previously suggested to develop as a compensatory condition when central nervous perfusion/oxygenation is compromised. Principal sympathoexcitatory C1 neurons of the rostral ventrolateral medulla oblongata (whose activation increases sympathetic drive and the arterial blood pressure) are highly sensitive to hypoxia, but the mechanisms of this O2 sensitivity remain unknown. Here, we investigated potential mechanisms linking brainstem hypoxia and high systemic arterial blood pressure in the spontaneously hypertensive rat. Brainstem parenchymal PO2 in the spontaneously hypertensive rat was found to be ≈15 mm Hg lower than in the normotensive Wistar rat at the same level of arterial oxygenation and systemic arterial blood pressure. Hypoxia-induced activation of rostral ventrolateral medulla oblongata neurons was suppressed in the presence of either an ATP receptor antagonist MRS2179 or a glycogenolysis inhibitor 1,4-dideoxy-1,4-imino-d-arabinitol, suggesting that sensitivity of these neurons to low PO2 is mediated by actions of extracellular ATP and lactate. Brainstem hypoxia triggers release of lactate and ATP which produce excitation of C1 neurons in vitro and increases sympathetic nerve activity and arterial blood pressure in vivo. Facilitated breakdown of extracellular ATP in the rostral ventrolateral medulla oblongata by virally-driven overexpression of a potent ectonucleotidase transmembrane prostatic acid phosphatase results in a significant reduction in the arterial blood pressure in the spontaneously hypertensive rats (but not in normotensive animals). These results suggest that in the spontaneously hypertensive rat, lower PO2 of brainstem parenchyma may be associated with higher levels of ambient ATP and L-lactate within the presympathetic circuits, leading to increased central sympathetic drive and concomitant sustained increases in systemic arterial blood pressure.

Control of ventricular excitability by neurons of the dorsal motor nucleus of the vagus nerve

Background The central nervous origins of functional parasympathetic innervation of cardiac ventricles remain controversial. Objective This study aimed to identify a population of vagal preganglionic neurons that contribute to the control of ventricular excitability. An animal model of synuclein pathology relevant to Parkinson’s disease was used to determine whether age-related loss of the activity of the identified group of neurons is associated with changes in ventricular electrophysiology. Methods In vivo cardiac electrophysiology was performed in anesthetized rats in conditions of selective inhibition of the dorsal vagal motor nucleus (DVMN) neurons by pharmacogenetic approach and in mice with global genetic deletion of all family members of the synuclein protein. Results In rats anesthetized with urethane (in conditions of systemic beta-adrenoceptor blockade), muscarinic and neuronal nitric oxide synthase blockade confirmed the existence of a tonic parasympathetic control of cardiac excitability mediated by the actions of acetylcholine and nitric oxide. Acute DVMN silencing led to shortening of the ventricular effective refractory period (vERP), a lowering of the threshold for triggered ventricular tachycardia, and prolongation of the corrected QT (QTc) interval. Lower resting activity of the DVMN neurons in aging synuclein-deficient mice was found to be associated with vERP shortening and QTc interval prolongation. Conclusion Activity of the DVMN vagal preganglionic neurons is responsible for tonic parasympathetic control of ventricular excitability, likely to be mediated by nitric oxide. These findings provide the first insight into the central nervous substrate that underlies functional parasympathetic innervation of the ventricles and highlight its vulnerability in neurodegenerative diseases.

Impact of variant pulmonary vein anatomy and image integration on long-term outcome after catheter ablation for atrial fibrillation.

AIMS To investigate the impact of variant pulmonary vein (PV) anatomy and the use of three-dimensional image integration (3D-II) on long-term efficacy of catheter ablation for atrial fibrillation (AF). METHODS Consecutive procedures from 2002 to 2007 were analysed from a prospective database. All patients underwent wide area circumferential ablation, with linear lesions added and complex fractionated electrograms targeted for persistent AF. Imaging was segmented on Carto to assess PV anatomy. RESULTS Three hundred and fifty patients underwent 1.9 ± 0.9 procedures. The mean age was 57 ± 11 years, 73% males, and 55% paroxysmal AF. Freedom from AF/atrial tachycardia was 42% for paroxysmal AF and 20% for persistent AF at 3.1 years after the first procedure, or 86 and 66%, respectively, at 2.5 years after the last procedure. The Kaplan-Meier analysis showed a trend towards improved single-procedure efficacy with 3D-II (8.9% difference, P = 0.087) and a reduction in the number of procedures per patient from 2.1 ± 1.1 to 1.8 ± 0.9 (P < 0.0001). The use of 3D-II improved single-procedure efficacy with Carto (13.3% difference, P = 0.018), but not with Ensite NavX. Variant PV anatomy was identified in 28% and was associated with a lower single-procedure efficacy (10.0% difference, P = 0.024) but with no effect on final outcome. Multivariate analysis confirmed the impact of 3D-II [hazard ratio (HR) for recurrence of AF 0.67, P = 0.020] and variant PV anatomy (HR 1.37, P = 0.044). CONCLUSION The use of 3D-II improves single-procedure efficacy of PV isolation for AF. Variant PV anatomy was associated with a lower single-procedure success rate.

The ATP-Sensitive Potassium Channel Subunit, Kir6.1, in Vascular Smooth Muscle Plays a Major Role in Blood Pressure Control

ATP-sensitive potassium channels (KATP) regulate a range of biological activities by coupling membrane excitability to the cellular metabolic state. In particular, it has been proposed that KATP channels and specifically, the channel subunits Kir6.1 and SUR2B, play an important role in the regulation of vascular tone. However, recent experiments have suggested that KATP channels outside the vascular smooth muscle compartment are the key determinant of the observed behavior. Thus, we address the importance of the vascular smooth muscle KATP channel, using a novel murine model in which it is possible to conditionally delete the Kir6.1 subunit. Using a combination of molecular, electrophysiological, in vitro, and in vivo techniques, we confirmed the absence of Kir6.1 and KATP currents and responses specifically in smooth muscle. Mice with conditional deletion of Kir6.1 showed no obvious arrhythmic phenotype even after provocation with ergonovine. However, these mice were hypertensive and vascular smooth muscle cells failed to respond to vasodilators in a normal fashion. Thus, Kir6.1 underlies the vascular smooth muscle KATP channel and has a key role in vascular reactivity and blood pressure control.

The Role of Inhibitory G Proteins and Regulators of G Protein Signaling in the in vivo Control of Heart Rate and Predisposition to Cardiac Arrhythmias.

Inhibitory heterotrimeric G proteins and the control of heart rate. The activation of cell signaling pathways involving inhibitory heterotrimeric G proteins acts to slow the heart rate via modulation of ion channels. A large number of Regulators of G protein signalings (RGSs) can act as GTPase accelerating proteins to inhibitory G proteins and thus it is important to understand the network of RGS\G-protein interaction. We will review our recent findings on in vivo heart rate control in mice with global genetic deletion of various inhibitory G protein alpha subunits. We will discuss potential central and peripheral contributions to the phenotype and the controversies in the literature.

Brain metabolic sensing and metabolic signaling at the level of an astrocyte

Astrocytes support neuronal function by providing essential structural and nutritional support, neurotransmitter trafficking and recycling and may also contribute to brain information processing. In this article we review published results and report new data suggesting that astrocytes function as versatile metabolic sensors of central nervous system (CNS) milieu and play an important role in the maintenance of brain metabolic homeostasis. We discuss anatomical and functional features of astrocytes that allow them to detect and respond to changes in the brain parenchymal levels of metabolic substrates (oxygen and glucose), and metabolic waste products (carbon dioxide). We report data suggesting that astrocytes are also sensitive to circulating endocrine signals—hormones like ghrelin, glucagon‐like peptide‐1 and leptin, that have a major impact on the CNS mechanisms controlling food intake and energy balance. We discuss signaling mechanisms that mediate communication between astrocytes and neurons and consider how these mechanisms are recruited by astrocytes activated in response to various metabolic challenges. We review experimental data suggesting that astrocytes modulate the activities of the respiratory and autonomic neuronal networks that ensure adaptive changes in breathing and sympathetic drive in order to support the physiological and behavioral demands of the organism in ever‐changing environmental conditions. Finally, we discuss evidence suggesting that altered astroglial function may contribute to the pathogenesis of disparate neurological, respiratory and cardiovascular disorders such as Rett syndrome and systemic arterial hypertension.

Pulmonary vein measurements on pre-procedural CT/MR imaging can predict difficult pulmonary vein isolation and phrenic nerve injury during cryoballoon ablation for paroxysmal atrial fibrillation

OBJECTIVE We tested the hypothesis that pulmonary vein (PV) measurements on pre-procedural CT/MR imaging can predict difficulty in isolation and phrenic nerve (PN) injury during cryoballoon ablation for paroxysmal atrial fibrillation (AF). METHODS Consecutive patients with paroxysmal AF who had pre-procedural CT/MRI and underwent cryoballoon ablation as part of a randomized trial were studied. Imaging was anonymized for blinded analysis of: (1) maximum ostial diameter, (2) minimum ostial diameter, (3) ostial area and (4) ratio of maximum over minimum ostial diameter (eccentricity index). Veins that required more than 2 freezes of at least 200 s duration to isolate or not isolated were defined as difficult to isolate. Loss of PN pacing during right-sided ablation was defined as PN injury. Logistic regression was used to analyze the predictive effect of the measurements on the 2 outcomes. RESULTS 148 PVs in 38 patients (aged 60 ± 11 years, 76% male) were analyzed. Left inferior PV (LIPV) was most difficult to isolate with 23 out of 37 PVs (62%), and PN injury occurred in 3 of 38 (8%) right superior PV (RSPV). Greater eccentricity index predicted difficulty in isolating LIPV, OR 40.33 (95% CI 1.40 to 1160, p = 0.03) and smaller eccentricity index predicted PN injury in RSPV, OR 0.01 (95% CI 0.01-0.16, p = 0.001). CONCLUSIONS Eccentricity index measured from pre-procedural CT/MR imaging can predict difficulty of PV isolation and PN injury during cryoballoon ablation for paroxysmal AF.

The in vivo regulation of heart rate in the murine sinoatrial node by stimulatory and inhibitory heterotrimeric G proteins

Reciprocal physiological modulation of heart rate is controlled by the sympathetic and parasympathetic systems acting on the sinoatrial (SA) node. However, there is little direct in vivo work examining the role of stimulatory and inhibitory G protein signaling in the SA node. Thus, we designed a study to examine the role of the stimulatory (Gαs) and inhibitory G protein (Gαi2) in in vivo heart rate regulation in the SA node in the mouse. We studied mice with conditional deletion of Gαs and Gαi2 in the conduction system using cre-loxP technology. We crossed mice in which cre recombinase expression was driven by a tamoxifen-inducible conduction system-specific construct with “Gαs floxed” and “Gαi2 floxed” mice. We studied the heart rate responses of adult mice compared with littermate controls by using radiotelemetry before and after administration of tamoxifen. The mice with conditional deletion of Gαs and Gαi2 had a loss of diurnal variation and were bradycardic or tachycardic, respectively, in the daytime. In mice with conditional deletion of Gαs, there was a selective loss of low-frequency power, while with deletion of Gαi2, there was a loss of high-frequency power in power spectral analysis of heart rate variability. There was no evidence of pathological arrhythmia. Pharmacological modulation of heart rate by isoprenaline was impaired in the Gαs mice, but a muscarinic agonist was still able to slow the heart rate in Gαi2 mice. We conclude that Gαs- and Gαi2-mediated signaling in the sinoatrial node is important in the reciprocal regulation of heart rate through the autonomic nervous system.

The Hot and the Cold: Radiofrequency Versus Cryoballoon Ablation for Atrial Fibrillation

Catheter ablation is superior to antiarrhythmic drugs in maintaining sinus rhythm for patients with atrial fibrillation (AF). Pulmonary vein (PV) isolation is the cornerstone of any AF ablation procedure. Conventionally, this is achieved by performing point by point lesions using radiofrequency (RF) energy. However, this is technically challenging, time consuming and is associated with a number of complications. Long-term durability of PV isolation is also a concern. To address these issues, ‘one-shot’ energy delivery systems and alternative energy sources have been developed. The cryoballoon system has emerged as the most commonly used alternative to point by point RF technology. In this paper, we compare the technology, biophysics and clinical data of cryoballoon to conventional RF ablation for AF. The safety and efficacy of cryoballoon compared to RF ablation is critically reviewed. We conclude by looking at future applications of this technology.

Highlights in basic autonomic neurosciences: remote ischaemic preconditioning as an autonomic reflex--a question of timing and circumstances?

Myocardial ischaemia–reperfusion injury was induced in pentobarbital anaesthetised rats by ligating the left anterior descending (LAD) coronary artery for 30 min followed by 60 min of reperfusion. Remote ischaemic conditioning (RIc) was performed by temporary bilateral occlusion of femoral arteries for 15 min. The investigators found a reduction in infarct size by about 50% when RIc stimulus was applied 25 min prior to myocardial ischaemia, 10 or 25 min after the onset of ischaemia and 10 min after the onset of reperfusion. There was no reduction in infarct sizewhen RIcwas performed 30 min into the reperfusion period. Prior to the onset of myocardial ischaemia, capsaicin administration to the hind paw mimicked the cardioprotective effect of RIc whereas sectioning of sciatic and femoral nerves or bilateral vagotomy both abrogated the cardioprotective effect of RIc. These manoeuvres had no effect on RIc-induced cardioprotectionwhen femoral artery occlusion was applied 10 min after the onset of reperfusion.