Remodeling of cardiac cholinergic innervation and control of heart rate in mice with streptozotocin-induced diabetes

Molecular and cellular neurocardiology in heart disease

This paper updates and builds on a previous White Paper in this journal that some of us contributed to concerning the molecular and cellular basis of cardiac neurobiology of heart disease. Here we focus on recent findings that underpin cardiac autonomic development, novel intracellular pathways and neuroplasticity. Throughout we highlight unanswered questions and areas of controversy. Whilst some neurochemical pathways are already demonstrating prognostic viability in patients with heart failure, we also discuss the opportunity to better understand sympathetic impairment by using patient specific stem cells that provides pathophysiological contextualization to study 'disease in a dish'. Novel imaging techniques and spatial transcriptomics are also facilitating a road map for target discovery of molecular pathways that may form a therapeutic opportunity to treat cardiac dysautonomia.

Cardiac innervations in diabetes mellitus-Anatomical evidence of neuropathy

The extensive innervations of the heart include a complex network of sympathetic, parasympathetic, and sensory nerves connected in loops that serve to regulate cardiac output. Metabolic dysfunction in diabetes affects many different organ systems, including the cardiovascular system; it causes cardiac arrhythmias, silent myocardial ischemia, and sudden cardiac death, among others. These conditions are associated with damage to the nerves that innervate the heart, cardiac autonomic neuropathy (CAN), which is caused by various pathophysiological mechanisms. In this review, the main facts about the anatomy of cardiac innervations and the current knowledge of CAN, its pathophysiological mechanisms, and its diagnostic approach are discussed. In addition, anatomical evidence for CAN from human and animal studies has been summarized.

Atrial Fibrillation and Diabetes Mellitus: Dangerous Liaisons or Innocent Bystanders?

Atrial fibrillation (AF) is the most common arrhythmia in adults and diabetes mellitus (DM) is a major risk factor for cardiovascular diseases. However, the relationship between both pathologies has not been fully documented and new evidence supports the existence of direct and independent links. In the myocardium, a combination of structural, electrical, and autonomic remodeling may lead to AF. Importantly, patients with AF and DM showed more dramatic alterations than those with AF or DM alone, particularly in mitochondrial respiration and atrial remodeling, which alters conductivity, thrombogenesis, and contractile function. In AF and DM, elevations of cytosolic Ca²⁺ and accumulation of extra cellular matrix (ECM) proteins at the interstitium can promote delayed afterdepolarizations. The DM-associated low-grade inflammation and deposition/infiltration of epicardial adipose tissue (EAT) enforce abnormalities in Ca²⁺ handling and in excitation-contraction coupling, leading to atrial myopathy. This atrial enlargement and the reduction in passive emptying volume and fraction can be key for AF maintenance and re-entry. Moreover, the stored EAT can prolong action of potential durations and progression from paroxysmal to persistent AF. In this way, DM may increase the risk of thrombogenesis as a consequence of increased glycation and oxidation of fibrinogen and plasminogen, impairing plasmin conversion and resistance to fibrinolysis. Additionally, the DM-associated autonomic remodeling may also initiate AF and its re-entry. Finally, further evidence of DM influence on AF development and maintenance are based on the anti-arrhythmogenic effects of certain anti-diabetic drugs like SGLT2 inhibitors. Therefore, AF and DM may share molecular alterations related to Ca²⁺ mobility, mitochondrial function and ECM composition that induce atrial remodeling and defects in autonomic stimulation and conductivity. Likely, some specific therapies could work against the associated cardiac damage to AF and/or DM.

Effects of obesity and diabesity on heart rhythm in the Zucker rat

Obesity and type 2 diabetes mellitus are risk factors for hypertension, coronary heart disease, cardiac arrhythmias including atrial fibrillation, heart failure and sudden cardiac death. The effects of obesity and diabesity on heart rhythm were investigated in the Zucker diabetic fatty (ZDF) and Zucker fatty (ZF) compared to the Zucker lean (ZL) control rat. In vivo biotelemetry techniques were used to assess the electrocardiogram and other cardiac and metabolic parameters. ZDF rats were characterized by age-dependent elevations in fasting and non-fasting blood glucose, glucose intolerance and weight gain and ZF rats were characterized by smaller elevations in fasting and non-fasting blood glucose and greater weight gain compared to ZL rats. Heart rate (HR) was progressively reduced in ZDF, ZF and ZL rats. At 195 days (6.5 months) of age there were significant differences in HR between ZDF (265 ± 8 bpm, n = 10), ZF (336 ± 9 bpm, n = 10) and ZL (336 ± 10 bpm, n = 10) rats and significant differences in HRV between ZDF (22 ± 1 bpm, n = 10), ZF (27 ± 1 bpm, n = 10) and ZL (31 ± 1 bpm, n = 10) rats. Power spectral analysis revealed no significant (P > 0.05) differences in HRV at low frequencies, reduced HRV at high frequencies and increased sympathovagal balance in ZDF compared to ZF and ZL rats. HR was reduced by ageing and additionally reduced by diabesity in the absence of changes in physical activity and body temperature. Reductions in HRV associated with altered sympathovagal drive might partly underlie disturbed HR in the ZDF rat. Possible explanations for reduced HR and future mechanistic studies are discussed.

Diabetes, and its treatment, as an effector of autonomic nervous system circuits and its functions

Diabetes increases the risk of cardiovascular complications, including heart failure, hypertension, and stroke. There is a strong involvement of autonomic dysfunction in individuals with diabetes that exhibit clinical manifestations of cardiovascular diseases (CVD). Still, the mechanisms by which diabetes and its treatments alter autonomic function and subsequently affect cardiovascular complications remain elusive. For this reason, understanding the brainstem circuits involved in sensing metabolic state(s) and enacting autonomic control of the cardiovascular system are important to develop more comprehensive therapies for individuals with diabetes at increased risk for CVD. We review how autonomic nervous system circuits change during these disease states and discuss their potential role in current pharmacotherapies that target diabetic states. Overall, this review proposes that the brainstem circuits provide an integrative sensorimotor network capable of responding to metabolic cues to regulate cardiovascular function and this network is modified by, and in turn affects, diabetes-induced CVD and its treatment.

Cardiac innervation in acute myocardial ischaemia/reperfusion injury and cardioprotection

Acute myocardial infarction (AMI) and the heart failure (HF) that often complicates this condition, are among the leading causes of death and disability worldwide. To reduce myocardial infarct (MI) size and prevent heart failure, novel therapies are required to protect the heart against the detrimental effects of acute ischaemia/reperfusion injury (IRI). In this regard, targeting cardiac innervation may provide a novel therapeutic strategy for cardioprotection. A number of cardiac neural pathways mediate the beneficial effects of cardioprotective strategies such as ischaemic preconditioning and remote ischaemic conditioning, and nerve stimulation may therefore provide a novel therapeutic strategy for cardioprotection. In this article, we provide an overview of cardiac innervation and its impact on acute myocardial IRI, the role of extrinsic and intrinsic cardiac neural pathways in cardioprotection, and highlight peripheral and central nerve stimulation as a cardioprotective strategy with therapeutic potential for reducing MI size and preventing HF following AMI. This article is part of a Cardiovascular Research Spotlight Issue entitled 'Cardioprotection Beyond the Cardiomyocyte', and emerged as part of the discussions of the European Union (EU)-CARDIOPROTECTION Cooperation in Science and Technology (COST) Action, CA16225.

Aberrant sinus node firing during β-adrenergic stimulation leads to cardiac arrhythmias in diabetic mice

AIM

Cardiovascular complications, including cardiac arrhythmias, result in high morbidity and mortality in patients with type-2 diabetes mellitus (T2DM). Clinical and experimental data suggest electrophysiological impairment of the natural pacemaker of the diabetic heart. The present study examined sinoatrial node (SAN) arrhythmias in a mouse model of T2DM and physiologically probed their underlying cause.

METHODS

Electrocardiograms were obtained from conscious diabetic db/db and lean control db/+ mice. In vivo SAN function was probed through pharmacological autonomic modulation with isoprenaline, atropine and carbachol. Blood pressure stability and heart rate variability (HRV) were evaluated. Intrinsic SAN function was evaluated through ex vivo imaging of spontaneous Ca²⁺ transients in isolated SAN preparations.

RESULTS

While lean control mice showed constant RR intervals during isoprenaline challenge, the diabetic mice experienced SAN arrhythmias with large RR fluctuations in a dose-dependent manner. These arrhythmias were completely abolished by atropine pre-treatment, while carbachol pretreatment significantly increased SAN arrhythmia frequency in the diabetic mice. Blood pressure and HRV were comparable in db/db and db/+ mice, suggesting that neither augmented baroreceptor feedback nor autonomic neuropathy is a likely arrhythmia mechanism. Cycle length response to isoprenaline was comparable in isolated SAN preparations from db/db and db/+ mice; however, Ca²⁺ spark frequency was significantly increased in db/db mice compared to db/+ at baseline and after isoprenaline.

CONCLUSION

Our results demonstrate a dysfunction of cardiac pacemaking in an animal model of T2DM upon challenge with a β-adrenergic agonist. Ex vivo, higher Ca²⁺ spark frequency is present in diabetic mice, which may be directly linked to in vivo arrhythmias.

Pulmonary Vein Ganglia Are Remodeled in the Diabetic Heart

Background Cardiac autonomic neuropathy is thought to cause adverse cardiovascular effects in diabetes mellitus. Pulmonary vein ganglia ( PVG ), which have been implicated in normal and abnormal heart rhythm regulation, have not been fully investigated in type 1 diabetes mellitus (T1D). We examined the functional and anatomical effects of T1D on PVG and studied the details of T1D-induced remodeling on the PVG structure and function. Methods and Results We used a mouse model of T1D (Akita mouse), immunofluorescence, isolated Langendorff-perfused hearts, and mathematical simulations to explore the effects of T1D on PVG . Whole-mount atrial immunofluorescence of choline acetyltransferase and tyrosine hydroxylase labeling showed that sympathetic and parasympathetic somas of the PVG neurons were significantly hypotrophied in T1D hearts versus wild type. Stimulation of PVG in isolated Langendorff-perfused hearts caused more pronounced P-P interval prolongation in wild type compared with Akita hearts. Propranolol resulted in a comparable P-P prolongation in both phenotypes, and atropine led to more pronounced P-P interval shortening in wild type compared with Akita hearts. Numerical modeling using network simulations revealed that a decrease in the sympathetic and parasympathetic activities of PVG in T1D could explain the experimental results. Conclusions T1D leads to PVG remodeling with hypotrophy of sympathetic and parasympathetic cell bodies and a concomitant decrease in the PVG sympathetic and parasympathetic activities.

Increased arrhythmia susceptibility in type 2 diabetic mice related to dysregulation of ventricular sympathetic innervation

Patients with type 2 diabetes mellitus (T2DM) have a greater risk of developing life-threatening cardiac arrhythmias. Because the underlying mechanisms and potential influence of diabetic autonomic neuropathy are not well understood, we aimed to assess the relevance of a dysregulation in cardiac autonomic tone. Ventricular arrhythmia susceptibility was increased in Langendorff-perfused hearts isolated from mice with T2DM (db/db). Membrane properties and synaptic transmission were similar at cardiac postganglionic parasympathetic neurons from diabetic and control mice; however, a greater asynchronous neurotransmitter release was present at sympathetic postganglionic neurons from the stellate ganglia of db/db mice. Western blot analysis showed a reduction of tyrosine hydroxylase (TH) from the ventricles of db/db mice, which was confirmed with confocal imaging as a heterogeneous loss of TH-immunoreactivity from the left ventricular wall but not the apex. In vivo stimulation of cardiac parasympathetic (vagus) or cardiac sympathetic (stellate ganglion) nerves induced similar changes in heart rate in control and db/db mice, and the kinetics of pacing-induced Ca²⁺ transients (recorded from isolated cardiomyocytes) were similar in control and db/db cells. Antagonism of cardiac muscarinic receptors did not affect the frequency or severity of arrhythmias in db/db mice, but sympathetic blockade with propranolol completely inhibited arrhythmogenicity. Collectively, these findings suggest that the increased ventricular arrhythmia susceptibility of type 2 diabetic mouse hearts is due to dysregulation of the sympathetic ventricular control.NEW & NOTEWORTHY Patients with type 2 diabetes mellitus have greater risk of suffering from sudden cardiac death. We found that the increased ventricular arrhythmia susceptibility in type 2 diabetic mouse hearts is due to cardiac sympathetic dysfunction. Sympathetic dysregulation is indicated by an increased asynchronous release at stellate ganglia, a heterogeneous loss of tyrosine hydroxylase from the ventricular wall but not apex, and inhibition of ventricular arrhythmias in db/db mice after β-sympathetic blockade.

Data Linking Diabetes Mellitus and Atrial Fibrillation-How Strong Is the Evidence? From Epidemiology and Pathophysiology to Therapeutic Implications

According to estimates, around 5% of the world population has hazel eyes. And there are about as many people with diabetes mellitus (DM). Red hair occurs naturally in up to 2% of the human population. And about as many people are estimated to have atrial fibrillation (AF). If a hazel eyed person with red hair does not surprise us, should a diabetic patient with AF? Accumulating epidemiologic data suggest, however, that the DM-AF association may be more than a simple coincidence. But, how strong is this evidence? Experimental studies bring evidence for a DM-induced atrial proarrhythmic remodelling. But how relevant are these data for the clinical setting? In this review, we aim to provide a critical analysis of the existing clinical and experimental, epidemiologic, and mechanistic data that bridge DM and AF, we emphasize a number of questions that remain to be answered, and we identify hotspots for future research. The therapeutic implications of the DM-AF coexistence are also discussed, with a focus on rhythm control and on conventional and DM-specific upstream therapies for AF management.

Zymosan-Induced Peritonitis: Effects on Cardiac Function, Temperature Regulation, Translocation of Bacteria, and Role of Dectin-1

Zymosan-induced peritonitis is a model commonly used to study systemic inflammatory response syndrome and multiple organ dysfunction syndrome. However, effects of zymosan on cardiac function have not been reported. We evaluated cardiac responses to zymosan in mice and the role of β-glucan and dectin-1 in mediating these responses. Temperature and cardiac function were evaluated before and after intraperitoneal (i.p.) injection of zymosan (100 or 500 mg/kg) or saline. Chronotropic and dromotropic functions were measured using electrocardiograms (ECGs) collected from conscious mice. Cardiac inotropic function was determined by echocardiography. High-dose zymosan caused a rapid and maintained hypothermia along with visual signs of illness. Baseline heart rate (HR) was unaffected but HR variability (HRV) increased, and there was a modest slowing of ventricular conduction. High-dose zymosan also caused prominent decreases in cardiac contractility at 4 and 24 h. Because zymosan is known to cause gastrointestinal tract pathology, peritoneal wash and blood samples were evaluated for bacteria at 24 h after zymosan or saline injection. Translocation of bacterial occurred in all zymosan-treated mice (n = 3), and two had bacteremia. Purified β-glucan (50 and 125 mg/kg, i.p.) had no effect on temperature or ECG parameters. However, deletion of dectin-1 modified the ECG responses to high-dose zymosan; slowing of ventricular conduction and the increase in HRV were eliminated but a marked bradycardia appeared at 24 h after zymosan treatment. Zymosan-treated dectin-1 knockout mice also showed hypothermia and visual signs of illness. Fecal samples from dectin-1 knockout mice contained more bacteria than wild types, but zymosan caused less translocation of bacteria. Collectively, these findings demonstrate that zymosan-induced systemic inflammation causes cardiac dysfunction in mice. The data suggest that dectin-1-dependent and -independent mechanisms are involved. Although zymosan treatment causes translocation of bacteria, this effect does not have a major role in the overall systemic response to zymosan.

Engineered Context-Sensitive Agonism: Tissue-Selective Drug Signaling through a G Protein-Coupled Receptor

Drug discovery strives for selective ligands to achieve targeted modulation of tissue function. Here we introduce engineered context-sensitive agonism as a postreceptor mechanism for tissue-selective drug action through a G protein-coupled receptor. Acetylcholine M₂-receptor activation is known to mediate, among other actions, potentially dangerous slowing of the heart rate. This unwanted side effect is one of the main reasons that limit clinical application of muscarinic agonists. Herein we show that dualsteric (orthosteric/allosteric) agonists induce less cardiac depression ex vivo and in vivo than conventional full agonists. Exploration of the underlying mechanism in living cells employing cellular dynamic mass redistribution identified context-sensitive agonism of these dualsteric agonists. They translate elevation of intracellular cAMP into a switch from full to partial agonism. Designed context-sensitive agonism opens an avenue toward postreceptor pharmacologic selectivity, which even works in target tissues operated by the same subtype of pharmacologic receptor.

Deletion of neurturin impairs development of cholinergic nerves and heart rate control in postnatal mouse hearts

The neurotrophic factor neurturin is required for normal cholinergic innervation of adult mouse heart and bradycardic responses to vagal stimulation. Our goals were to determine effects of neurturin deletion on development of cardiac chronotropic and dromotropic functions, vagal baroreflex response, and cholinergic nerve density in nodal regions of postnatal mice. Experiments were performed on postnatal C57BL/6 wild-type (WT) and neurturin knockout (KO) mice. Serial electrocardiograms were recorded noninvasively from conscious pups using an ECGenie apparatus. Mice were treated with atenolol to evaluate and block sympathetic effects on heart rate (HR) and phenylephrine (PE) to stimulate the baroreflex. Immunohistochemistry was used to label cholinergic nerves in paraffin sections. WT and KO mice showed similar age-dependent increases in HR and decreases in PR interval between postnatal days (P) 2.5 and 21. Treatment with atenolol reduced HR significantly in WT and KO pups at P7.5. PE caused a reflex bradycardia that was significantly smaller in KO pups. Cholinergic nerve density was significantly less in nodal regions of P7.5 KO mice. We conclude that cholinergic nerves have minimal influence on developmental changes in HR and PR, QRS, and QTc intervals in mouse pups. However, cholinergic nerves mediate reflex bradycardia by 1 week postnatally. Deletion of neurturin impairs cholinergic innervation of the heart and the vagal efferent component of the baroreflex early during postnatal development.

Glutamatergic drive facilitates synaptic inhibition of dorsal vagal motor neurons after experimentally induced diabetes in mice

The role of central regulatory circuits in modulating diabetes-associated glucose dysregulation has only recently been under rigorous investigation. One brain region of interest is the dorsal motor nucleus of the vagus (DMV), which contains preganglionic parasympathetic motor neurons that regulate subdiaphragmatic visceral function. Previous research has demonstrated that glutamatergic and GABAergic neurotransmission are independently remodeled after chronic hyperglycemia/hypoinsulinemia. However, glutamatergic circuitry within the dorsal brain stem impinges on GABAergic regulation of the DMV. The present study investigated the role of glutamatergic neurotransmission in synaptic GABAergic control of DMV neurons after streptozotocin (STZ)-induced hyperglycemia/hypoinsulinemia by using electrophysiological recordings in vitro. The frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) was elevated in DMV neurons from STZ-treated mice. The effect was abolished in the presence of the ionotropic glutamate receptor blocker kynurenic acid or the sodium channel blocker tetrodotoxin, suggesting that after STZ-induced hyperglycemia/hypoinsulinemia, increased glutamatergic receptor activity occurs at a soma-dendritic location on local GABA neurons projecting to the DMV. Although sIPSCs in DMV neurons normally demonstrated considerable amplitude variability, this variability was significantly increased after STZ-induced hyperglycemia/hypoinsulinemia. The elevated amplitude variability was not related to changes in quantal release, but rather correlated with significantly elevated frequency of sIPSCs in these mice. Taken together, these findings suggest that GABAergic regulation of central vagal circuitry responsible for the regulation of energy homeostasis undergoes complex functional reorganization after several days of hyperglycemia/hypoinsulinemia, including both glutamate-dependent and -independent forms of plasticity.

Molecular and cellular neurocardiology: development, and cellular and molecular adaptations to heart disease

The nervous system and cardiovascular system develop in concert and are functionally interconnected in both health and disease. This white paper focuses on the cellular and molecular mechanisms that underlie neural-cardiac interactions during development, during normal physiological function in the mature system, and during pathological remodelling in cardiovascular disease. The content on each subject was contributed by experts, and we hope that this will provide a useful resource for newcomers to neurocardiology as well as aficionados.

Heart Rate Variability as Early Biomarker for the Evaluation of Diabetes Mellitus Progress

According to the American Diabetes Association (ADA), the side effects of diabetes mellitus have recently increased the global health expenditure each year. Of these, the early diagnostic can contribute to the decrease on renal, cardiovascular, and nervous systems complications. However, the diagnostic criteria, which are commonly used, do not suggest the diabetes progress in the patient. In this study, the streptozotocin model in mice (cDM) was used as early diagnostic criterion to reduce the side effects related to the illness. The results showed some clinical signs similarly to five-year diabetes progress without renal injury, neuropathies, and cardiac neuropathy autonomic in the cDM-model. On the other hand, the electrocardiogram was used to determine alterations in heart rate and heart rate variability (HRV), using the Poincaré plot to quantify the HRV decrease in the cDM-model. Additionally, the SD1/SD2 ratio and ventricular arrhythmias showed increase without side effects of diabetes. Therefore, the use of HRV as an early biomarker contributes to evaluating diabetes mellitus complications from the diagnostic.

Differential impact of type-1 and type-2 diabetes on control of heart rate in mice

AIMS

Cardiac autonomic dysfunction is a serious complication of diabetes. One consequence is disruption of the normal beat-to-beat regulation of heart rate (HR), i.e. HR variability (HRV). However, our understanding of the disease process has been limited by inconsistent HR/HRV data from previous animal studies. We hypothesized that differences in the method of measurement, time of day, and level of stress account for the differing results across studies. Thus, our aim was to systematically assess HR and HRV in two common diabetic mouse models.

METHODS

ECG radiotelemetry devices were implanted into db/db (type-2 diabetic), STZ-treated db/+ (type-1 diabetic), and control db/+ mice (n=4 per group). HR and HRV were analyzed over 24 h and during treadmill testing.

RESULTS

24 h analysis revealed that db/db mice had an altered pattern of circadian HR changes, and STZ-treated mice had reduced HR throughout. HRV measures linked to sympathetic control were reduced in db/db mice in the early morning and early afternoon, and partially reduced in STZ-treated mice. HR response to treadmill testing was blunted in both models.

CONCLUSIONS

It is important to consider both time of day and level of stress when assessing HR and HRV in diabetic mice. db/db mice may have altered circadian rhythm of sympathetic control of HR, whereas STZ-treated mice have a relative reduction. This study provides baseline data and a framework for HR analysis that may guide future investigations.

Impaired heart rate regulation and depression of cardiac chronotropic and dromotropic function in polymicrobial sepsis

The scope of cardiac pathophysiology in sepsis has not been fully defined. Accordingly, we evaluated the effects of sepsis on heart rate (HR), HR variability, and conduction parameters in a murine model of sepsis. Electrocardiograms were recorded noninvasively from conscious mice before and after cecal ligation and puncture (CLP) or sham surgery. Responses of isolated atria to tyramine and isoproterenol were quantified to assess the functional state of sympathetic nerves and postjunctional sensitivity to adrenergic stimulation. Cecal ligation and puncture mice had lower HR compared with sham at 16 to 18 h postsurgery (sham, 741 ± 7 beats/min; CLP, 557 ± 31 beats/min; n = 6/group; P < 0.001), and there was significant prolongation of the PR, QRS, and QTc intervals. Slowing of HR and conduction developed within 4 to 6 h after CLP and were preceded by a decrease in HR variability. Treatment of CLP mice with isoproterenol (5 mg/kg, intraperitoneally) at 25 h after surgery failed to increase HR or decrease conduction intervals. The lack of in vivo response to isoproterenol cannot be attributed to hypothermia because robust chronotropic and inotropic responses to isoproterenol were evoked from isolated atria at 25 °C and 30 °C. These findings demonstrate that impaired regulation of HR (i.e., reduced HR variability) develops before the onset of overt cardiac rate and conduction changes in septic mice. Subsequent time-dependent decreases in HR and cardiac conduction can be attributed to hypothermia and would contribute to decreased cardiac output and organ perfusion. Because isolated atria from septic mice showed normal responsiveness to adrenergic stimulation, we conclude that impaired effectiveness of isoproterenol in vivo can be attributed to reversible effects of systemic factors on adrenergic receptors and/or postreceptor signaling.

Diabetes mellitus and atrial remodeling: mechanisms and potential upstream therapies

Atrial fibrillation (AF) is the most common cardiac arrhythmia in clinical practice, and its prevalence has increasing substantially over the last decades. Recent data suggest that there is an increased risk of AF among the patients with diabetes mellitus (DM). However, the potential molecular mechanisms regarding DM-related AF and diabetic atrial remodeling are not fully understood. In this comprehensive review, we would like to summarize the potential relationship between diabetes and atrial remodeling, including structural, electrical, and autonomic remodeling. Also, some upstream therapies, such as thiazolidinediones, probucol, ACEI/ARBs, may play an important role in the prevention and treatment of AF. Therefore, large prospective randomized, controlled trials and further experimental studies should be challengingly continued.

Diabetes induces GABA receptor plasticity in murine vagal motor neurons

Autonomic dysregulation accompanies type-1 diabetes, and synaptic regulation of parasympathetic preganglionic motor neurons in the dorsal motor nucleus of the vagus (DMV) is altered after chronic hyperglycemia/hypoinsulinemia. Tonic gamma-aminobutyric acid A (GABAA) inhibition prominently regulates DMV neuron activity, which contributes to autonomic control of energy homeostasis. This study investigated persistent effects of chronic hyperglycemia/hypoinsulinemia on GABAA receptor-mediated inhibition in the DMV after streptozotocin-induced type-1 diabetes using electrophysiological recordings in vitro, quantitative (q)RT-PCR, and immunohistochemistry. Application of the nonspecific GABAA receptor agonist muscimol evoked an outward current of significantly larger amplitude in DMV neurons from diabetic mice than controls. Results from application of 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride (THIP), a δ-subunit agonist, suggested that GABAA receptors containing δ-subunits contributed to the enhanced inducible tonic GABA current in diabetic mice. Sensitivity to THIP of inhibitory postsynaptic currents in DMV neurons from diabetic mice was also increased. Results from qRT-PCR and immunohistochemical analyses indicated that the altered GABAergic inhibition may be related to increased trafficking of GABAA receptors that contain the δ-subunit, rather than an expression change. Overall these findings suggest increased sensitivity of δ-subunit containing GABAA receptors after several days of hyperglycemia/hypoinsulinemia, which dramatically alters GABAergic inhibition of DMV neurons and could contribute to diabetic autonomic dysregulation.

Sinoatrial node dysfunction induces cardiac arrhythmias in diabetic mice

BACKGROUND

The aim of this study was to probe cardiac complications, including heart-rate control, in a mouse model of type-2 diabetes. Heart-rate development in diabetic patients is not straight forward: In general, patients with diabetes have faster heart rates compared to non-diabetic individuals, yet diabetic patients are frequently found among patients treated for slow heart rates. Hence, we hypothesized that sinoatrial node (SAN) dysfunction could contribute to our understanding of the mechanism behind this conundrum and the consequences thereof.

METHODS

Cardiac hemodynamic and electrophysiological characteristics were investigated in diabetic db/db and control db/+ mice.

RESULTS

We found improved contractile function and impaired filling dynamics of the heart in db/db mice, relative to db/+ controls. Electrophysiologically, we observed comparable heart rates in the two mouse groups, but SAN recovery time was prolonged in diabetic mice. Adrenoreceptor stimulation increased heart rate in all mice and elicited cardiac arrhythmias in db/db mice only. The arrhythmias emanated from the SAN and were characterized by large RR fluctuations. Moreover, nerve density was reduced in the SAN region.

CONCLUSIONS

Enhanced systolic function and reduced diastolic function indicates early ventricular remodeling in obese and diabetic mice. They have SAN dysfunction, and adrenoreceptor stimulation triggers cardiac arrhythmia originating in the SAN. Thus, dysfunction of the intrinsic cardiac pacemaker and remodeling of the autonomic nervous system may conspire to increase cardiac mortality in diabetic patients.

Effect of Guizhi Decoction ([symbols; see text]) on heart rate variability and regulation of cardiac autonomic nervous imbalance in diabetes mellitus rats

OBJECTIVE

To observe abnormalities in heart rate variability (HRV) in diabetic rats and to explore the effects of treatment with Guizhi Decoction ([symbols; see text]) on cardiac autonomic nervous (CAN) imbalance.

METHODS

A radio-telemetry system for monitoring physiological parameters was implanted into rats to record electrocardiac signals and all indictors of HRV [time domain measures: standard deviation of all RR intervals in 24 h (SDNN), root mean square of successive differences (RMSSD), percentage of differences between adjacent RR intervals greater than 50 ms (PNN50), and standard deviation of the averages of RR intervals (SDANN); frequency domain measures: low frequency (LF), high frequency (HF), total power (TP), and LF/HF ratio]. The normal group was randomly selected, and the remaining rats were used to establish streptozocin (STZ)-induced diabetic model. After 4 weeks, the model rats were divided into the model group, the methycobal group, and the Guizhi Decoction group, 9 rats in each group. Four weeks after intragastric administration of the corresponding drugs, the right atria of the rats were collected for immunohistochemical staining of tyrosine hydroxylase (TH) and choline acetyltransferase (CHAT) to observe the distribution of the sympathetic and vagus nerves in the right atrium. The myocardial homogenate from the interventricular septum and the left ventricle was used for determination of TH, CHAT, growth-associated protein 43 (GAP-43), nerve growth factor (NGF), and ciliary neurotrophic factor (CNTF) levels using an enzyme-linked immunosorbent assay.

RESULTS

(1) STZ rats had elevated blood glucose levels, reduced body weight, and decreased heart rate; there was no difference between the model group and the drug treated groups. (2) Compared with the model group, only RMSSD and TP increased in the methycobal group significantly (P<0.05); SDNN, RMSSD, PNN50, LF, HF, and TP increased, LF/HF decreased (P<0.05), and SDANN just showed a decreasing trend in the Guizhi Decoction group (P>0.05). TH increased, CHAT decreased, and TH/CHAT increased in the myocardial homogenate of the model group (P<0.05). Compared with the model group, left ventricular TH reduced in the methycobal group; and in the Guizhi Decoction group CHAT increased, while TH and TH/CHAT decreased (P<0.05). Compared with the model group, CNTF in the interventricular septum increased in the methycobal group (P<0.05); GAP-43 increased, NGF decreased, and CNTF increased (P<0.05) in the Guizhi Decoction group. There were significant differences in the reduction of NGF and elevation of CNTF between the Guizhi Decoction group and the methycobal group (P<0.05). (3) Immunohistochemical results showed that TH expression significantly increased and CHAT expression significantly decreased in the myocardia of the model group, whereas TH expression decreased and CHAT expression increased in the Guizhi Decoction group (P<0.05).

CONCLUSION

Guizhi Decoction was effective in improving the function of the vagus nerve, and it could alleviate autonomic nerve damage.

Temporal dystrophic remodeling within the intrinsic cardiac nervous system of the streptozotocin-induced diabetic rat model

Introduction

The pathogenesis of heart failure (HF) in diabetic individuals, called “diabetic cardiomyopathy”, is only partially understood. Alterations in the cardiac autonomic nervous system due to oxidative stress have been implicated. The intrinsic cardiac nervous system (ICNS) is an important regulatory pathway of cardiac autonomic function, however, little is known about the alterations that occur in the ICNS in diabetes. We sought to characterize morphologic changes and the role of oxidative stress within the ICNS of diabetic hearts. Cultured ICNS neuronal cells from the hearts of 3- and 6-month old type 1 diabetic streptozotocin (STZ)-induced diabetic Sprague-Dawley rats and age-matched controls were examined. Confocal microscopy analysis for protein gene product 9.5 (PGP 9.5) and amino acid adducts of (E)-4-hydroxy-2-nonenal (4-HNE) using immunofluorescence was undertaken. Cell morphology was then analyzed in a blinded fashion for features of neuronal dystrophy and the presence of 4-HNE adducts.

Results

At 3-months, diabetic ICNS neuronal cells exhibited 30% more neurite swellings per area (p = 0.01), and had a higher proportion with dystrophic appearance (88.1% vs. 50.5%; p = <0.0001), as compared to control neurons. At 6-months, diabetic ICNS neurons exhibited more features of dystrophy as compared to controls (74.3% vs. 62.2%; p = 0.0448), with 50% more neurite branching (p = 0.0015) and 50% less neurite outgrowth (p = <0.001). Analysis of 4-HNE adducts in ICNS neurons of 6-month diabetic rats demonstrated twice the amount of reactive oxygen species (ROS) as compared to controls (p = <0.001).

Conclusion

Neuronal dystrophy occurs in the ICNS neurons of STZ-induced diabetic rats, and accumulates temporally within the disease process. In addition, findings implicate an increase in ROS within the neuronal processes of ICNS neurons of diabetic rats suggesting an association between oxidative stress and the development of dystrophy in cardiac autonomic neurons.

Institutional animal care and use committee considerations for animal models of peripheral neuropathy

Peripheral neuropathy and neuropathic pain are debilitating, life-altering conditions that affect a significant proportion of the human population. Animal models, used to study basic disease mechanisms and treatment modalities, are diverse and provide many challenges for institutional animal care and use committee (IACUC) review and postapproval monitoring. Items to consider include regulatory and ethical imperatives in animal models that may be designed to study pain, the basic mechanism of neurodegeneration, and different disease processes for which neuropathic pain is a side effect. Neuropathic pain can be difficult to detect or quantify in many models, and pain management is often unsuccessful in both humans and animals, inspiring the need for more research. Design of humane endpoints requires clear communication of potential adverse outcomes and solutions. Communication with the IACUC, researchers, and veterinary staff is also key for successful postapproval monitoring of these challenging models.

Remodelling of the intracardiac ganglia in diabetic Goto-Kakizaki rats: an anatomical study

Background

Although cardiac autonomic neuropathy is one of major complications of diabetes mellitus (DM), anatomical data on cardiac innervation of diabetic animal models is scant and controversial. We performed this study to check whether long-term diabetic state impacts the anatomy of intracardiac ganglia in Goto-Kakizaki (GK) rats, a genetic model of type 2 DM.

Methods

Twelve GK rats (276 ± 17 days of age; mean ± standard error) and 13 metabolically healthy Wistar rats (262 ± 5 days of age) as controls were used for this study. Blood glucose was determined using test strips, plasma insulin by radioimmunoassay. Intrinsic ganglia and nerves were visualized by acetylcholinesterase histochemistry on whole hearts. Ganglion area was measured, and the neuronal number was assessed according to ganglion area.

Results

The GK rats had significantly elevated blood glucose level compared to controls (11.0 ± 0.6 vs. 5.9 ± 0.1 mmol/l, p < 0.001), but concentration of plasma insulin did not differ significantly between the two groups (84.0 ± 9.8 vs. 67.4 ± 10.9 pmol/l, p = 0.17). The GK rats contained significantly fewer intracardiac ganglia, decreased total area of intracardiac ganglia (1.4 ± 0.1 vs. 2.2 ± 0.1 mm², p < 0.001) and smaller somata of ganglionic neurons. Mean total number of intracardiac neurons in GK rats was 1461 ± 62, while this number in control rats was higher by 39% and reached 2395 ± 110 (p < 0.001).

Conclusions

Results of our study demonstrate the decreased number of intracardiac neurons in GK rats compared to metabolically healthy Wistar rats of similar age. It is likely that the observed structural remodelling of intracardiac ganglia in GK rats is caused by a long-term diabetic state.

Autonomic cardiac innervation: development and adult plasticity

Autonomic cardiac neurons have a common origin in the neural crest but undergo distinct developmental differentiation as they mature toward their adult phenotype. Progenitor cells respond to repulsive cues during migration, followed by differentiation cues from paracrine sources that promote neurochemistry and differentiation. When autonomic axons start to innervate cardiac tissue, neurotrophic factors from vascular tissue are essential for maintenance of neurons before they reach their targets, upon which target-derived trophic factors take over final maturation, synaptic strength and postnatal survival. Although target-derived neurotrophins have a central role to play in development, alternative sources of neurotrophins may also modulate innervation. Both developing and adult sympathetic neurons express proNGF, and adult parasympathetic cardiac ganglion neurons also synthesize and release NGF. The physiological function of these “non-classical” cardiac sources of neurotrophins remains to be determined, especially in relation to autocrine/paracrine sustenance during development.

 

Cardiac autonomic nerves are closely spatially associated in cardiac plexuses, ganglia and pacemaker regions and so are sensitive to release of neurotransmitter, neuropeptides and trophic factors from adjacent nerves. As such, in many cardiac pathologies, it is an imbalance within the two arms of the autonomic system that is critical for disease progression. Although this crosstalk between sympathetic and parasympathetic nerves has been well established for adult nerves, it is unclear whether a degree of paracrine regulation occurs across the autonomic limbs during development. Aberrant nerve remodeling is a common occurrence in many adult cardiovascular pathologies, and the mechanisms regulating outgrowth or denervation are disparate. However, autonomic neurons display considerable plasticity in this regard with neurotrophins and inflammatory cytokines having a central regulatory function, including in possible neurotransmitter changes. Certainly, neurotrophins and cytokines regulate transcriptional factors in adult autonomic neurons that have vital differentiation roles in development. Particularly for parasympathetic cardiac ganglion neurons, additional examinations of developmental regulatory mechanisms will potentially aid in understanding attenuated parasympathetic function in a number of conditions, including heart failure.

PAC₁ receptors mediate positive chronotropic responses to PACAP-27 and VIP in isolated mouse atria

PACAP and VIP have prominent effects on cardiac function in several species, but little is known about their influence on the murine heart. Accordingly, we evaluated the expression of PACAP/VIP receptors in mouse heart and the response of isolated atria to peptide agonists. Quantitative PCR demonstrated that PAC₁, VPAC₁, and VPAC₂ receptor mRNAs are present throughout the mouse heart. Expression of all three receptor transcripts was low, PAC₁ being the lowest. No regional differences in expression were detected for individual receptor mRNAs after normalization to L32. Pharmacological effects of PACAP-27, VIP, and the selective PAC₁ agonist maxadilan were evaluated in isolated, spontaneously beating atria from C57BL/6 mice of either sex. Incremental additions of PACAP-27 at 1 min intervals caused a concentration-dependent tachycardia with a logEC₅₀=-9.08 ± 0.15 M (n=7) and a maximum of 96.3 ± 5.9% above baseline heart rate. VIP and maxadilan also caused tachycardia but their potencies were about two orders of magnitude less. Increasing the dosing interval to 5 min caused a leftward shift of the concentration-response curve to maxadilan but no changes in the curves for PACAP-27 or VIP. Under this condition, neither the potency nor the efficacy of maxadilan differed from those of PACAP-27. Neither PACAP-27 nor maxadilan caused tachyphylaxis, and maximal responses to maxadilan were maintained for at least 2 h. We conclude that all three VIP/PACAP family receptors are expressed by mouse cardiac tissue, but only PAC₁ receptors mediate positive chronotropic responses to PACAP-27 and VIP.

Diabetic cardiac autonomic neuropathy: insights from animal models

Cardiac autonomic neuropathy (CAN) is a relatively common and often devastating complication of diabetes. The major clinical signs are tachycardia, exercise intolerance, and orthostatic hypotension, but the most severe aspects of this complication are high rates of cardiac events and mortality. One of the earliest manifestations of CAN is reduced heart rate variability, and detection of this, along with abnormal results in postural blood pressure testing and/or the Valsalva maneuver, are central to diagnosis of the disease. The treatment options for CAN, beyond glycemic control, are extremely limited and lack evidence of efficacy. The underlying molecular mechanisms are also poorly understood. Thus, CAN is associated with a poor prognosis and there is a compelling need for research to understand, prevent, and reverse CAN. In this review of the literature we examine the use and usefulness of animal models of CAN in diabetes. Compared to other diabetic complications, the number of animal studies of CAN is very low. The published studies range across a variety of species, methods of inducing diabetes, and timescales examined, leading to high variability in study outcomes. The lack of well-characterized animal models makes it difficult to judge the relevance of these models to the human disease. One major advantage of animal studies is the ability to probe underlying molecular mechanisms, and the limited numbers of mechanistic studies conducted to date are outlined. Thus, while animal models of CAN in diabetes are crucial to better understanding and development of therapies, they are currently under-used.

Atrial Fibrillation and Metabolic Syndrome: Understanding the Connection

Metabolic syndrome, a constellation of conditions including obesity, dyslipidemia, hypertension and insulin resistance, has increased to epidemic proportions. Metabolic syndrome has been recognized as a risk factor for cardiovascular morbidity and is likely related to the epidemic of cardiovascular diseases. Perhaps not coincidentally, its growth in incidence has paralleled that of atrial fibrillation. Various components of metabolic syndrome have been known to have a role in the pathogenesis of atrial fibrillation. With the conglomeration of components seen in the metabolic syndrome, the risk for atrial fibrillation increases greatly. Several studies have elucidated the role of metabolic syndrome in the development of atrial fibrillation. Its role on the atrial substrate makes it an important determinant of progression of disease and failure of therapeutic strategies such as catheter ablation. Control of the various components of metabolic syndrome may ultimately lead to better outcomes in atrial fibrillation patients.

Exercise training mitigates aberrant cardiac protein O-GlcNAcylation in streptozotocin-induced diabetic mice

AIMS

Increased protein O-GlcNAcylation occurs in response to increased availability of glucose and fatty acids and is a hallmark of diabetes. Previous studies have demonstrated an improvement in heart function associated with decreased protein O-GlcNAcylation. Our group has recently demonstrated a capacity for exercise to decrease protein O-GlcNAcylation in the heart of normal mice; however, the impact of such training under diabetic conditions has not been examined.

MAIN METHODS

Diabetes was induced in mice through injection of streptozotocin. Animals either remained sedentary or were subjected to 6 weeks of swim training protocol. At the end of 6 weeks in vivo cardiac function was assessed and the hearts were harvested for gene expression and Western blotting in relation to O-GlcNAcylation

KEY FINDINGS

Diabetes resulted in elevated blood glucose relative to non-diabetic mice. Relative to the sedentary diabetic group, the rate of relaxation (Tau) was significantly improved in the exercised group. Western blot analysis revealed an increase in protein O-GlcNAcylation in the diabetic group which was reversed through exercise despite persistent hyperglycemia. No change in the expression of O-GlcNAc transferase (OGT) was noted between sedentary and exercised diabetic mice; however an increase in the expression and activity of O-GlcNAcase (OGA) was apparent in the exercised group.

SIGNIFICANCE

This study demonstrates the potential for exercise training to decrease intracellular protein O-GlcNAcylation in the heart even under conditions of persistent hyperglycemia associated with diabetes. Our results suggest the beneficial effects of regular aerobic exercise extend beyond simple regulation of blood glucose levels.

Chronic activation of endogenous angiotensin-converting enzyme 2 protects diabetic rats from cardiovascular autonomic dysfunction

In this study, we evaluated whether the activation of endogenous angiotensin-converting enzyme 2 (ACE2) would improve the cardiovascular autonomic dysfunction of diabetic rats. Ten days after induction of type 1 diabetes (streptozotocin, 50 mg kg(-1) i.v.), the rats were treated orally with 1-[(2-dimethylamino)ethylamino]-4-(hydroxymethyl)-7-[(4-methylphenyl) sulfonyl oxy]-9H-xanthene-9-one (XNT), a newly discovered ACE2 activator (1 mg kg(-1) day(-1)), or saline (equivalent volume) for 30 days. Autonomic cardiovascular parameters were evaluated in conscious animals, and an isolated heart preparation was used to analyse cardiac function. Diabetes induced a significant decrease in the baroreflex bradycardia sensitivity, as well as in the chemoreflex chronotropic response and parasympathetic tone. The XNT treatment improved these parameters by ≈ 76% [0.82 ± 0.09 versus 1.44 ± 0.17 Ratio between changes in pulse interval and changes in mean arterial pressure (ΔPI/ΔmmHg)], ∼85% (-57 ± 9 versus -105 ± 10 beats min(-1)) and ≈ 205% (22 ± 2 versus 66 ± 12 beats min(-1)), respectively. Also, XNT administration enhanced the bradycardia induced by the chemoreflex activation by v 74% in non-diabetic animals (-98 ± 16 versus -170 ± 9 Δbeats min(-1)). No significant changes were observed in the mean arterial pressure, baroreflex tachycardia sensitivity, chemoreflex pressor response and sympathetic tone among any of the groups. Furthermore, chronic XNT treatment ameliorated the cardiac function of diabetic animals. However, the coronary vasoconstriction observed in diabetic rats was unchanged by ACE2 activation. These findings indicate that XNT protects against the autonomic and cardiac dysfunction induced by diabetes. Thus, our results provide evidence for the viability and effectiveness of oral administration of an ACE2 activator for the treatment of the cardiovascular autonomic dysfunction caused by diabetes.