Heterogeneous cardiac sympathetic denervation and decreased myocardial nerve growth factor in streptozotocin-induced diabetic rats: implications for cardiac sympathetic dysinnervation complicating 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.

Dysregulation of hypoxia-inducible factor 1α in the sympathetic nervous system accelerates diabetic cardiomyopathy

BACKGROUND

An altered sympathetic nervous system is implicated in many cardiac pathologies, ranging from sudden infant death syndrome to common diseases of adulthood such as hypertension, myocardial ischemia, cardiac arrhythmias, myocardial infarction, and heart failure. Although the mechanisms responsible for disruption of this well-organized system are the subject of intensive investigations, the exact processes controlling the cardiac sympathetic nervous system are still not fully understood. A conditional knockout of the Hif1a gene was reported to affect the development of sympathetic ganglia and sympathetic innervation of the heart. This study characterized how the combination of HIF-1α deficiency and streptozotocin (STZ)-induced diabetes affects the cardiac sympathetic nervous system and heart function of adult animals.

METHODS

Molecular characteristics of Hif1a deficient sympathetic neurons were identified by RNA sequencing. Diabetes was induced in Hif1a knockout and control mice by low doses of STZ treatment. Heart function was assessed by echocardiography. Mechanisms involved in adverse structural remodeling of the myocardium, i.e. advanced glycation end products, fibrosis, cell death, and inflammation, was assessed by immunohistological analyses.

RESULTS

We demonstrated that the deletion of Hif1a alters the transcriptome of sympathetic neurons, and that diabetic mice with the Hif1a-deficient sympathetic system have significant systolic dysfunction, worsened cardiac sympathetic innervation, and structural remodeling of the myocardium.

CONCLUSIONS

We provide evidence that the combination of diabetes and the Hif1a deficient sympathetic nervous system results in compromised cardiac performance and accelerated adverse myocardial remodeling, associated with the progression of diabetic cardiomyopathy.

Association of P-Wave Axis With Incident Atrial Fibrillation in Diabetes Mellitus (from the ACCORD Trial)

Abnormal P-wave axis may reflect preclinical atrial dysfunction and has been associated with an increased risk of incident atrial fibrillation (AF) in the general population. Patients with diabetes mellitus (DM) have a higher prevalence of AF, but the association of abnormal P-wave axis and the risk of incident AF in those with diabetes has not been previously explored. For this analysis, we included 8,965 eligible participants from the Action to Control Cardiovascular Risk in Diabetes (ACCORD) trial. P-wave axis was automatically measured on study electrocardiogram and visually confirmed, with the normal range being between 0° and 75°. At baseline, 8% of the study population had an abnormal P-wave axis. During 43,856 person-years of follow-up, there were 145 cases of incident AF. Using multivariable-adjusted Cox proportional hazards models, participants with abnormal P-wave axis had an increased risk of incident AF (hazard ratio 2.65, 95% confidence interval 1.76 to 3.99, p < 0.0001). Findings were similar in prespecified subgroups, without evidence of effect modification. Both left- and right-axis deviation of the P-wave were associated with incident AF. Our results suggest that abnormal P-wave axis is associated with incident AF in those with DM and that this relation is conserved in prespecified subgroups. There may be utility in considering P-wave axis values from routine ECGs in these patients.

α-Amylase and α-glucosidase inhibitor effects and pancreatic response to diabetes mellitus on Wistar rats of Ephedra alata areal part decoction with immunohistochemical analyses

Ephedra alata, known as a medicinal plant in China, was used in this study as aqueous extract from aerial parts, for diabetes mellitus treatment. This study was carried out on two parts, in vitro, we tested the effect of the studied extract on the inhibition of α-glucosidase and α-amylase activities, and in vivo on Wistar male rats receiving alloxan intraperitoneally at a rate of 125 mg/kg. Extract (100, 200, and 300 mg/kg of body weight) was administrated for 28 days by oral gavage. Blood glucose, amylase, lipase, and lipid profile level were determined. Oxidative stress was evaluated by enzymatic activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx), and by estimation of lipid peroxidation and protein carbonyl (PC) level. Histopathological changes in pancreas were investigated under photonic microscopy using immunohistochemical procedure. Our findings showed that aqueous extract inhibited in vitro both α-glucosidase and α-amylase activities and its use in vivo at 300 mg/kg of body weight restored pancreas weight and weight gain, ameliorated significantly (p ˂ 0.05) biochemical parameters; it prevented the increase in lipid and protein oxidation and the decrease in enzymatic and non-enzymatic defense system. Histological study of treated animals showed a comparable healed regeneration of beta cells.

Prevalence of arrhythmias in patients with type 2 diabetes and the role of structural changes in myocardium in their development

OBJECTIVE

The aim of the study was to evaluate the prevalence of arrhythmias in patients with type 2 diabetes and their relationships with the structural parameters of the heart.

METHODS

A retrospective case-control study was conducted using clinical and biochemical profiles of patients with diabetes at the Endocrinology Centre and City Clinical Hospital No. 40, Ekaterinburg, Russia.

RESULTS

The total study sample included 75 subjects. The average age (SD) was 48.2 (5.6) years, and the mean duration of diabetes (SD) was 6.2 (2.4) years. The most common type of extrasystoles were the single supraventricular extrasystoles, observed in 72.29% of cases (vs 38.89% in the control group) and paired supraventricular extrasystoles, which were identified in 30% of cases (vs 19.44% in the control group). Ventricular cardiac arrhythmias in the form of ventricular extrasystoles (VE) were identified in 25.71% of cases (13.89% in the control group).

CONCLUSIONS

This study revealed the signs of the morphological restructuring of the right chambers of the heart and a relatively high prevalence of supraventricular arrhythmias in the early stages of type 2 diabetes. Moreover, according to the results, the incidence of some types of supraventricular arrhythmias and the occurrence of tachycardia episodes in patients with type 2 diabetes mostly depends on the restructuring of the right chambers of the heart, which may be caused by the peculiarities of the cardiac innervation, with the higher density of choline and adrenergic plexuses in the right chambers.

Changes in neurofilament 200 and tyrosine hydroxylase expression in the cardiac innervation of diabetic rats during aging

Changes in sensory and sympathetic innervation during diabetes mellitus (DM) can be a predictor of arrhythmias, silent myocardial ischemia, and chronic heart failure, but knowledge about these changes is still unsatisfactory. We analyzed whether prolonged DM induces changes in density of sensory and sympathetic nerve terminals of rat's heart and whether it contributes to cardiomyopathy during aging. DM was induced by i/p injecting 55 mg/kg streptozotocin to male Sprague-Dawley rats, while a control group received a citrate buffer. DM in the rats was validated by measuring blood glucose level. Animals were sacrificed after 2 weeks, 2 months, 6 months, and 12 months. Five areas of cardiac sections were analyzed. Antibodies raised against tyrosine hydroxylase (TH) and neurofilament 200 kDa (NF 200) were used to detect sympathetic and sensory fibers. TH immunoreactive fiber density increased in DM groups 2 weeks after induction, reaching a peek after 2 months, while in the later stages of DM (6 and 12 months), there was no significant difference compared to control. NF 200 immunoreactive fiber density increased 2 weeks after induction compared to control. There was no consistent pattern of change during the given period in both the DM or control groups. In the DM group, we found thickening of the left ventricle wall (P<.05) as the sign of cardiomyopathy. Our findings suggest that hyperglycemia as a hallmark of DM in early stages can lead to proliferation of sympathetic and sensory nerve terminals. This finding can contribute to a better understanding of the occurrence of arrhythmias and silent myocardial ischemia in DM.

Norepinephrine transporter expression is inversely associated with glycaemic indices: a pilot study in metabolically diverse persons with overweight and obesity

Objective

The objective of this study was to examine the cross‐sectional relationship between the expression of norepinephrine transporter (NET), the protein responsible for neuronal uptake‐1, and indices of glycaemia and hyperinsulinaemia, in overweight and obese individuals.

Methods

Thirteen non‐medicated, non‐smoking subjects, aged 58 ± 1 years (mean ± standard error of the mean), body mass index (BMI) 31.4 ± 1.0 kg m⁻², with wide‐ranging plasma glucose and haemoglobin A1c (HbA1c, range 5.1% to 6.5%) participated. They underwent forearm vein biopsy to access sympathetic nerves for the quantification of NET by Western blot, oral glucose tolerance test (OGTT), euglycaemic hyperinsulinaemic clamp, echocardiography and assessments of whole‐body norepinephrine kinetics and muscle sympathetic nerve activity.

Results

Norepinephrine transporter expression was inversely associated with fasting plasma glucose (r = −0.62, P = 0.02), glucose area under the curve during OGTT (AUC_0–120, r = −0.65, P = 0.02) and HbA1c (r = −0.67, P = 0.01), and positively associated with steady‐state glucose utilization during euglycaemic clamp (r = 0.58, P = 0.04). Moreover, NET expression was inversely related to left ventricular posterior wall dimensions (r = −0.64, P = 0.02) and heart rate (r = −0.55, P = 0.05). Indices of hyperinsulinaemia were not associated with NET expression. In stepwise linear regression analysis adjusted for age, body mass index and blood pressure, HbA1c was an independent inverse predictor of NET expression, explaining 45% of its variance.

Conclusions

Hyperglycaemia is associated with reduced peripheral NET expression. Further studies are required to identify the direction of causality.

Heart Development, Diseases, and Regeneration - New Approaches From Innervation, Fibroblasts, and Reprogramming

It is well known that cardiac function is tightly controlled by neural activity; however, the molecular mechanism of cardiac innervation during development and the relationship with heart disease remain undetermined. My work has revealed the molecular networks that govern cardiac innervation and its critical roles in heart diseases such as silent myocardial ischemia and arrhythmias. Cardiomyocytes proliferate during embryonic development, but lose their proliferative capacity after birth. Cardiac fibroblasts are a major source of cells during fibrosis and induce cardiac hypertrophy after myocardial injury in the adult heart. Despite the importance of fibroblasts in the adult heart, the role of fibroblasts in embryonic heart development was previously not determined. I demonstrated that cardiac fibroblasts play important roles in myocardial growth and cardiomyocyte proliferation during embryonic development, and I identified key paracrine factors and signaling pathways. In contrast to embryonic cardiomyocytes, adult cardiomyocytes have little regenerative capacity, leading to heart failure and high mortality rates after myocardial infarction. Leveraging the knowledge of developmental biology, I identified cardiac reprogramming factors that can directly convert resident cardiac fibroblasts into cardiomyocytes for heart regeneration. These findings greatly improved our understanding of heart development and diseases, and provide a new strategy for heart regenerative therapy. (Circ J 2016; 80: 2081-2088).

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.

PET Radiotracers of the Cardiovascular System

Cardiovascular PET provides exquisite measurements of key aspects of the cardiovascular system and as a consequence it plays central role in cardiovascular investigation. Moreover, PET is now playing an ever increasing role in the management of the cardiac patient. Central to the success of PET is the development and use of novel radiotracers that permit measurements of key aspects of cardiovascular health such as myocardial perfusion, metabolism, and neuronal function. Moreover, the development of molecular imaging radiotracers is now permitting the interrogation of cellular and sub cellular processes. This article highlights these various radiotracers and their role in both cardiovascular research and potential clinical applications.

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.

The biology of neurotrophins: cardiovascular function

This chapter addresses the role of neurotrophins in the development of the heart, blood vessels, and neural circuits that control cardiovascular function, as well as the role of neurotrophins in the mature cardiovascular system. The cardiovascular system includes the heart and vasculature whose functions are tightly controlled by the nervous system. Neurons, cardiomyocytes, endothelial cells, vascular smooth muscle cells, and pericytes are all targets for neurotrophin action during development. Neurotrophin expression continues throughout life, and several common pathologies that impact cardiovascular function involve changes in the expression or activity of neurotrophins. These include atherosclerosis, hypertension, diabetes, acute myocardial infarction, and heart failure. In many of these conditions, altered expression of neurotrophins and/or neurotrophin receptors has direct effects on vascular endothelial and smooth muscle cells in addition to effects on nerves that modulate vascular resistance and cardiac function. This chapter summarizes the effects of neurotrophins in cardiovascular physiology and pathophysiology.

Early diabetes treatment does not prevent sympathetic dysinnervation in the streptozotocin diabetic rat heart

BACKGROUND

Positron emission tomography (PET) studies have demonstrated reduced sympathetic neuronal integrity in high-fat diet fed streptozotocin insulin-resistant diabetic rats in parallel with abnormal early-to-atrial transmitral velocity. We hypothesized that administration of anti-glycemic drugs early after diabetes induction would prevent sympathetic neuronal dysfunction.

METHODS AND RESULTS

Male Sprague-Dawley rats fed high-fat diet were administered streptozotocin (45 mg·kg(-1), ip, n = 23) to induce diabetes or vehicle alone (n = 6). Diabetic rats were randomized to receive insulin (4 U·day(-1)), metformin (650 mg·kg(-1)·day(-1)), rosiglitazone (4 mg·kg(-1)·day(-1)), or no treatment 1 week after streptozotocin. Small animal PET imaging using the norepinephrine analog [(11)C]meta-hydroxyephedrine (HED) at baseline and 8 weeks of diabetes determined sympathetic neuronal integrity. Echocardiography assessed cardiac function. Plasma norepinephrine levels were determined in parallel. Ex vivo immunoblotting was performed at the end of the experiment to compare the relative expression of various proteins involved in metabolic and noradrenergic signaling. Insulin restored blood glucose and lipid levels to normal. Despite improved plasma lipid levels, neither metformin nor rosiglitazone reduced blood glucose. At 8 weeks, untreated and treated diabetics displayed a 39%-42% reduction in myocardial HED standardized uptake values (P < .05). In all diabetic groups, plasma norepinephrine was elevated (2.3- to 3.3-fold, P < .05) and norepinephrine reuptake transporter expression reduced (28%-35%, P < .05) compared to non-diabetics. Doppler echocardiography revealed delayed development of prolonged mitral valve deceleration and elevated early-to-atrial filling velocity ratio among treated diabetic rats.

CONCLUSION

Early glycemic treatment of insulin-resistant diabetic rats did not prevent deterioration of sympathetic neuronal integrity though ventricular filling abnormalities were delayed.

The co-occurrence of myocardial dysfunction and peripheral insensate neuropathy in a streptozotocin-induced rat model of diabetes

BACKGROUND

Cardiomyopathy and distal symmetrical polyneuropathy (DSPN), including sensory and autonomic dysfunction, often co-occur in diabetic mellitus (DM) patients. However, the temporal relationship and progression between these two complications has not been investigated. Using a streptozotocin DM animal model that develops insensate neuropathy, our aim was to examine in parallel the development of DSPN and DM-associated changes in cardiac structure and function as well as potential mechanisms, such as autonomic dysfunction, evaluated by changes in urinary and myocardial norepinephrine content and myocardial neuronal markers.

METHODS

Sensory neuropathy was measured by behavioral tests using Von Frey filaments and Hargreaves methods. Echocardiography was used to evaluate myocardial structure and function. Autonomic function was evaluated by measuring urinary and myocardial norepinephrine (NE) levels by enzyme-linked immunosorbent assay and high-performance liquid chromatography/mass spectrometry. Quantitative immunohistochemistry was used to measure the myocardial neuronal markers, calcitonin gene-related peptide (CGRP) and general neuronal protein gene product 9.5 (PGP 9.5).

RESULTS

The DM group developed tactile and thermal insensate neuropathy 4-5 weeks after DM onset. Cardiovascular changes were found between 4 and 12 weeks after DM onset and included bradycardia, diastolic and systolic dysfunction and cardiac dilation. There was a 2.5-fold reduction in myocardial NE levels and a 5-fold increase in urinary NE levels in the DM group. Finally, there was a 2.3-fold increase in myocardial CGRP levels in the DM group and no change in PGP9.5 levels.

CONCLUSIONS

Cardiovascular structural and functional changes developed early in the course of DM and in combination with insensate neuropathy. In parallel, signs of cardiac autonomic dysfunction were also found and included decreased myocardial NE levels and altered CGRP levels. These results may indicate the need for early cardiovascular evaluation in DM patients with insensate neuropathy.

Autonomic neuropathy in experimental models of diabetes mellitus

Autonomic neuropathy complicates diabetes by increasing patient morbidity and mortality. Surprisingly, considering its importance, development and exploitation of animal models has lagged behind the wealth of information collected for somatic symmetrical sensory neuropathy. Nonetheless, animal studies have resulted in a variety of insights into the pathogenesis, neuropathology, and pathophysiology of diabetic autonomic neuropathy (DAN) with significant and, in some cases, remarkable correspondence between rodent models and human disease. Particularly in the study of alimentary dysfunction, findings in intrinsic intramural ganglia, interstitial cells of Cajal and the extrinsic parasympathetic and sympathetic ganglia serving the bowel vie for recognition as the chief mechanism. A body of work focused on neuropathologic findings in experimental animals and human subjects has demonstrated that axonal and dendritic pathology in sympathetic ganglia with relative neuron preservation represents one of the neuropathologic hallmarks of DAN but it is unlikely to represent the entire story. There is a surprising selectivity of the diabetic process for subpopulations of neurons and nerve terminals within intramural, parasympathetic, and sympathetic ganglia and innervation of end organs, afflicting some while sparing others, and differing between vascular and other targets within individual end organs. Rather than resulting from a simple deficit in one limb of an effector pathway, autonomic dysfunction may proceed from the inability to integrate portions of several complex pathways. The selectivity of the diabetic process appears to confound a simple global explanation (e.g., ischemia) of DAN. Although the search for a single unifying pathogenetic hypothesis continues, it is possible that autonomic neuropathy will have multiple pathogenetic mechanisms whose interplay may require therapies consisting of a cocktail of drugs. The role of multiple neurotrophic substances, antioxidants (general or pathway specific), inhibitors of formation of advanced glycosylation end products and drugs affecting the polyol pathway may be complex and therapeutic elements may have both salutary and untoward effects. This review has attempted to present the background and current findings and hypotheses, focusing on autonomic elements including and beyond the typical parasympathetic and sympathetic nervous systems to include visceral sensory and enteric nervous systems.

Mechanisms of disease: role of neurotrophins in diabetes and diabetic neuropathy

Neuropathy is an insidious and devastating consequence of diabetes. Early studies provided a strong rationale for deficient neurotrophin support in the pathogenesis of diabetic neuropathy in a number of critical tissues and organs. It has now been over a decade since the first failed human neurotrophin supplementation clinical trials, but mounting evidence still implicates these trophic factors in diabetic neuropathy. Since then, tremendous advances have been made in our understanding of the complexities of neurotrophin signaling and processing and how the diabetic milieu might impact this. This in turn changes both our perception of how the altered trophic environment contributes to the etiology of diabetic neuropathy and the design of future neurotrophin therapeutic interventions. This chapter summarizes some of these findings and attempts to integrate neurotrophin actions on the nervous system with an increasing appreciation of their role in the regulation of metabolic processes in diabetes that impact the diabetic neuropathic state.

Exogenous nerve growth factor promotes the repair of cardiac sympathetic heterogeneity and electrophysiological instability in diabetic rats

OBJECTIVES

Diabetic cardiac autonomic neuropathy can lead to an increased incidence of ventricular arrhythmias (VAs). However, few data are available regarding the pathogenesis and therapy of the VAs accompanying diabetic cardiac autonomic neuropathy. We aimed to explore whether or not exogenous nerve growth factor (NGF) can reduce the sympathetic heterogeneity and the incidence of VAs in diabetes mellitus (DM).

METHODS

Male Wistar rats were randomly divided into 3 groups: controls, rats with DM with saline infused into the left stellate ganglion (LSG), i.e. the DS group and rats with DM with NGF infused into the LSG, i.e. the DN group. After 28 weeks, all rats were subjected to electrophysiological experiments. Sympathetic innervations and NGF were studied by immunostaining, RT-PCR or Western blot analysis.

RESULTS

The incidence of inducible VAs was significantly higher in the DS group than in the control group, but was markedly decreased in the DN group. In the DS group, the tyrosine hydroxylase (TH) and NGF expression were significantly lower than in the other groups, and significant proximal-distal heterogeneities existed regarding the TH and NGF expression in the left ventricle, but were markedly repaired in the DN group.

CONCLUSIONS

NGF intervention in the LSG can reduce the heterogeneity of cardiac sympathetic innervations and the incidence of VAs in diabetic rats.

Insulin restores myocardial presynaptic sympathetic neuronal integrity in insulin-resistant diabetic rats

BACKGROUND

Diabetes is associated with increased sympathetic activity, elevated norepinephrine, impaired heart rate variability, and the added risk of cardiovascular mortality. The temporal development of sympathetic neuronal dysfunction, response to therapy, and relation to ventricular function is not well characterized.

METHODS AND RESULTS

Sympathetic neuronal integrity was serially investigated in high fat diet-fed streptozotocin diabetic rats using [(11)C]meta-hydroxyephedrine (HED) positron emission tomography at baseline, 8 weeks of diabetes, and after a further 8 weeks of insulin or insulin-sensitizing metformin therapy. Myocardial HED retention was reduced in diabetic rats (n = 16) compared to non-diabetics (n = 6) at 8 weeks by 52-57% (P = .01) with elevated plasma and myocardial norepinephrine levels. Echocardiography pulse-wave Doppler measurements demonstrated prolonged mitral valve deceleration and increased early-to-atrial filling velocity, consistent with diastolic dysfunction. Insulin but not metformin evoked recovery of HED retention and plasma norepinephrine (P < .05), whereas echocardiography measurements of diastolic function were not improved by either treatment. Relative expressions of norepinephrine reuptake transporter and β-adrenoceptors were lower in metformin-treated as compared to insulin-treated diabetic and non-diabetic rats. Diabetic rats exhibited depressed heart rate variability and impaired diastolic function which persisted despite insulin treatment.

CONCLUSIONS

HED imaging provides sound estimation of sympathetic function. Effective glycemic control can recover sympathetic function in diabetic rats without the corresponding recovery of echocardiography indicators of diastolic dysfunction. HED positron emission tomography imaging may be useful in stratifying cardiovascular risk among diabetic patients and in evaluating the effect of glycemic therapy on the heart.

Diabetes, Obesity and Atrial Fibrillation: Epidemiology, Mechanisms and Interventions

Body mass index (BMI) is a powerful predictor of death, type 2 diabetes (T2DM) and cardiovascular (CV) morbidity and mortality. Over the last few decades, we have witnessed a global rise in adult obesity of epidemic proportions. Similarly, there has been a parallel increase in the incidence of atrial fibrillation (AF), itself a significant cause of cardiovascular morbidity and mortality. This may be partly attributable to advances in the treatment of coronary heart disease (CHD) and heart failure (HF) improving life expectancy, however, epidemiological studies have demonstrated an independent association between obesity, diabetes and AF, suggesting possible common pathophysiological mechanisms and risk factors. Indeed, cardiac remodeling, haemodynamic alterations, autonomic dysfunction, and diastolic dysfunction have been reported in obese and diabetic cohorts. Moreover, diabetic cardiomyopathy is characterized by an adverse structural and functional cardiac phenotype, which may predispose to the development of AF. In this review, we discuss the pathophysiological and mechanistic relationships between obesity, diabetes and AF, and some of the challenges posed in the management of this high-risk group of individuals.

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.

Obesity, diabetes and atrial fibrillation; epidemiology, mechanisms and interventions

The last few decades have witnessed a global rise in adult obesity of epidemic proportions. The potential impact of this is emphasized when one considers that body mass index (BMI) is a powerful predictor of death, type 2 diabetes (T2DM) and cardiovascular (CV) morbidity and mortality [1, 2]. Similarly we have witnessed a parallel rise in the incidence of atrial fibrillation (AF), the commonest sustained cardiac arrhythmia, which is also a significant cause of cardiovascular morbidity and mortality. Part of this increase is attributable to advances in the treatment of coronary heart disease (CHD) and heart failure (HF) improving life expectancy and consequently the prevalence of AF. However, epidemiological studies have demonstrated an independent association between obesity and AF, possibly reflecting common pathophysiology and risk factors for both conditions. Indeed, weight gain and obesity are associated with structural and functional changes of the cardiovascular system including left atrial and ventricular remodeling, haemodynamic alterations, autonomic dysfunction, and diastolic dysfunction. Moreover, diabetic cardiomyopathy is characterized by an adverse structural and functional cardiac phenotype which may predispose to the development of AF [3]. In this review, we discuss the pathophysiological and mechanistic relationships between obesity, diabetes and AF, and the challenges posed in the management of this high-risk group of individuals.

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.

Assessment of cardiac autonomic neuronal function using PET imaging

The autonomic nervous system is the primary extrinsic control of cardiac performance, and altered autonomic activity has been recognized as an important factor in the progression of various cardiac pathologies. Molecular imaging techniques have been developed for global and regional interrogation of pre- and postsynaptic targets of the cardiac autonomic nervous system. Building on established work with the guanethidine analogue ¹²³I-metaiodobenzylguanidine (MIBG) for single-photon emission tomography (SPECT), development of radiotracers and protocols for positron emission tomography (PET) investigation of autonomic signaling has expanded. PET is limited in availability and requires specialized centers for radiosynthesis and interpretation, but the higher resolution allows for improved regional analysis and kinetic modeling provides more true quantification than is possible with SPECT. A wider array of radiolabeled catecholamines, analogues of catecholamines, and receptor ligands have been characterized and evaluated. Sympathetic neuronal PET tracers have shown promise in the identification of several cardiac pathologies. In particular, recent studies have elucidated a mechanistic role for heterogeneous sympathetic innervation in the development of lethal ventricular arrhythmias. Evaluation of cardiomyocyte adrenergic receptor expression and the parasympathetic nervous system has been slower to develop, with clinical studies beginning to emerge. This review summarizes the clinical and the experimental PET tracers currently available for autonomic imaging and discusses their application in health and cardiovascular disease, with particular emphasis on the major findings of the last decade.

QT interval variability in type 2 diabetic patients with cardiac sympathetic dysinnervation assessed by 123I-metaiodobenzylguanidine scintigraphy

QT Variability and Sympathetic Dysinnervation. 

INTRODUCTION

The mechanism of adverse prognosis attributable to proarrhythmic cardiac sympathetic dysinnervation in patients with type 2 diabetes is incompletely understood. This study sought the association of cardiac sympathetic dysinnervation with temporal instability of ventricular repolarization assessed by beat-to-beat QT interval variability.

METHODS AND RESULTS

 (123) I-metaiodobenzylguanidine ((123) I-MIBG) scintigraphy was analyzed in 31 type 2 diabetic patients for cardiac sympathetic dysinnervation (4-hour heart-to-mediastinum ratio <1.8) and regional sympathetic integrity and washout rate (from 15-minute (123) I-MIBG uptake). Relative QT variability was defined from a continuous 5-minute ECG in the supine position (n = 31) and standing position (subgroup; n = 15) by the log ratio of absolute QT variability (QT variance divided by the mean QT interval squared) to heart rate (HR) variability (HR variance divided by the mean HR squared). Patients with (n = 16; 52%) versus without cardiac sympathetic dysinnervation demonstrated higher relative QT variability in the supine position (P < 0.001), owing to lower HR variability. However, on standing, absolute QT variability was significantly raised in these patients (P = 0.009) despite similar HR variability in the 2 groups. Correlations of heart-to-mediastinum ratio with standing QT variability (relative [r =-0.63, P = 0.013] and absolute [r =-0.79, P = 0.001]) were superior to corresponding supine measures (relative [r =-0.47, P = 0.008] and absolute [P = NS]). No associations of QT variability with washout rate or regional (123) I-MIBG uptake were identified.

CONCLUSION

Elevated QT variability is associated with cardiac sympathetic dysinnervation in type 2 diabetes and may contribute to adverse prognosis. Moreover, QT variability may be more specific for cardiac sympathetic innervation when measured in the context of sympathetic activation. (J Cardiovasc Electrophysiol, Vol. 24, pp. 305-313, March 2013).

Nerve growth factor gene therapy using adeno-associated viral vectors prevents cardiomyopathy in type 1 diabetic mice

Diabetes is a cause of cardiac dysfunction, reduced myocardial perfusion, and ultimately heart failure. Nerve growth factor (NGF) exerts protective effects on the cardiovascular system. This study investigated whether NGF gene transfer can prevent diabetic cardiomyopathy in mice. We worked with mice with streptozotocin-induced type 1 diabetes and with nondiabetic control mice. After having established that diabetes reduces cardiac NGF mRNA expression, we tested NGF gene therapies with adeno-associated viral vectors (AAVs) for the capacity to protect the diabetic mouse heart. To this aim, after 2 weeks of diabetes, cardiac expression of human NGF or β-Gal (control) genes was induced by either intramyocardial injection of AAV serotype 2 (AAV2) or systemic delivery of AAV serotype 9 (AAV9). Nondiabetic mice were given AAV2-β-Gal or AAV9-β-Gal. We found that the diabetic mice receiving NGF gene transfer via either AAV2 or AAV9 were spared the progressive deterioration of cardiac function and left ventricular chamber dilatation observed in β-Gal-injected diabetic mice. Moreover, they were additionally protected from myocardial microvascular rarefaction, hypoperfusion, increased deposition of interstitial fibrosis, and increased apoptosis of endothelial cells and cardiomyocytes, which afflicted the β-Gal-injected diabetic control mice. Our data suggest therapeutic potential of NGF for the prevention of cardiomyopathy in diabetic subjects.

Sympathetic nervous dysregulation in the absence of systolic left ventricular dysfunction in a rat model of insulin resistance with hyperglycemia

Background

Diabetes mellitus is strongly associated with cardiovascular dysfunction, derived in part from impairment of sympathetic nervous system signaling. Glucose, insulin, and non-esterified fatty acids are potent stimulants of sympathetic activity and norepinephrine (NE) release. We hypothesized that sustained hyperglycemia in the high fat diet-fed streptozotocin (STZ) rat model of sustained hyperglycemia with insulin resistance would exhibit progressive sympathetic nervous dysfunction in parallel with deteriorating myocardial systolic and/or diastolic function.

Methods

Cardiac sympathetic nervous integrity was investigated in vivo via biodistribution of the positron emission tomography radiotracer and NE analogue [¹¹C]meta-hydroxyephedrine ([¹¹C]HED). Cardiac systolic and diastolic function was evaluated by echocardiography. Plasma and cardiac NE levels and NE reuptake transporter (NET) expression were evaluated as correlative measurements.

Results

The animal model displays insulin resistance, sustained hyperglycemia, and progressive hypoinsulinemia. After 8 weeks of persistent hyperglycemia, there was a significant 13-25% reduction in [¹¹C]HED retention in myocardium of STZ-treated hyperglycemic but not euglycemic rats as compared to controls. There was a parallel 17% reduction in immunoblot density for NE reuptake transporter, a 1.2 fold and 2.5 fold elevation of cardiac and plasma NE respectively, and no change in sympathetic nerve density. No change in ejection fraction or fractional area change was detected by echocardiography. Reduced heart rate, prolonged mitral valve deceleration time, and elevated transmitral early to atrial flow velocity ratio measured by pulse-wave Doppler in hyperglycemic rats suggest diastolic impairment of the left ventricle.

Conclusions

Taken together, these data suggest that sustained hyperglycemia is associated with elevated myocardial NE content and dysregulation of sympathetic nervous system signaling in the absence of systolic impairment.

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

Cardiac autonomic neuropathy is a frequent complication of diabetes and often presents as impaired cholinergic regulation of heart rate. Some have assumed that diabetics have degeneration of cardiac cholinergic nerves, but basic knowledge on this topic is lacking. Accordingly, our goal was to evaluate the structure and function of cardiac cholinergic neurons and nerves in C57BL/6 mice with streptozotocin-induced diabetes. Electrocardiograms were obtained weekly from conscious control and diabetic mice for 16 weeks. Resting heart rate decreased in diabetic mice, but intrinsic heart rate was unchanged. Power spectral analysis of electrocardiograms revealed decreased high frequency and increased low frequency power in diabetic mice, suggesting a relative reduction of parasympathetic tone. Negative chronotropic responses to right vagal nerve stimulation were blunted in 16-week diabetic mice, but postjunctional sensitivity of isolated atria to muscarinic agonists was unchanged. Immunohistochemical analysis of hearts from diabetic and control mice showed no difference in abundance of cholinergic neurons, but cholinergic nerve density was increased at the sinoatrial node of diabetic mice (16 weeks: 14.9±1.2% area for diabetics versus 8.9±0.8% area for control, P<0.01). We conclude that disruption of cholinergic function in diabetic mice cannot be attributed to a loss of cardiac cholinergic neurons and nerve fibers or altered cholinergic sensitivity of the atria. Instead, decreased responses to vagal stimulation might be caused by a defect of preganglionic cholinergic neurons and/or ganglionic neurotransmission. The increased density of cholinergic nerves observed at the sinoatrial node of diabetic mice might be a compensatory response.

Fluvastatin attenuates diabetes-induced cardiac sympathetic neuropathy in association with a decrease in oxidative stress

BACKGROUND

Increased oxidative stress might contribute to diabetic (DM) neuropathy, so the effects of long-term treatment with fluvastatin (FL) on myocardial oxidative stress and cardiac sympathetic neural function were investigated in diabetic rats.

METHODS AND RESULTS

FL (10 mg . kg(-1) . day(-1), DM-FL) or vehicle (DM-VE) was orally administered for 2 weeks to streptozotocin-induced DM rats. Cardiac oxidative stress was determined by myocardial 8-iso-prostaglandin F(2alpha) (PGF(2alpha)) and NADPH oxidase subunit p22(phox) mRNA expression. Sympathetic neural function was quantified by autoradiography using (131)I- and (125)I-metaiodobenzylguanidine (MIBG). FL did not affect plasma glucose levels but remarkably decreased PGF(2alpha) levels compared with DM-VE rats (13.8+/-9.2 vs 175.0+/-93.9 ng/g tissue), although PGF(2alpha) levels were below the detection limit in non-DM rats. FL significantly reduced myocardial p22(phox) mRNA expression. Cardiac (131)I-MIBG uptake was lower in DM-VE rats than in non-DM rats, but the decrease was attenuated in DM-FL rats (1.31+/-0.08, 1.88+/-0.22, and 1.58+/-0.18 %kg dose/g, respectively, P<0.01). Cardiac MIBG clearance was not affected by the induction of DM or by FL, indicating that the reduced MIBG uptake in DM rats might result from impaired neural function.

CONCLUSIONS

FL ameliorates cardiac sympathetic neural dysfunction in DM rats in association with attenuation of increased myocardial oxidative stress.

Influences of autonomic nervous system on atrial arrhythmogenic substrates and the incidence of atrial fibrillation in diabetic heart

Diabetes mellitus (DM) is clinically associated with an increased incidence of atrial fibrillation (AF), but the underlying mechanism remains unclear. We hypothesized that neural remodeling enhances AF vulnerability in diabetic hearts. Eight weeks after creating streptozotocin-induced diabetic rats (DM rats) or control rats, the hearts were perfused according to the Langendorff method. Inducibility of AF was evaluated by 5 times burst pacing from the right atrium and the atrial effective refractory period (AERP) was measured. The protocol was repeated during sympathetic nerve stimulation (SNS) or parasympathetic nerve stimulation (PNS). In tissue samples taken from the right atrium, the density of nerves positive for tyrosine hydroxylase (TH) and acetylcholinesterase (AChE) were determined. SNS significantly increased the incidence of AF in DM rats (14 +/- 6 to 30 +/- 8%, P < 0.01), but not in control rats (11 +/- 4 to 14 +/- 6%, NS). Although AERP was significantly decreased by SNS in both rats (each P < 0.01), increased heterogeneity of AERP by SNS was seen only in DM rats. PNS significantly decreased AERP and increased the incidence of AF (9 +/- 5 to 30 +/- 5% in control rats, 12 +/- 6 to 27 +/- 6% in DM rats, each P < 0.01) in both rats. The density of TH-positive nerves was heterogeneous in DM rats compared with control rats, whereas the heterogeneity of AChE-positive nerves was not different in the rats. The prevalence of AF was enhanced by adrenergic activation in diabetic hearts, in which heterogeneous sympathetic innervation was evident. These results suggest that neural remodeling may play a crucial role for increased AF vulnerability in DM.

Cardiac innervation and sudden cardiac death

The heart is extensively innervated and its performance is tightly controlled by the nervous system. Cardiac innervation density varies in diseased hearts leading to unbalanced neural activation and lethal arrhythmia. Diabetic sensory neuropathy causes silent myocardial ischemia, characterized by loss of pain perception during myocardial ischemia, which is a major cause of sudden cardiac death in diabetes mellitus (DM). Despite its clinical importance, the mechanisms underlying the control and regulation of cardiac innervation remain poorly understood.We found that cardiac innervation is determined by the balance between neural chemoattractants and chemorepellents within the heart. Nerve growth factor (NGF), a potent chemoattractant, is induced by endothelin-1 upregulation during development and is highly expressed in cardiomyocytes. By comparison, Sema3a, a neural chemorepellent, is highly expressed in the subendocardium of early stage embryos, and is suppressed during development. The balance of expression between NGF and Seme3a leads to epicardial-to-endocardial transmural sympathetic innervation patterning. We also found that downregulation of cardiac NGF leads to diabetic neuropathy, and that NGF supplementation rescues silent myocardial ischemia in DM. Cardiac innervation patterning is disrupted in Sema3a-deficient and Sema3a-overexpressing mice, leading to sudden death or lethal arrhythmias. The present review focuses on the regulatory mechanisms underlying cardiac innervation and the critical role of these processes in cardiac performance.

Effects of cyclooxygenase-2 gene inactivation on cardiac autonomic and left ventricular function in experimental diabetes

Glucose-mediated oxidative stress and the upregulation of cyclooxygenase (COX)-2 pathway activity have been implicated in the pathogenesis of several vascular complications of diabetes including diabetic neuropathy. However, in nondiabetic subjects, the cardiovascular safety of selective COX-2 inhibition is controversial. The aim of this study was to explore the links between hyperglycemia, oxidative stress, activation of the COX-2 pathway, cardiac sympathetic integrity, and the development of left ventricular (LV) dysfunction in experimental diabetes. R wave-to-R wave interval (R-R interval) and parameters of LV function measured by echocardiography using 1% isoflurane, LV sympathetic nerve fiber density, LV collagen content, and markers of myocardial oxidative stress, inflammation, and PG content were assessed after 6 mo in control and diabetic COX-2-deficient (COX-2(-/-)) and littermate, wild-type (COX-2(+/+)) mice. There were no differences in blood glucose, LV echocardiographic measures, collagen content, sympathetic nerve fiber density, and markers of oxidative stress and inflammation between nondiabetic (ND) COX-2(+/+) and COX-2(-/-) mice at baseline and thereafter. After 6 mo, diabetic COX-2(+/+) mice developed significant deteriorations in the R-R interval and signs of LV dysfunction. These were associated with a loss of LV sympathetic nerve fiber density, increased LV collagen content, and a significant increase in myocardial oxidative stress and inflammation compared with those of ND mice. Diabetic COX-2(-/-) mice were protected against all these biochemical, structural, and functional deficits. These data suggest that in experimental diabetes, selective COX-2 inactivation confers protection against sympathetic denervation and LV dysfunction by reducing intramyocardial oxidative stress, inflammation, and myocardial fibrosis.

Impaired activation of celiac ganglion neurons in vivo after damage to their sympathetic nerve terminals

Because damage to sympathetic nerve terminals occurs in a variety of diseases, we tested the hypothesis that nerve terminal damage per se is sufficient to impair ganglionic neurotransmission in vivo. First, we measured the effect of nerve terminal damage produced by the sympathetic nerve terminal toxin 6-hydroxydopamine (6-OHDA) on ganglionic levels of several neurotrophins thought to promote neurotransmission. 6-OHDA-induced nerve terminal damage did not decrease the expression of neurotrophin-4 or brain-derived neurotrophic factor mRNA in the celiac ganglia but did decrease the ganglionic content of both nerve growth factor protein (nadir = -63%) and the mRNA of the alpha-3 subunit of the nicotinic cholinergic receptor (nadir = -49%), a subunit required for neurotransmission. Next, we tested whether this degree of receptor deficiency was sufficient to impair activation of celiac ganglia neurons. Impaired fos mRNA responses to nicotine administration in the celiac ganglia of 6-OHDA-pretreated rats correlated temporally with suppressed expression of functional nicotinic receptors. We verified by Fos protein immunohistochemistry that this ganglionic impairment was specific to principal ganglionic neurons. Last, we tested whether centrally initiated ganglionic neurotransmission is also impaired following nerve terminal damage. The principal neurons in rat celiac ganglia were reflexively activated by 2-deoxy-glucose-induced glucopenia, and the Fos response in the celiac ganglia was markedly inhibited by pretreatment with 6-OHDA. We conclude that sympathetic nerve terminal damage per se is sufficient to impair ganglionic neurotransmission in vivo and that decreased nicotinic receptor production is a likely mediator.

[Cardiovascular impact of the autonomic neuropathy of diabetes mellitus]

The neuropathic complications related to Diabetes may affect the somatic, sympathetic and parasympathetic nervous system. As a result, there are several clinical manifestations of diabetic neuropathy. They can be related to nervous system lesions of the genital, urinary, gastro-intestinal, skin and cardiovascular tissues. The results of these alterations are loss in the quality of life as well as increase of mortality indexes related to sudden death with cardiac arrhythmias and other causes. The cardiovascular autonomic neuropathy probably contributes to the bad prognosis of the coronary heart disease and of the heart failure in type 1 and type 2 diabetic patients. For diabetologists, the nervous complications of diabetes are the result of an increase influx of glucose to the neuronal and endothelial cells. Evidences show that, with the aim of preventing these complications, the diabetic patients should receive a precocious diagnosis and be instructed for having a good metabolic and blood pressure control. Use of angiotensin converting enzyme inhibitors and beta adrenergic blockers are probably of impact in the prevention of the cardiac autonomic complications of diabetes.

Cardiac sympathetic neuronal imaging using PET

INTRODUCTION

Balance of the autonomic nervous system is essential for adequate cardiac performance, and alterations seem to play a key role in the development and progression of various cardiac diseases.

PET AS AN IMAGING TOOL

PET imaging of the cardiac autonomic nervous system has advanced extensively in recent years, and multiple pre- and postsynaptic tracers have been introduced. The high spatial and temporal resolution of PET enables noninvasive quantification of neurophysiologic processes at the tissue level. Ligands for catecholamine receptors, along with radiolabeled catecholamines and catecholamine analogs, have been applied to determine involvement of sympathetic dysinnervation at different stages of heart diseases such as ischemia, heart failure, and arrhythmia.

REVIEW

This review summarizes the recent findings in neurocardiological PET imaging. Experimental studies with several radioligands and clinical findings in cardiac dysautonomias are discussed.

Evidence of left ventricular contractile asynchrony by echocardiographic phase imaging in patients with type 2 diabetes mellitus and without clinically evident heart disease

Left ventricular electromechanical asynchrony has been shown to predict cardiac events in patients with heart failure. This study investigated whether left ventricular asynchrony is present in patients with type 2 diabetes mellitus (DM) with no clinically evident heart disease and normal QRS durations. Asynchrony was evaluated in 24 patients with DM, 15 nondiabetic control subjects, and 20 patients with left bundle branch block (LBBB) due to cardiomyopathy serving as positive controls by conventional tissue Doppler imaging and by a novel method, echocardiographic phase imaging. Asynchrony was significantly higher in patients with DM than in controls and significantly lower than in patients with LBBB. This was shown by tissue Doppler imaging: the SD of time to peak myocardial velocity was 13 +/- 10 ms in controls, compared with 30 +/- 19 ms in patients with DM (p <0.01) and 68 +/- 28 ms in those with LBBB (p <0.001). Similar data were obtained using echocardiographic phase imaging: the SD of phase degrees was 25 degrees +/- 8 degrees in controls, compared with 44 degrees +/- 21 degrees in patients with DM (p = 0.02) and 76 degrees +/- 25 degrees in those with LBBB (p <0.001). Tissue Doppler imaging correlated with echocardiographic phase imaging (r = 0.79, p <0.0001) but was more time consuming (15.5 +/- 4.5 vs 4.5 +/- 2.2 min/patient, p <0.05) and showed higher intraobserver variability (5.6% vs 3.2%, p <0.05). In conclusion, this is the first study showing increased left ventricular asynchrony in patients with DM and no clinical evidence of heart disease.

Nerve Growth Factor Is Critical for Cardiac Sensory Innervation and Rescues Neuropathy in Diabetic Hearts

BACKGROUND

Molecular mechanisms regulating the cardiac sensory nervous system remain poorly understood. Cardiac sensory nerve impairment causes silent myocardial ischemia, a main cause of sudden death in diabetes mellitus (DM). The present study focused on the roles of nerve growth factor (NGF) in the regulation of the cardiac sensory nervous system and analyzed the mechanism of silent myocardial ischemia in DM.

METHODS AND RESULTS

We screened neurotrophic factors and found that cardiac sensory nerves developed in parallel with NGF synthesized in the heart. Cardiac nociceptive sensory nerves that were immunopositive for calcitonin gene-related peptide, dorsal root ganglia (DRG), and the dorsal horn were markedly retarded in NGF-deficient mice, whereas cardiac-specific overexpression of NGF rescued these deficits. DM was induced with streptozotocin in wild-type and transgenic mice overexpressing NGF in the heart. Downregulation of NGF, calcitonin gene-related peptide-immunopositive cardiac sensory denervation, and atrophic changes in DRG were observed in DM-induced wild-type mice, whereas these deteriorations were reversed in DM-induced NGF transgenic mice. Cardiac sensory function, measured by myocardial ischemia-induced c-Fos expression in DRG, was also downregulated by DM in the wild-type mice but not in NGF transgenic mice. Direct gene transfer of NGF in the diabetic rat hearts improved impaired cardiac sensory innervation and function, determined by electrophysiological activity of cardiac afferent nerves during myocardial ischemia.

CONCLUSIONS

These findings demonstrate that the development and regulation of the cardiac sensory nervous system are dependent on NGF synthesized in the heart and that DM-induced NGF reduction causes cardiac sensory neuropathy.

Quantitation of cardiac sympathetic innervation in rabbits using 11C-hydroxyephedrine PET: relation to 123I-MIBG uptake

PURPOSE

Although (11)C-hydroxyephedrine ((11)C-HED) PET is used to map cardiac sympathetic innervation, no studies have shown the feasibility of quantitation of (11)C-HED PET in small- to medium-sized animals. Furthermore, its relation to (123)I-MIBG uptake, the most widely used sympathetic nervous tracer, is unknown. The aims of this study were to establish in vivo sympathetic nerve imaging in rabbits using (11)C-HED PET, and to compare the retention of (11)C-HED with that of (123)I-MIBG.

METHODS

Twelve rabbits were assigned to three groups; control (n=4), chemical denervation by 6-hydroxydopamine (6-OHDA) (n=4) and reserpine treated to inhibit vesicular uptake (n=4). After simultaneous injection of (11)C-HED and (123)I-MIBG, all animals underwent dynamic (11)C-HED PET for 40 min with arterial blood sampling. The (11)C-HED retention fraction and normalised (11)C-HED activity measured by tissue sampling were compared with those measured by PET.

RESULTS

Both the (11)C-HED retention fraction and the normalised (11)C-HED activity measured by PET correlated closely with those measured by tissue sampling (R=0.96027, p<0.001 and R=0.97282, p<0.001, respectively). Inhibition study by 6-OHDA resulted in a significant reduction in retention (90%) for both (11)C-HED and (123)I-MIBG. Reserpine pretreatment reduced (11)C-HED retention by 50%, but did not reduce (123)I-MIBG retention at 40 min after injection.

CONCLUSION

Non-invasive quantitation of cardiac sympathetic innervation using (11)C-HED PET is feasible and gives reliable estimates of cardiac sympathetic innervation in rabbits. Additionally, although both (11)C-HED and (123)I-MIBG are specific for sympathetic neurons, (11)C-HED may be more specific for intravesicular uptake than (123)I-MIBG in some situations, such as that seen in reserpine pretreatment.

Heterogenous changes in neuropeptide Y, norepinephrine and epinephrine concentrations in the hearts of diabetic rats

The changes in concentrations of neuropeptide Y (NPY), norepinephrine and epinephrine were investigated in the rat hearts 1, 2, 4, 6, 9 and 12 months after administration of streptozotocin (STZ; 65 mg/kg i.v.). About 30% of diabetic animals displayed symptoms of partial spontaneous recovery, i.e. decreasing blood glucose levels and increasing insulin concentrations in the plasma and pancreas. NPY concentrations in the atria of diabetic rats did not differ from those in age-matched control rats 1, 2, 4, 6 months in the right atria and even 9 months after STZ in the left atria. However, uncompensated diabetes led to a significant decrease in NPY levels 9 and 12 months after STZ administration in the right and left atria, respectively. In the ventricles, NPY concentrations were significantly decreased 6 months after the onset of diabetes. Interestingly, partial spontaneous recovery of diabetes was associated with increased NPY levels in the atria. Myocardial norepinephrine concentrations increased 1 month after STZ and then declined reaching approximately 60% of the respective control values 12 months after the onset of the disease. Partial spontaneous recovery of diabetes had no effect on norepinephrine concentrations. Myocardial epinephrine concentrations did not differ from those found in controls till month 9 of the disease and they became significantly lower at month 12. Partial recovery of diabetes resulted in epinephrine concentrations not differing from the control values at month 12 of diabetes. Regarding to preferential localization of norepinephrine in the sympathetic postganglionic fibers and that of NPY also in intrinsic ganglion neurons, intrinsic neuronal circuits seem to be less susceptible to STZ-induced damage than extrinsic nerves and they might be able to recover after amelioration of diabetes.

Autonomic dysfunction in rheumatic diseases

Patients who have rheumatic diseases often present with dysfunctions that are related to the autonomic nervous system (ANS) and are due to peripheral autonomic neuropathy or central changes. This article describes the prevalence of autonomic dysfunctions in patients who have rheumatic diseases. In the second part of this article, another form of ANS dysfunction-complex regional pain syndromes-is demonstrated. Clinically, these syndromes are characterized by pain (spontaneous, hyperalgesia, allodynia); active movement disorders, including an increased physiologic tremor, abnormal regulation of blood flow and sweating, edema of skin and subcutaneous tissues; and trophic changes of skin, appendages of skin, and subcutaneous tissues. In conclusion, this discussion shows that alterations of the ANS occur in rheumatic and related diseases, that these alterations may be involved in the pathogenesis of these diseases, and that we need more refined methods to study the changes that are related to the ANS.

Sympathetic dysfunction in type 1 diabetes: association with impaired myocardial blood flow reserve and diastolic dysfunction

OBJECTIVES

This study was designed to explore the relationships of early diabetic microangiopathy to alterations of cardiac sympathetic tone and myocardial blood flow (MBF) regulation in subjects with stable type 1 diabetes.

BACKGROUND

In diabetes, augmented cardiac sympathetic tone and abnormal MBF regulation may predispose to myocardial injury and enhanced cardiac risk.

METHODS

Subject groups comprised healthy controls (C) (n = 10), healthy diabetic subjects (DC) (n = 12), and diabetic subjects with very early diabetic microangiopathy (DMA+) (n = 16). [(11)C]meta-hydroxyephedrine ([(11)C]HED) and positron emission tomography (PET) were used to explore left ventricular (LV) sympathetic integrity and [(13)N]ammonia-PET to assess MBF regulation in response to cold pressor testing (CPT) and adenosine infusion.

RESULTS

Deficits of LV [(11)C]HED retention were extensive and global in the DMA+ subjects (36 +/- 31% vs. 1 +/- 1% in DC subjects; p < 0.01) despite preserved autonomic reflex tests. On CPT, plasma norepinephrine excursions were two-fold greater than in C and DC subjects (p < 0.05), and basal LV blood flow decreased (-12%, p < 0.05) in DMA+ but not in C or DC subjects (+45% and +51%, respectively). On adenosine infusion, compared with C subjects, MBF reserve decreased by approximately 45% (p < 0.05) in DMA+ subjects. Diastolic dysfunction was detected by two-dimensional echocardiography in 5 of 8 and 0 of 8 consecutively tested DMA+ and DC subjects, respectively.

CONCLUSIONS

Augmented cardiac sympathetic tone and responsiveness and impaired myocardial perfusion may contribute to myocardial injury in diabetes.

Assessment of cardiac sympathetic neuronal function using PET imaging

The autonomic nervous system plays a key role for regulation of cardiac performance, and the importance of alterations of innervation in the pathophysiology of various heart diseases has been increasingly emphasized. Nuclear imaging techniques have been established that allow for global and regional investigation of the myocardial nervous system. The guanethidine analog iodine 123 metaiodobenzylguanidine (MIBG) has been introduced for scintigraphic mapping of presynaptic sympathetic innervation and is available today for imaging on a broad clinical basis. Not much later than MIBG, positron emission tomography (PET) has also been established for characterizing the cardiac autonomic nervous system. Although PET is methodologically demanding and less widely available, it provides substantial advantages. High spatial and temporal resolution along with routinely available attenuation correction allows for detailed definition of tracer kinetics and makes noninvasive absolute quantification a reality. Furthermore, a series of different radiolabeled catecholamines, catecholamine analogs, and receptor ligands are available. Those are often more physiologic than MIBG and well understood with regard to their tracer physiologic properties. PET imaging of sympathetic neuronal function has been successfully applied to gain mechanistic insights into myocardial biology and pathology. Available tracers allow dissection of processes of presynaptic and postsynaptic innervation contributing to cardiovascular disease. This review summarizes characteristics of currently available PET tracers for cardiac neuroimaging along with the major findings derived from their application in health and disease.

Diabetic cardiomyopathy: evidence, mechanisms, and therapeutic implications

The presence of a diabetic cardiomyopathy, independent of hypertension and coronary artery disease, is still controversial. This systematic review seeks to evaluate the evidence for the existence of this condition, to clarify the possible mechanisms responsible, and to consider possible therapeutic implications. The existence of a diabetic cardiomyopathy is supported by epidemiological findings showing the association of diabetes with heart failure; clinical studies confirming the association of diabetes with left ventricular dysfunction independent of hypertension, coronary artery disease, and other heart disease; and experimental evidence of myocardial structural and functional changes. The most important mechanisms of diabetic cardiomyopathy are metabolic disturbances (depletion of glucose transporter 4, increased free fatty acids, carnitine deficiency, changes in calcium homeostasis), myocardial fibrosis (association with increases in angiotensin II, IGF-I, and inflammatory cytokines), small vessel disease (microangiopathy, impaired coronary flow reserve, and endothelial dysfunction), cardiac autonomic neuropathy (denervation and alterations in myocardial catecholamine levels), and insulin resistance (hyperinsulinemia and reduced insulin sensitivity). This review presents evidence that diabetes is associated with a cardiomyopathy, independent of comorbid conditions, and that metabolic disturbances, myocardial fibrosis, small vessel disease, cardiac autonomic neuropathy, and insulin resistance may all contribute to the development of diabetic heart disease.