Inhomogeneous derangement of cardiac autonomic nerve control in diabetic rats

Heart rate variability in male rats

The cardiovascular system is primarily controlled by the autonomic nervous system, and any changes in sympathetic or parasympathetic activity also have an impact on myocardial activity. Heart rate variability (HRV) is a readily available metric used to assess heart rate control by the autonomic nervous system. HRV can provide information about neural (parasympathetic, sympathetic, reflex) and humoral (hormones, thermoregulation) control of myocardial activity. Because there are no relevant reference values for HRV parameters in rats in the scientific literature, all experimental results are only interpreted on the basis of changes from currently measured control or baseline HRV values, which are, however, significantly different in individual studies. Considering the significant variability of published HRV data, the present study focused primarily on comparing control or baseline HRV values under different conditions in in vivo experiments involving rats. The aim of the study was therefore to assess whether there are differences in the starting values before the experiment itself.

3,5-Diiodothyronine protects against cardiac ischaemia-reperfusion injury in male rats

NEW FINDINGS

What is the central question of this study? 3,5-Diiodothyronine (3,5-T2) administration increases resting metabolic rate, prevents or treats liver steatosis in rodent models, and ameliorates insulin resistance: what are its effects on cardiac electrical and contractile properties and autonomic regulation? What is the main finding and its importance? Chronic 3,5-T2 administration has no adverse effects on cardiac function. Remarkably, 3,5-T2 improves the autonomous control of the rat heart and protects against ischaemia-reperfusion injury.

ABSTRACT

The use of 3,5,3'-triiodothyronine (T3) and thyroxine (T4) to treat metabolic diseases has been hindered by potential adverse effects on liver, lipid metabolism and cardiac electrical properties. It is recognized that 3,5-diiodothyronine (3,5-T2) administration increases resting metabolic rate, prevents or treats liver steatosis in rodent models and ameliorates insulin resistance, suggesting 3,5-T2 as a potential therapeutic tool. However, a comprehensive assessment of cardiac electrical and contractile properties has not been made so far. Three-month-old Wistar rats were daily administered vehicle, 3,5-T2 or 3,5-T2+T4 and no signs of atrial or ventricular arrhythmia were detected in non-anaesthetized rats during 90 days. Cardiac function was preserved as heart rate, left ventricle diameter and shortening fraction in 3,5-T2-treated rats compared to vehicle and 3,5-T2+T4 groups. Power spectral analysis indicated an amelioration of the heart rate variability only in 3,5-T2-treated rats. An increased baroreflex sensitivity at rest was observed in both 3,5-T2-treated groups. Finally, 3,5-T2 Langendorff-perfused hearts presented a significant recovery of left ventricular function and remarkably smaller infarction area after ischaemia-reperfusion injury. In conclusion, chronic 3,5-T2 administration ameliorates tonic cardiac autonomic control and confers cardioprotection against ischaemia-reperfusion injury in healthy male rats.

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.

The initial level of heart rate variability determines the dynamics of the neuropathy of the autonomic nervous system and temperature sensitivity in rats with streptozotocin diabetes

BACKGROUND: The development of diabetes mellitus (DM) is accompanied by hyperglycemia, which leads to the development of neuropathy. We assume that the individual characteristics of the organization of the autonomic nervous system (ANS) in humans affect not only the ability to withstand stress, but can determine the course of diseases, including diabetes. AIMS: The study of the dynamics of heart rate variability and temperature sensitivity in rats against the background of hyperglycemia depending on the organization of the initial regulation of the heart under ANS. MATERIALS AND METHODS: 70 male rats were randomized by weight and level of total heart rate variability (HRV) on animals with initially low and high levels of total HRV. Diabetes was modeled by a single i.p. injection of streptozotocin (STZ). The control group received a single i.p. injection of citrate buffer (CB). Before the induction of diabetes, as well as on 21, 42 and 70 days after the injection of STZ, a comprehensive examination of rats condition was carried out: 1) assessment of HRV; 2) analysis of temperature-pain sensitivity; 3) assessment of glucose and ketone bodies in the blood. Similarly, animals in the CB group were examined. RESULTS: The change in HRV and temperature-pain sensitivity in rats depends on the initial level of HRV. In rats with low variability, the reaction time in the pain test increased by 832% from the 28th day of the experiment, from 21 days the bradycardia increased and the decrease in individual HRV indices at rest, but not the response of these parameters to cold stress. CONCLUSION: The development of ANSs lesion depends on the initial level of HRV. Low variability animals are more resistant to hyperglycemia: the normal ranges of CP reactions from the side of HR, the parameters of total HRV and the CVHS contribution to it are longer, but there is a loss of temperature sensitivity. Initially, highly variable rats with the development of diabetes do not lose temperature sensitivity, but demonstrate an imbalance in the regulatory circuits of heart rate and HRV.

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.

Gastrointestinal motility changes and myenteric plexus alterations in spontaneously diabetic biobreeding rats

BACKGROUND/AIMS

Type 1 diabetes is often accompanied by gastrointestinal motility disturbances. Vagal neuropathy, hyperglycemia, and alterations in the myenteric plexus have been proposed as underlying mechanism. We therefore studied the relationship between vagal function, gastrointestinal motiliy and characteristics of the enteric nervous system in the biobreeding (BB) rat known as model for spontaneous type 1 diabetes.

METHODS

Gastric emptying breath test, small intestinal electromyography, relative risk-interval variability, histology and immunohistochemistry on antral and jejunal segments were performed at 1, 8 and 16 weeks after diabetes onset and on age-matched controls.

RESULTS

We observed no consistent changes in relative risk-interval variability and gastric emptying rate. There was however, a loss of phases 3 with longer duration of diabetes on small intestinal electromyography. We found a progressive decrease of nitrergic neurons in the myenteric plexus of antrum and jejunum, while numbers of cholinergic nerve were not altered. In addition, a transient inflammatory infiltrate in jejunal wall was found in spontaneous diabetic BB rats at 8 weeks of diabetes.

CONCLUSIONS

In diabetic BB rats, altered small intestinal motor control associated with a loss of myenteric nitric oxide synthase expression occurs, which does not depend on hyperglycemia or vagal dysfunction, and which is preceded by transient intestinal inflammation.

Synaptic degradation of cardiac autonomic nerves in streptozotocin-induced diabetic rats

BACKGROUND

Cardiac autonomic neuropathy (CAN) is a common complication in type I diabetes mellitus (DM). Nevertheless, the relationship between functional and structural disturbances of cardiac autonomic nerves remains unclear.

METHODS AND RESULTS

To clarify this relationship, we studied heart rate variability (HRV) and ultrastructural changes of cardiac autonomic nerves in streptozotocin (STZ)-induced DM in rats. STZ was injected (65mg/kg intravenous) into the tail vein of male Wistar rats to destroy β cells in the pancreatic islets. After STZ injection, fasting blood sugar (FBS) increased from baseline values of 75±3mg/dl up to 328±12mg/dl within 1week and it reached up to 353±24mg/dl within 17weeks. HR in these rats was decreased within 20days and low HR was maintained for the observation period. TP and HF power started decreasing 20days after STZ injection, and this decrease progressed throughout the observation period. The L/H power ratio was decreased 80days after STZ. Electron microscopic findings indicated a depletion of neurotransmitter vesicles and degradation of parasympathetic nerve endings but not of sympathetic ones in the SA node region of the heart in the early stages of DM. In the late stages of DM, the same region showed degradation of both sympathetic and parasympathetic nerve endings.

CONCLUSION

Synaptic degradation in parasympathetic nerves immediately after the onset of DM, and in sympathetic nerves much later in the development of DM is consistent with functional derangements in cardiac autonomic nerve activities assessed by HRV analysis.

Extended longitudinal analysis of arterial pressure and heart rate control in unanesthetized rats with type 1 diabetes

We recorded arterial pressure (BP) and heart rate (HR) in type-1 diabetic rats vs. controls for >6 months. Diabetic rats (DIAB) were maintained on insulin from the day glucose >250 mg/dl ("Day 0"). Weight was similar between groups until ~3 weeks before Day 0 when the weight in DIAB transiently lagged the controls (CONT); this difference was maintained throughout the study, but both groups otherwise gained weight in parallel. Plasma glucose attained 371 ± 109 (SD) mg/dl by day 1 in DIAB. Mean BP was similar across groups, and declined through the initial 4-6 months in both the CONT (at -0.06 ± 0.04 mmHg/day) and in the DIAB (at -0.14 ± 0.21 mmHg/day; NS vs. CONT). HR in the CONT (Month 1: 341 ± 13 bpm) exceeded DIAB (325 ± 25 bpm) through ~6 months after Day 0, and also decreased progressively over this period in CONT (-0.19 ± 0.14 bpm/day) and DIAB (-0.29 ± 0.23 bpm/day; NS vs. CONT) before leveling. The BP power within 0.35-0.45 Hz changed during the 90 min before vs. after the transition from dark to light, and light to dark; there were no between group differences. The slope of the log-log linear portion of the BP power spectrum between 1.0/h and 1/min was similar across groups, and increased in both from month 1 to month 6. Regulatory mechanisms maintain similar profiles in BP and HR in diabetic vs. control animals through the initial half year of the disease.

Increase of ATP-sensitive potassium (K(ATP)) channels in the heart of type-1 diabetic rats

Background

An impairment of cardiovascular function in streptozotocin (STZ)-diabetic rats has been mentioned within 5 days-to-3 months of induction. ATP-sensitive potassium (K_ATP) channels are expressed on cardiac sarcolemmal membranes. It is highly responsive to metabolic fluctuations and can have effects on cardiac contractility. The present study attempted to clarify the changes of cardiac K_ATP channels in diabetic disorders.

Methods

Streptozotocin-induced diabetic rats and neonatal rat cardiomyocytes treated with a high concentration of glucose (a D-glucose concentration of 30 mM was used and cells were cultured for 24 hr) were used to examine the effect of hyperglycemia on cardiac function and the expression of K_ATP channels. K_ATP channels expression was found to be linked to cardiac tonic dysfunction, and we evaluated the expression levels of K_ATP channels by Western blot and Northern blot analysis.

Results

The result shows diazoxide produced a marked reduction of heart rate in control group. Furthermore, the methods of Northern blotting and Western blotting were employed to identify the gene expression of K_ATP channel. Two subunits of cardiac K_ATP channel (SUR2A and kir 6.2) were purchased as indicators and showed significantly decreased in both diabetic rats and high glucose treated rat cardiac myocytes. Correction of hyperglycemia by insulin or phlorizin restored the gene expression of cardiac K_ATP in these diabetic rats.

Conclusions

Both mRNA and protein expression of cardiac K_ATP channels are decreased in diabetic rats induced by STZ for 8 weeks. This phenomenon leads to result in desensitization of some K_ATP channel drugs.

Noninvasive method for electrocardiogram recording in conscious rats: feasibility for heart rate variability analysis

Heart rate variability (HRV) analysis consists in a well-established tool for the assessment of cardiac autonomic control, both in humans and in animal models. Conventional methods for HRV analysis in rats rely on conscious state electrocardiogram (ECG) recording based on prior invasive surgical procedures for electrodes/transmitters implants. The aim of the present study was to test a noninvasive and inexpensive method for ECG recording in conscious rats, assessing its feasibility for HRV analysis. A custom-made elastic cotton jacket was developed to fit the rat's mean thoracic circumference, with two pieces of platinum electrodes attached on its inner surface, allowing ECG to be recorded noninvasively in conscious, restrained rats (n=6). Time- and frequency-domain HRV analyses were conducted, under basal and autonomic blockade conditions. High-quality ECG signals were obtained, being feasible for HRV analysis. As expected, mean RR interval was significantly decreased in the presence of atropine (p <0.05) and increased in the presence of propranolol (p<0.001). Also, reinforcing the reliability of the method, low- and high-frequency HRV spectral powers were significantly decreased in the presence of propranolol (p <0.05) and atropine (p< 0.001), respectively. In summary, the present work describes a novel, inexpensive and noninvasive method for surface ECG recording in conscious rats.

Role of hyperglycaemia in the pathogenesis of hypotension observed in type-1 diabetic rats

The role of hyperglycaemia in the pathogenesis of hypotension in diabetic disorders was investigated using the changes in cardiac M(2)-muscarinic receptor (M(2)-mAChR) gene expression in type-1-like diabetic rats and cultured cardiomyocytes. Blood pressure was markedly decreased in diabetic rats following the intravenous injection of streptozotocin (STZ) for 8 weeks. Also, the baroreflex sensitivity (Delta HR/Delta BP), as measured by the changes in heart rate (Delta HR) and mean blood pressure (Delta BP) 1 min after the intravenous injection of phenylephrine (10 microg/kg), was significantly increased. Arecaidine propargyl ester (APE), a M(2)-mAChR agonist produced a marked reduction in heart rate in these diabetic rats. Normalization of plasma glucose in diabetic rats using insulin (0.5 IU) or phlorizin (1 mg/kg) injection attenuated the blood pressure reduction and reversed the mRNA and protein levels of cardiac M(2)-mAChR. A high concentration of glucose (20 mmol/l) directly influenced the increase in gene expression of M(2)-mAChR in the H9c2 cardiac cell line. Hyperglycaemia induced an increase in cardiac M(2)-mAChR gene expression, suggesting a role in the pathogenesis of hypotension in diabetic disorders.

Effect of beta-adrenoblockers on heart rate variability in awake and narcotized rats

In narcotized rats, beta-adrenoceptor blockers changed all parameters of heart rate variability indicating up-regulation of parasympathetic activity and down-regulation of the sympathetic one. In immobilized awake rats, the temporal and geometrical parameters varied similarly, while spectral analysis indicated possible activation of other neurohumoral mechanisms against the background of beta-blocker treatment.

Cardiac autonomic dysfunction in rats chronically treated with anabolic steroid

To date no published data exist regarding the effects of chronic high-dose anabolic-androgenic steroid administration on tonic cardiac autonomic control. The aim of this study was to evaluate, by power spectral analysis of heart rate variability (HRV), the effects of chronic treatment with supraphysiological doses of nandrolone decanoate (DECA) on tonic cardiac autonomic regulation in sedentary rats. Male Wistar rats were treated weekly with 10 mg kg(-1) of DECA (n=7) or vehicle (CONTROL, n=7) for 10 weeks. At the 8th week of treatment, electrocardiogram was recorded in the conscious state, for time- and frequency-domain HRV analysis. Parasympathetic indexes were reduced in DECA group: high-frequency power (CONTROL=11.1+/-3.0 ms2 vs. DECA=3.8+/-0.6 ms2, P<0.05), RMSSD (CONTROL=5.9+/-0.9 ms vs. DECA 3.5+/-0.3 ms; P<0.05) and pNN5 (CONTROL=31.5+/-7.5 ms vs. DECA=13.2+/-2.6 ms; P<0.05). The sympathetic index LF/HF tended to be higher in DECA group (CONTROL=0.65+/-0.15 vs. DECA=1.17+/-0.26, P=0.0546). In conclusion, chronic treatment with DECA, in rats, impairs tonic cardiac autonomic regulation, which may provide a key mechanism for anabolic steroid-induced arrhythmia and sudden cardiac death.

The role of sorbitol pathway and treatment effect of aldose reductase inhibitor ONO2235 in the up-regulation of cardiac M2-muscarinic receptors in streptozotocin-induced diabetic rats

This study investigated the role of the sorbitol pathway on the genetic up-regulation of the cardiac M(2) muscarinic receptor (M(2)-mAChR) in streptozotocin (STZ)-induced diabetic rats. Three-month-old male Wistar rats were divided into five groups: (1) normal controls; (2) rats rendered diabetic by streptozotocin; (3) rats fed with glucose; (4) rats injected with sorbitol; and (5) diabetic rats treated with ONO-2235, an aldose reductase inhibitor. The M(2) muscarinic receptor (M(2)-mAChR) protein and mRNA densities of the heart tissue were measured by Western immunoblotting and Northern blotting, respectively. The densities of M(2)-mAChR protein and mRNA in the heart were significantly increased in diabetic rats, and rats given either glucose or sorbitol. When diabetic rats were treated with ONO-2235, the increases in heart M(2)-mAChR protein and mRNA were significantly reduced. The findings suggest that hyperglycemia and the sorbitol pathway are involved in the pathogenesis of diabetic heart disease in STZ-induced diabetic rats. Aldose reductase inhibitors may be useful in the treatment and prevention of cardiac complication in diabetes.

Cardiovascular responses in unrestrained WKY rats to inhaled ultrafine carbon particles

Based on epidemiologic observations, the issue of adverse health effects of inhaled ultrafine particles (UFP) is currently under intensive discussion. We therefore examined cardiovascular effects of UFP in a controlled animal exposure on young, healthy WKY rats. Short-term exposure (24 h) to carbon UFPs (38 nm, 180 microg m (-3)), generated by spark discharging, induced a mild but consistent increase in heart rate (18 bpm, 4.8%), which was associated with a significant decrease in heart-rate variability during particle inhalation. The timing and the transient character of these responses point to a particle induced alteration of cardiac autonomic balance, mediated by a pulmonary receptor activation. After 24 h of inhalation exposure, bronchoalveolar lavage revealed significant but low-grade pulmonary inflammation (clean air 1.9% vs. UFPs 6.9% polymorphonuclear cells) and on histopathology sporadic accumulation of particle-laden macrophages was found in the alveolar region. There was no evidence of an inflammation-mediated increase in blood coagulability, as UFP inhalation did not induce any significant changes in plasma fibrinogen or factor VIIa levels and there were no prothrombotic changes in the lung or the heart at both the protein and mRNA level. Histological analysis revealed no signs of cardiac inflammation or cardiomyopathy. This study therefore provides toxicological evidence for UFP-associated pulmonary and cardiac effects in healthy rats. Our findings suggest that the observed changes are mediated by an altered sympatho-vagal balance in response to UFP inhalation, but do not support the concept of an inflammation-mediated prothrombotic state by UFP.

Characteristics of autonomic nervous function in Zucker-fatty rats: investigation by power spectral analysis of heart rate variability

We investigated the characteristics of autonomic nervous function in Zucker-fatty and Zucker-lean rats. For this purpose, a long-term electrocardiogram (ECG) was recorded from conscious and unrestrained rats using a telemetry system, and the autonomic nervous function was investigated by power spectral analysis of heart rate variability (HRV). Although heart rate (HR) in Zucker-fatty rats was lower than that in Zucker-lean rats throughout 24 h, apparent diurnal variation in HR was observed in both strains and HR during the dark period was significantly higher than that in light period. Diurnal variation in locomotor activity (LA) in Zucker-fatty rats was also observed, but LA was lower than that in Zucker lean rats, especially during the dark period. There were no significant differences, however, in high-frequency (HF) power, low-frequency (LF) power, and the LF/HF ratio between Zucker-fatty and Zucker-lean rats. The circadian rhythm of these parameters was mostly preserved in both strains of rats. Moreover, the effect of autonomic blockades on HRV was nearly the same in Zucker-fatty and Zucker-lean rats. These results suggest that the autonomic nervous function of insulin-resistant Zucker-fatty rats remain normal, from the aspect of power spectral analysis of HRV.