Assessment of diabetic cardiac autonomic neuropathy in type I diabetic mice

Streptozotocin-induced type 1 and 2 diabetes in rodents: a model for studying diabetic cardiac autonomic neuropathy

Background

Several animal models are continually being developed to study diabetic complication. Several conflicting regimen for diabetes induction exist in the literature with varying dose strength and regimen for different study interest in diabetes. This study aims to show the effect of high dose streptozotocin (STZ) on the one hand compared with multiple low doses after high fat diet induction on diabetic cardiac autonomic neuropathy (DCAN).

Methodology

Eighty-four Wistar rats were used to demonstrate DCAN induction using 2 approaches one for T1DM (STZ 50mg/kg) and the other for T2DM (HFD for 8 weeks with STZ 25mg/Kg daily for five days). DCAN features were assessed using invasive biomarkers, histology patterns and cardiac nerve densities.

Results

Diabetes induction rate was 76% and 89% in T1DM and T2DM model respectively. T1DM group had significant weight loss, reduced c-peptide, and insulin level post induction. The T2DM additionally showed significantly higher total cholesterol and Homeostatic model assessment (HOMA) compared with control. Serum levels of catecholamine, choactase, nerve growth factor and cardiac nerve density confirms development of DCAN.

Conclusion

High single dose of STZ and HFD with multiple low doses of STZ may be recommended for DCAN study in T1DM and T2DM rat model respectively.

Effects of respiratory muscle training on parasympathetic activity in diabetes mellitus

This study verified the effects of respiratory muscle training (RMT) on hemodynamics, heart rate (HR) variability, and muscle morphology in rats with streptozotocin-induced diabetes mellitus (DM). Thirty-six male Wistar rats were randomized into 4 groups and 34 completed the study: i) sham-sedentary (Sham-ST; n=9); ii) sham-RMT (Sham-RMT; n=9); iii) DM-sedentary (DM-ST; n=8); and iv) DM-RMT (DM-RMT; n=8). Hemodynamics were assessed by central cannulation, and R-R intervals were measured by electrocardiogram. In addition, the effects of RMT on the cross-sectional area of the diaphragm, anterior tibial, and soleus muscles were analyzed. The induction of DM by streptozotocin resulted in weight loss, hyperglycemia, reduced blood pressure, and attenuated left ventricular contraction and relaxation (P<0.05). We also observed a decrease in root mean square of successive differences between adjacent RR intervals (RMSSD) index and in the cross-sectional area of the muscles assessed, specifically the diaphragm, soleus, and anterior tibial muscles in diabetic rats (P<0.05). Interestingly, RMT led to an increase in RMSSD in rats with DM (P<0.05). The induction of DM produced profound deleterious changes in the diaphragmatic and peripheral muscles, as well as impairments in cardiovascular hemodynamics and autonomic control. Nevertheless, RMT may beneficially attenuate autonomic changes and improve parasympathetic modulation.

Sensory and autonomic function and structure in footpads of a diabetic mouse model

Sensory and autonomic neuropathy affects the majority of type II diabetic patients. Clinically, autonomic evaluation often focuses on sudomotor function yet this is rarely assessed in animal models. We undertook morphological and functional studies to assess large myelinated and small unmyelinated axons in the db/db type II diabetes mouse model. We observed that autonomic innervation of sweat glands in the footpads was significantly reduced in db/db mice compared to control db/+ mice and this deficit was greater compared to reductions in intraepidermal sensory innervation of adjacent epidermis. Additionally, db/db mice formed significantly fewer sweat droplets compared to controls as early as 6 weeks of age, a time when no statistical differences were observed electrophysiologically between db/db and db/+ mice studies of large myelinated sensory and motor nerves. The rate of sweat droplet formation was significantly slower and the sweat droplet size larger and more variable in db/db mice compared to controls. Whereas pilocarpine and glycopyrrolate increased and decreased sweating, respectively, in 6 month-old controls, db/db mice did not respond to pharmacologic manipulations. Our findings indicate autonomic neuropathy is an early and prominent deficit in the db/db model and have implications for the development of therapies for peripheral diabetic neuropathy.

Autonomic remodeling may be responsible for decreased incidence of aortic dissection in STZ-induced diabetic rats via down-regulation of matrix metalloprotease 2

Background

Epidemiological studies reported that diabetic patients had a lower incidence of aortic dissection (AD), but the definite mechanism is unknown. We aim to investigate the possible protective effect of diabetes mellitus (DM) on AD formation with an emphasis on autonomic remodeling.

Methods

Streptozotocin (STZ) intraperitoneal injection was applied to induce diabetes, unilateral renal artery stenosis (URAS) together with β-amino propionitrile (BAPN) oral treatment was used to induce AD. Sixty SD rats were equally and randomly divided into four groups (normal group, DM group, URAS + BAPN oral treatment group, DM + URAS + BAPN oral treatment group). Rats were fed for 6 weeks, the number of AD was recorded and remained rats were sacrificed. Thoracic aorta were harvested, morphological changes were assessed. Expression of tyrosine hydroxylase (TH), choline acetylase (ChAT), matrix metalloprotease 2 (MMP2) and matrix metalloprotease 9 (MMP9) were evaluated.

Results

A total of 7 AD was noted in S + B group, DM rats did not develop AD. Diabetic rats had a lower incidence of AD (P < 0.01). In dissected aorta, collagen deposition increased while elastic fiber became fragmented. These pathological changes diminished in diabetic rats. Diabetic rats had a lower expression of ChAT (P < 0.01). URAS + BAPN treatment elevated expression of TH in normal rat and ChAT in diabetic rats (P < 0.001). Expression of MMP2 and MMP9 elevated in all the rats after URAS + BAPN, but the elevation range of MMP2 in diabetic rats was smaller (P < 0.001).

Conclusions

STZ-induced diabetic rats have a lower incidence of AD after URAS and BAPN treatment, this protective effect could be possibly attributed to autonomic innervation modification and possible related down-regulation of MMP2.

High Intensity Aerobic Exercise Training Improves Deficits of Cardiovascular Autonomic Function in a Rat Model of Type 1 Diabetes Mellitus with Moderate Hyperglycemia

Indices of cardiovascular autonomic neuropathy (CAN) in experimental models of Type 1 diabetes mellitus (T1DM) are often contrary to clinical data. Here, we investigated whether a relatable insulin-treated model of T1DM would induce deficits in cardiovascular (CV) autonomic function more reflective of clinical results and if exercise training could prevent those deficits. Sixty-four rats were divided into four groups: sedentary control (C), sedentary T1DM (D), control exercise (CX), or T1DM exercise (DX). Diabetes was induced via multiple low-dose injections of streptozotocin and blood glucose was maintained at moderate hyperglycemia (9-17 mM) through insulin supplementation. Exercise training consisted of daily treadmill running for 10 weeks. Compared to C, D had blunted baroreflex sensitivity, increased vascular sympathetic tone, increased serum neuropeptide Y (NPY), and decreased intrinsic heart rate. In contrast, DX differed from D in all measures of CAN (except NPY), including heart rate variability. These findings demonstrate that this T1DM model elicits deficits and exercise-mediated improvements to CV autonomic function which are reflective of clinical T1DM.

Basic Science Evidence for the Link Between Erectile Dysfunction and Cardiometabolic Dysfunction

INTRODUCTION

Although clinical evidence supports an association between cardiovascular/metabolic diseases (CVMD) and erectile dysfunction (ED), scientific evidence for this link is incompletely elucidated.

AIM

This study aims to provide scientific evidence for the link between CVMD and ED.

METHODS

In this White Paper, the Basic Science Committee of the Sexual Medicine Society of North America assessed the current literature on basic scientific support for a mechanistic link between ED and CVMD, and deficiencies in this regard with a critical assessment of current preclinical models of disease.

RESULTS

A link exists between ED and CVMD on several grounds: the endothelium (endothelium-derived nitric oxide and oxidative stress imbalance); smooth muscle (SM) (SM abundance and altered molecular regulation of SM contractility); autonomic innervation (autonomic neuropathy and decreased neuronal-derived nitric oxide); hormones (impaired testosterone release and actions); and metabolics (hyperlipidemia, advanced glycation end product formation).

CONCLUSION

Basic science evidence supports the link between ED and CVMD. The Committee also highlighted gaps in knowledge and provided recommendations for guiding further scientific study defining this risk relationship. This endeavor serves to develop novel strategic directions for therapeutic interventions.

Sinoatrial node dysfunction induces cardiac arrhythmias in diabetic mice

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Glycogen synthase kinase-3β inhibition ameliorates cardiac parasympathetic dysfunction in type 1 diabetic Akita mice

Decreased heart rate variability (HRV) is a major risk factor for sudden death and cardiovascular disease. We previously demonstrated that parasympathetic dysfunction in the heart of the Akita type 1 diabetic mouse was due to a decrease in the level of the sterol response element-binding protein (SREBP-1). Here we demonstrate that hyperactivity of glycogen synthase kinase-3β (GSK3β) in the atrium of the Akita mouse results in decreased SREBP-1, attenuation of parasympathetic modulation of heart rate, measured as a decrease in the high-frequency (HF) fraction of HRV in the presence of propranolol, and a decrease in expression of the G-protein coupled inward rectifying K(+) (GIRK4) subunit of the acetylcholine (ACh)-activated inward-rectifying K(+) channel (IKACh), the ion channel that mediates the heart rate response to parasympathetic stimulation. Treatment of atrial myocytes with the GSK3β inhibitor Kenpaullone increased levels of SREBP-1 and expression of GIRK4 and IKACh, whereas a dominant-active GSK3β mutant decreased SREBP-1 and GIRK4 expression. In Akita mice treated with GSK3β inhibitors Li(+) and/or CHIR-99021, Li(+) increased IKACh, and Li(+) and CHIR-99021 both partially reversed the decrease in HF fraction while increasing GIRK4 and SREBP-1 expression. These data support the conclusion that increased GSK3β activity in the type 1 diabetic heart plays a critical role in parasympathetic dysfunction through an effect on SREBP-1, supporting GSK3β as a new therapeutic target for diabetic autonomic neuropathy.

Autonomic cardiac innervation: development and adult plasticity

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

 

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