Effects of streptozotocin-induced diabetes on heart rate, blood pressure and cardiac autonomic nervous control

New Insights into IGF-1 Signaling in the Heart

Insulin-like growth factor-1 (IGF-1) signaling has multiple physiological roles in cellular growth, metabolism, and aging. Myocardial hypertrophy, cell death, senescence, fibrosis, and electrical remodeling are hallmarks of various heart diseases and contribute to the progression of heart failure. This review highlights the critical role of IGF-1 and its cognate receptor in cardiac hypertrophy, aging, and remodeling.

Cardioprotective and Antifibrotic Effects of Low-Dose Renin-Angiotensin-Aldosterone System Inhibitors in Type 1 Diabetic Rat Model

Diabetic cardiovascular complications are associated with up to 50% mortality, and current therapies are not effective enough. Renin-angiotensin-aldosterone system inhibitors (RAASis) are the standard of care for diabetic patients with hypertension and albuminuria. Based on our previous studies reporting the renoprotective effects of low-dose RAASis, here, we hypothesized that low-dose RAASi treatment has cardioprotective and antifibrotic benefits in type 1 diabetes mellitus (T1DM). After five weeks of T1DM, adult male Wistar rats received low doses of ramipril, losartan, or eplerenone for two weeks. Heart rate, blood pressure, and pulse wave velocity (PWV) were recorded. Aortic intima-media thickness (IMT), collagen accumulation, and myocardial fibrosis were assessed. All RAASis reduced PWV elevation, prevented the progression of myocardial fibrosis, and normalized B-type natriuretic peptide, troponin I, and fibroblast growth factor 23 levels without affecting blood pressure. Interestingly, only eplerenone reversed the decline in Klotho levels and reduced IMT and fibrosis in the media of the aorta. Our comparative analysis suggests that mineralocorticoid receptor antagonists, particularly eplerenone, may offer superior efficacy in halting both the arterial and the myocardial injuries in T1DM compared to angiotensin-converting enzyme inhibitors or angiotensin II type 1 receptor blockers.

The principle of 'brain energy on demand' and its predictive power for stress, sleep, stroke, obesity and diabetes

Does the brain actively draw energy from the body when needed? There are different schools of thought regarding energy metabolism. In this study, the various theoretical models are classified into one of two categories: (1) conceptualizations of the brain as being purely passively supplied, which we call 'P-models,' and (2) models understanding the brain as not only passively receiving energy but also actively procuring energy for itself on demand, which we call 'A-models.' One prominent example of such theories making use of an A-model is the selfish-brain theory. The ability to make predictions was compared between the A- and P-models. A-models were able to predict and coherently explain all data examined, which included stress, sleep, caloric restriction, stroke, type-1-diabetes mellitus, obesity, and type-2-diabetes, whereas the predictions of P-models failed in most cases. The strength of the evidence supporting A-models is based on the coherence of accurate predictions across a spectrum of metabolic states. The theory test conducted here speaks to a brain that pulls its energy from the body on-demand.

SGLT2 inhibitor dapagliflozin prevents atherosclerotic and cardiac complications in experimental type 1 diabetes

Introduction

Cardiovascular disease (CVD) is two to five times more prevalent in diabetic patients and is the leading cause of death. Therefore, identification of novel therapeutic strategies that reduce the risk of CVD is a research priority. Clinical trials showed that reduction in the relative risk of heart failure by sodium-glucose cotransporter 2 inhibitors (SGLT2i) are partly beyond their glucose lowering effects, however, the molecular mechanisms are still elusive. Here we investigated the role of SGLT2i dapagliflozin (DAPA) in the prevention of diabetes-induced cardiovascular complications.

Methods

Type 1 diabetes was induced with streptozotocin (65 mg/bwkg, ip.) in adult, male Wistar rats. Following the onset of diabetes rats were treated for six weeks with DAPA (1 mg/bwkg/day, po.).

Results

DAPA decreased blood glucose levels (D: 37±2.7 vs. D+DAPA: 18±5.6 mmol/L; p<0.05) and prevented metabolic decline. Aortic intima-media thickening was mitigated by DAPA. DAPA abolished cardiac hypertrophy, and myocardial damage. Cardiac inflammation and fibrosis were also moderated after DAPA treatment.

Conclusions

These data support the preventive and protective role of SGLT2i in diabetes-associated cardiovascular disease. SGLT2i may provide novel therapeutic strategy to hinder the development of cardiovascular diseases in type 1 diabetes, thereby improve the outcomes.

The effects of subchronic intake of magnesium hydrocarbonate-rich mineral water on body weight and cardiovascular variables in rats with streptozotocin: Induced diabetes

Background/Aim: Optimal intake of magnesium minerals is essential in maintaining the coordinated physiological functions of cells, tissues and organs. The importance of this element is reflected in the fact that it is the fourth most abundant cation in the human body, participating as a cofactor in more than three hundred enzymatic reactions. Its presence is necessary for the proper functioning of a number of vital functions, such as glycaemic control, the work of the heart and the vascular system and it can potentially play a role in the regulation of body weight. Aim of this study was to investigate the effects of subchronic intake of magnesium hydrocarbonate-rich water on changes in body weight, organ weight and cardiovascular variables in rats with streptozotocin-induced diabetes. Methods: Wistar rats (n = 28) were divided into 4 groups: two control groups, on tap water (TW-C, n = 7) and magnesium hydrocarbonate-rich water (MW-C, n = 7); and two experimental groups with streptozotocin-induced diabetes, on tap water (TW-DM, n = 7) and magnesium hydrocarbonate-rich water (MW-DM, n = 7). The values of body weight, organ weight and cardiovascular parameters were compared after 6 weeks between control groups of rats on subchronic treatment with tap water (TW-C) and magnesium hydrocarbonate-rich water (MW-C) and between groups with streptozotocin-induced diabetes on tap water (TW-DM) and with magnesium hydrocarbonate-rich water (MW-DM). Results: By comparing the values of cardiovascular parameters between groups, significant (p < 0.05) positive effects of magnesium hydrocarbonate-rich water were registered on the values of systolic and pulse blood pressure in diabetic rats fed with magnesium hydrocarbonate-rich water (MW-DM) compared to those fed with tap water (TW-DM). In contrast, no significant effect of magnesium hydrocarbonate on changes in body weight and organ weight was observed. Conclusion: Based on the results, the beneficial effects of magnesium hydrocarbonate-rich water in the regulation of blood pressure can be clearly observed. Potential effects on other cardiovascular variables and body weight and organ weight should be further investigated.

Brain Mass (Energy) Resistant to Hyperglycaemic Oversupply: A Systematic Review

Cerebral energy supply is determined by the energy content of the blood. Accordingly, the brain is undersupplied during hypoglycaemia. Whether or not there is an additional cerebral energy demand that depends upon the energy content of the brain is considered differently in two opposing theoretical approaches. The Selfish-Brain theory postulates that the brain actively demands energy from the body when needed, while long-held theories, the gluco-lipostatic theory and its variants, deny such active brain involvement and view the brain as purely passively supplied. Here we put the competing theories to the test. We conducted a systematic review of a condition in which the rival theories make opposite predictions, i.e., experimental T1DM. The Selfish-Brain theory predicts that induction of experimental type 1 diabetes causes minor mass (energy) changes in the brain as opposed to major glucose changes in the blood. This prediction becomes our hypothesis to be tested here. A total of 608 works were screened by title and abstract, and 64 were analysed in full text. According to strict selection criteria defined in our PROSPERO preannouncement and complying with PRISMA guidelines, 18 studies met all inclusion criteria. Thirteen studies provided sufficient data to test our hypothesis. The 13 evaluable studies (15 experiments) showed that the diabetic groups had blood glucose concentrations that differed from controls by +294 ± 96% (mean ± standard deviation) and brain mass (energy) that differed from controls by −4 ± 13%, such that blood changes were an order of magnitude greater than brain changes (T = 11.5, df = 14, p < 0.001). This finding confirms not only our hypothesis but also the prediction of the Selfish-Brain theory, while the predictions of the gluco-lipostatic theory and its variants were violated. The current paper completes a three-part series of systematic reviews, the two previous papers deal with a distal and a proximal bottleneck in the cerebral brain supply, i.e., caloric restriction and cerebral artery occlusion. All three papers demonstrate that accurate predictions are only possible if one regards the brain as an organ that regulates its energy concentrations independently and occupies a primary position in a hierarchically organised energy metabolism.

Systematic Review Registration:https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=156816, PROSPERO, identifier: CRD42020156816.

Nephron-deficient HSRA rats exhibit renal injury with age but have limited renal damage from streptozotocin-induced hyperglycemia

Hypertension and diabetes are the greatest factors influencing the progression of chronic kidney disease (CKD). Investigation into the role of nephron number in CKD alone or with hypertension has revealed a strong inverse relationship between the two; however, not much is known about the connection between nephron number and diabetic kidney disease. The heterogeneous stock-derived model of unilateral renal agenesis (HSRA) rat, a novel model of nephron deficiency, provides a unique opportunity to study the association between nephron number and hypertension and diabetes on CKD. HSRA rats exhibit failure of one kidney to develop in 50-75% of offspring, whereas the remaining offspring are born with two kidneys. Rats born with one kidney (HSRA-S) develop significant renal injury with age compared with two-kidney littermates (HSRA-C). The induction of hypertension as a secondary stressor leads to significantly more renal injury in HSRA-S compared with HSRA-C rats and nephrectomized HSRA-C (HSRA-UNX) rats. The present study sought to address the hypothesis that nephron deficiency in the HSRA rat would hasten renal injury in the presence of a secondary stressor of hyperglycemia. HSRA animals did not exhibit diabetes-related traits at any age; thus, streptozotocin (STZ) was used to induce hyperglycemia in HSRA-S, HSRA-C, and HSRA-UNX rats. STZ- and vehicle-treated animals were followed for 15 wk. STZ-treated animals developed robust hyperglycemia, but in contrast to the response to hypertension, neither HSRA-S nor HSRA-UNX animals developed proteinuria compared with vehicle treatment. In total, our data indicate that hyperglycemia from STZ alone does not have a significant impact on the onset or progression of injury in young one-kidney HSRA animals.NEW & NOTEWORTHY The HSRA rat, a novel model of nephron deficiency, provides a unique opportunity to study the association between nephron number and confounding cardiovascular complications that impact kidney health. Although hypertension was previously shown to exacerbate renal injury in young HSRA animals, diabetic hyperglycemia did not lead to worse renal injury, suggesting that nephron number has limited impact on kidney injury, at least in this model.

Protective effects of morphine in a rat model of post-traumatic stress disorder: Role of hypothalamic-pituitary-adrenal axis and beta- adrenergic system

Post-traumatic stress disorder (PTSD) arises after tremendous traumatic experiences. Recently, we have reported that morphine has time-dependent protective effects against behavioral and morphological deficits in the single prolonged stress (SPS) as an experimental model of PTSD in adult male rats. To find the mechanisms underlying the protective effects of morphine against SPS-induced PTSD-like symptoms, the present study investigated the interaction between morphine and hypothalamic-pituitary-adrenal (HPA) axis and beta - adrenergic system, which crucially involved in the stress response, on PTSD-like symptoms in male rats. The animals were exposed to the SPS procedure (restraint for 2 h, forced swimming for 20 min, and ether anesthesia) and morphine (10 mg/kg) or saline was injected 24 h following the SPS. The glucocorticoid receptor antagonist RU486 (20 mg/kg), the mineralocorticoid receptor antagonist spironolactone (50 mg/kg), and the corticosterone synthesis inhibitor metyrapone (50 mg/kg) were injected 90 min before morphine administration to block the HPA axis activity. The beta - adrenergic receptor blocker propranolol (10 mg/kg) and the peripheral beta-adrenergic receptor blocker nadolol (5 mg/kg) were administered 30 min before morphine injection to block the beta - adrenergic system. Anxiety-like behaviors were evaluated using the elevated plus maze (EPM) 11 days after the SPS. After that, animals were conditioned in a fear-conditioning task and extinction training was performed on days 1, 2, 3, 4 and 11 after fear conditioning. SPS increased anxiety-like behaviors and impaired fear extinction. Morphine injection 24 h after SPS significantly improved anxiety-like behaviors and enhanced fear extinction. The RU486, spironolactone and metyrapone prevented the protective effects of morphine on both SPS-induced anxiety-like behaviors and impaired fear extinction. The propranolol, and nadolol did not prevent the effect of morphine on anxiety-like behaviors, but the propranolol prevented morphine effects on fear extinction in SPS animals. These findings together suggest that the protective effects of morphine on PTSD-like symptoms in rats require a certain level of the HPA axis and central beta - adrenergic activity and any alteration in the function of these systems can impede the protective effects of morphine.

The mechanisms of ameliorating effect of a green tea polyphenol on diabetic nephropathy based on diacylglycerol kinase α

Significant efforts have been made to ameliorate diabetic nephropathy (DN) by inhibiting protein kinase C. However, these efforts have not been successful in human trials, suggesting that novel therapeutic strategies are required. Thus far, it has been reported that green tea polyphenol epigallocatechin gallate (EGCg) improved albuminuria in DN in a human trial. Our previous study revealed that activation of diacylglycerol kinase α (DGKα) plays a crucial role in the amelioration of DN and that EGCg activates DGKα. Here, we investigated whether and how DGKα contributes to the amelioration of DN upon stimulation by EGCg by using streptozotocin-induced type 1 diabetic model mice. Our results revealed that EGCg ameliorated albuminuria in DN through DGKα in vivo, and methylated EGCg, which has higher absorption in the plasma improved albuminuria in DN effectively. Additionally, we showed that c-Src mediated EGCg-induced DGKα translocation and colocalized with the 67 kDa laminin receptor, which is an EGCg receptor. Furthermore, EGCg attenuated the loss of podocytes in DN by preventing a decrease in focal adhesion under high glucose conditions. Our results indicate that the DGKα pathway is an attractive therapeutic target and that activating this pathway is a novel strategy for treating DN.

Targeted Vagus Nerve Stimulation does not Disrupt Cardiac Function in the Diabetic Rat

In this study, we acutely identified a target branch of the vagus nerve known as the pancreatic branch of the vagus nerve, which exclusively innervates the pancreas by applying electrical stimulus to the known cervical vagus nerve and observing compound neural action potentials at the target nerve. In a set of chronically implanted rats, the target nerve was again cuffed using an electrode and also implanted with a continuous glucose monitor. A model of type 1 diabetes (T1D) was chemically induced and hyperglycemic state confirmed. After induction, stimulation was applied to the pancreatic branch of the vagus nerve and heart rate variability measured to assess the targeted nature of the stimulation. Pancreatic vagus nerve stimulation in a diabetic model was not found to influence heart rate demonstrating the ability of targeted stimulation to be used as for organ-specific neuromodulation while minimizing side effects.

The protection of resveratrol and its combination with glibenclamide, but not berberine on the diabetic hearts against reperfusion-induced arrhythmias: the role of myocardial KATP channel

CONTEXT

Cardiovascular dysfunctions such as life-threatening arrhythmias are one of the main reasons of mortality and morbidity in diabetic patients Objective: We aimed to investigate the long-term effects of resveratrol, berberine and glibenclamide combinations on the ischemia/reperfusion (I/R) induced arrhythmias in streptozotocin (STZ)-induced diabetic rats and to investigate the role of myocardial K_ATP channel in the possible anti-arrhythmic actions of the treatments.

METHODS

Two days after induction of diabetes, diabetic rats were treated with resveratrol [5 mg/kg, intraperitoneally (i.p.)], berberine (10 mg/kg, i.p) and glibenclamide (5 mg/kg, i.p) for 6 weeks. On the 43th day, experimental animals were subjected to 6-min ischemia and 6-min reperfusion in vivo.

RESULTS

The protein expression of Kir6.2 subunits was downregulated in the diabetic hearts. However, all drug treatments restored the protein expression of Kir6.2 subunits. Resveratrol alone and its combination with glibenclamide decreased the arrhythmia score, the arrhythmic period and the incidence of other types of arrhythmias during the reperfusion period.

CONCLUSIONS

The combination of resveratrol with glibenclamide may alleviate reperfusion-induced arrhythmias via an underlying mechanism not be only associated with the restoration of the protein expression of Kir6.2 subunits but also associated with the other subunits or ion channels underlying cardiac action potential.

Differential cardiac sympatho-inhibitory responses produced by the agonists B-HT 933, quinpirole and immepip in normoglycaemic and diabetic pithed rats

This study compared the cardiac sympatho-inhibitory responses produced by agonists at α₂ -adrenergic (B-HT 933), dopamine D₂ -like (quinpirole) and histamine H₃ /H₄ (immepip) receptors between normoglycaemic and streptozotocin-pretreated (diabetic) pithed rats. Intravenous (i.v.) continuous infusions of B-HT 933, quinpirole or immepip were used in normoglycaemic and diabetic pithed rats to analyse their sympatho-inhibitory effects on the electrically-stimulated cardioaccelerator sympathetic outflow. Both in normoglycaemic and diabetic animals, B-HT 933 (until 100 μg/kg per minute) and quinpirole (until 10 μg/kg per minute) inhibited the tachycardic responses to electrical sympathetic stimulation, but not those to i.v. bolus of exogenous noradrenaline. These sympatho-inhibitory responses were more pronounced in diabetic than in normoglycaemic animals. Accordingly, the areas under the curve for 100 μg/kg per minute B-HT 933 and 10 μg/kg per minute quinpirole in diabetic rats (1065 ± 70 and 920 ± 35, respectively) were significantly smaller (P < .05) than those in normoglycaemic rats (1220 ± 45 and 1360 ± 42, respectively). In contrast, immepip infusions produced cardiac sympatho-inhibition in normoglycaemic (until 10 μg/kg per minute), but not in diabetic (until 100 μg/kg per minute) animals. Our results suggest that in diabetic pithed rats: (i) the more pronounced cardiac sympatho-inhibition to B-HT 933 and quinpirole may be probably due to up-regulation of α₂ -adrenergic and dopamine D₂ -like receptors, respectively; (ii) the histamine H₃ /H₄ receptors do not seem to play a sympatho-inhibitory role; and (iii) there is a differential participation of α₂ -adrenergic and dopamine D₂ -like receptors, which may certainly represent therapeutic targets for the treatment of diabetic complications such as cardiovascular autonomic neuropathy.

Linalyl acetate restores endothelial dysfunction and hemodynamic alterations in diabetic rats exposed to chronic immobilization stress

Although stress is one of the risk factors of diabetes, few studies have assessed the effects of stress on diabetic rats. This study, therefore, analyzed differences in cardiovascular-related factors among control, nonstressed diabetic, and stressed diabetic rats as well as assessed the effects of linalyl acetate (LA) on stressed diabetic rats. Male Sprague-Dawley rats were subjected to immobilization stress throughout the experimental period, and diabetes was induced on day 15 by a single injection of streptozotocin. After confirming the induction of diabetes, stressed diabetic rats were administered LA (10 or 100 mg/kg) or metformin (500 mg/kg) for the last 7 days. Compared with nonstressed diabetic rats, stressed diabetic rats had significantly lower body weight, body fat percentage, ACh-induced vasorelaxation, systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), and NF-κB expression as well as increased serum nitrite concentration. Although metformin increased serum insulin concentration significantly, 100 mg/kg LA showed only an increasing tendency. However, treatment with 100 mg/kg LA not only reduced serum glucose and NF-κB expression, but also restored ACh-induced vasorelaxation, SBP, DBP, HR, AMP-activated protein kinase expression, and serum nitrite almost to control levels. Importantly, 100 mg/kg LA was more effective than metformin in ameliorating serum glucose, endothelial nitric oxide synthase expression, HR, and serum nitrite. These findings suggest that chronic stress can aggravate endothelial dysfunction and hemodynamic alterations in diabetes and that LA may have potent therapeutic efficacy in diabetic patients with cardiovascular disease complications or chronic stress.

NEW & NOTEWORTHY To our knowledge, this is the first study to assess the effects of linalyl acetate (LA) on cardiovascular-related factors in diabetic rats exposed to chronic stress. Treatment with LA restored acetylcholine-induced vasorelaxation, blood pressure, heart rate, and AMP-activated protein kinase and serum nitrite levels. The present results suggest that LA may have potent therapeutic efficacy in diabetic patients with complications of cardiovascular disease or chronic stress.

Severe Hypoglycemia-Induced Fatal Cardiac Arrhythmias Are Augmented by Diabetes and Attenuated by Recurrent Hypoglycemia

We previously demonstrated that insulin-mediated severe hypoglycemia induces lethal cardiac arrhythmias. However, whether chronic diabetes and insulin deficiency exacerbates, and whether recurrent antecedent hypoglycemia ameliorates, susceptibility to arrhythmias remains unknown. Thus, adult Sprague-Dawley rats were randomized into four groups: 1) nondiabetic (NONDIAB), 2) streptozotocin-induced insulin deficiency (STZ), 3) STZ with antecedent recurrent (3 days) hypoglycemia (∼40-45 mg/dL, 90 min) (STZ+RH), and 4) insulin-treated STZ (STZ+Ins). Following treatment protocols, all rats underwent hyperinsulinemic (0.2 units ⋅ kg^-1 ⋅ min^-1), severe hypoglycemic (10-15 mg/dL) clamps for 3 h with continuous electrocardiographic recordings. During matched nadirs of severe hypoglycemia, rats in the STZ+RH group required a 1.7-fold higher glucose infusion rate than those in the STZ group, consistent with the blunted epinephrine response. Second-degree heart block was increased 12- and 6.8-fold in the STZ and STZ+Ins groups, respectively, compared with the NONDIAB group, yet this decreased 5.4-fold in the STZ+RH group compared with the STZ group. Incidence of third-degree heart block in the STZ+RH group was 5.6%, 7.8-fold less than the incidence in the STZ group (44%). Mortality due to severe hypoglycemia was 5% in the STZ+RH group, 6.2-fold less than that in the STZ group (31%). In summary, severe hypoglycemia-induced cardiac arrhythmias were increased by insulin deficiency and diabetes and reduced by antecedent recurrent hypoglycemia. In this model, recurrent moderate hypoglycemia reduced fatal severe hypoglycemia-induced cardiac arrhythmias.

Reduced expression of HCN channels in the sinoatrial node of streptozotocin-induced diabetic rats

Diabetes mellitus (DM) is associated with an electrical remodeling of the heart, increasing the risk of arrhythmias. However, knowledge of electrical remodeling in the sinoatrial node (SAN) by DM is limited. We investigated the expression of HCN channel isoforms, HCN1-HCN4, in SAN from streptozotocin (STZ)-induced diabetic rats and the age-matched controls. We found that the STZ-induced diabetic rats have a lower intrinsic heart rate, a lengthened sinoatrial conduction time, and rate-corrected maximal sinoatrial node recovery time in vivo as well as a longer cycle length (CL) in vitro, as compared with the control. Optical mapping of the SAN demonstrated an inferior leading pacemaker site, reduced SAN conduction velocity and diastolic depolarization slope, and a longer action potential duration in the STZ-induced diabetic rats than in the control. The transcripts and proteins of HCN2 and HCN4 in diabetic SAN were reduced. Specific blockade of HCN channels by 3 μmol/L ivabradine significantly prolonged the CL of a Langendorff heart by 18% in the diabetic rats and 26% in the control. The reduced expression of HCN channel isoforms in the SAN of the STZ-induced diabetic rat may be an important contributor to the reduced SAN function in DM.

Clenbuterol activates the central IL-1 system via the β2-adrenoceptor without provoking inflammatory response related behaviours in rats

The long-acting, highly lipophilic, β2-adrenoceptor agonist clenbuterol may represent a suitable therapeutic agent for the treatment of neuroinflammation as it drives an anti-inflammatory response within the CNS. However, clenbuterol is also known to increase the expression of IL-1β in the brain, a potent neuromodulator that plays a role in provoking sickness related symptoms including anxiety and depression-related behaviours. Here we demonstrate that, compared to the immunological stimulus lipopolysaccharide (LPS, 250μg/kg), clenbuterol (0.5mg/kg) selectively up-regulates expression of the central IL-1 system resulting in a mild stress-like response which is accompanied by a reduction in locomotor activity and food consumption in rats. We provide further evidence that clenbuterol-induced activation of the central IL-1 system occurs in a controlled and selective manner in tandem with its negative regulators IL-1ra and IL-1RII. Furthermore, we demonstrate that peripheral β2-adrenoceptors mediate the suppression of locomotor activity and food consumption induced by clenbuterol and that these effects are not linked to the central induction of IL-1β. Moreover, despite increasing central IL-1β expression, chronic administration of clenbuterol (0.03mg/kg; twice daily for 21days) fails to induce anxiety or depressive-like behaviour in rats in contrast to reports of the ability of exogenously administered IL-1 to induce these symptoms in rodents. Overall, our findings suggest that clenbuterol or other selective β2-adrenoceptor agonists could have the potential to combat neuroinflammatory or neurodegenerative disorders without inducing unwanted symptoms of depression and anxiety.

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.

cAMP-dependent Protein Kinase (PKA) Signaling Is Impaired in the Diabetic Heart

Diabetes mellitus causes cardiac dysfunction and heart failure that is associated with metabolic abnormalities and autonomic impairment. Autonomic control of ventricular function occurs through regulation of cAMP-dependent protein kinase (PKA). The diabetic heart has suppressed β-adrenergic responsiveness, partly attributable to receptor changes, yet little is known about how PKA signaling is directly affected. Control and streptozotocin-induced diabetic mice were therefore administered 8-bromo-cAMP (8Br-cAMP) acutely to activate PKA in a receptor-independent manner, and cardiac hemodynamic function and PKA signaling were evaluated. In response to 8Br-cAMP treatment, diabetic mice had impaired inotropic and lusitropic responses, thus demonstrating postreceptor defects. This impaired signaling was mediated by reduced PKA activity and PKA catalytic subunit content in the cytoplasm and myofilaments. Compartment-specific loss of PKA was reflected by reduced phosphorylation of discrete substrates. In response to 8Br-cAMP treatment, the glycolytic activator PFK-2 was robustly phosphorylated in control animals but not diabetics. Control adult cardiomyocytes cultured in lipid-supplemented media developed similar changes in PKA signaling, suggesting that lipotoxicity is a contributor to diabetes-induced β-adrenergic signaling dysfunction. This work demonstrates that PKA signaling is impaired in diabetes and suggests that treating hyperlipidemia is vital for proper cardiac signaling and function.

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.

Caffeic acid phenethyl amide ameliorates ischemia/reperfusion injury and cardiac dysfunction in streptozotocin-induced diabetic rats

BACKGROUND

Caffeic acid phenethyl ester (CAPE) has been shown to protect the heart against ischemia/reperfusion (I/R) injury by various mechanisms including its antioxidant effect. In this study, we evaluated the protective effects of a CAPE analog with more structural stability in plasma, caffeic acid phenethyl amide (CAPA), on I/R injury in streptozotocin (STZ)-induced type 1 diabetic rats.

METHODS

Type 1 diabetes mellitus was induced in Sprague-Dawley rats by a single intravenous injection of 60 mg/kg STZ. To produce the I/R injury, the left anterior descending coronary artery was occluded for 45 minutes, followed by 2 hours of reperfusion. CAPA was pretreated intraperitoneally 30 minutes before reperfusion. An analog devoid of the antioxidant property of CAPA, dimethoxyl CAPA (dmCAPA), and a nitric oxide synthase (NOS) inhibitor (Nω-nitro-l-arginine methyl ester [l-NAME]) were used to evaluate the mechanism involved in the reduction of the infarct size following CAPA-treatment. Finally, the cardioprotective effect of chronic treatment of CAPA was analyzed in diabetic rats.

RESULTS

Compared to the control group, CAPA administration (3 and 15 mg/kg) significantly reduced the myocardial infarct size after I/R, while dmCAPA (15 mg/kg) had no cardioprotective effect. Interestingly, pretreatment with a NOS inhibitor, (L-NAME, 3 mg/kg) eliminated the effect of CAPA on myocardial infarction. Additionally, a 4-week CAPA treatment (1 mg/kg, orally, once daily) started 4 weeks after STZ-induction could effectively decrease the infarct size and ameliorate the cardiac dysfunction by pressure-volume loop analysis in STZ-induced diabetic animals.

CONCLUSIONS

CAPA, which is structurally similar to CAPE, exerts cardioprotective activity in I/R injury through its antioxidant property and by preserving nitric oxide levels. On the other hand, chronic CAPA treatment could also ameliorate cardiac dysfunction in diabetic animals.

Inner retinal oxygen delivery and metabolism in streptozotocin diabetic rats

PURPOSE

The purpose of the study is to report global measurements of inner retinal oxygen delivery (DO2_IR) and oxygen metabolism (MO2_IR) in streptozotocin (STZ) diabetic rats.

METHODS

Phosphorescence lifetime and blood flow imaging were performed in rats 4 (STZ/4 wk; n = 10) and 6 (STZ/6 wk; n = 10) weeks following injection of STZ to measure retinal arterial (O2A) and venous (O2V) oxygen contents and total retinal blood flow (F). DO2_IR and MO2_IR were calculated from measurements of F and O2A and of F and the arteriovenous oxygen content difference, respectively. Data in STZ rats were compared to those in healthy control rats (n = 10).

RESULTS

Measurements of O2A and O2V were not significantly different among STZ/4 wk, STZ/6 wk, and control rats (P ≥ 0.28). Likewise, F was similar among all groups of rats (P = 0.81). DO2_IR measurements were 941 ± 231, 956 ± 232, and 973 ± 243 nL O2/min in control, STZ/4 wk, and STZ/6 wk rats, respectively (P = 0.95). MO2_IR measurements were 516 ± 175, 444 ± 103, and 496 ± 84 nL O2/min in control, STZ/4 wk, and STZ/6 wk rats, respectively (P = 0.37).

CONCLUSIONS

Global inner retinal oxygen delivery and metabolism were not significantly impaired in STZ rats in early diabetes.

The afterload-dependent peak efficiency of the isolated working rat heart is unaffected by streptozotocin-induced diabetes

Background

Diabetes is known to alter the energy metabolism of the heart. Thus, it may be expected to affect the efficiency of contraction (i.e., the ratio of mechanical work output to metabolic energy input). The literature on the subject is conflicting. The majority of studies have reported a reduction of myocardial efficiency of the diabetic heart, yet a number of studies have returned a null effect. We propose that these discrepant findings can be reconciled by examining the dependence of myocardial efficiency on afterload.

Methods

We performed experiments on streptozotocin (STZ)-induced diabetic rats (7-8 weeks post-induction), subjecting their (isolated) hearts to a wide range of afterloads (40 mmHg to maximal, where aortic flow approached zero). We measured work output and oxygen consumption, and their suitably scaled ratio (i.e., myocardial efficiency).

Results

We found that myocardial efficiency is a complex function of afterload: its value peaks in the mid-range and decreases on either side. Diabetes reduced the maximal afterload to which the hearts could pump (105 mmHg versus 150 mmHg). Thus, at high afterloads (for example, 90 mmHg), the efficiency of the STZ heart was lower than that of the healthy heart (10.4% versus 14.5%) due to its decreased work output. Diabetes also reduced the afterload at which peak efficiency occurred (optimal afterload: 63 mmHg versus 83 mmHg). Despite these negative effects, the peak value of myocardial efficiency (14.7%) was unaffected by diabetes.

Conclusions

Diabetes reduces the ability of the heart to pump at high afterloads and, consequently, reduces the afterload at which peak efficiency occurs. However, the peak efficiency of the isolated working rat heart remains unaffected by STZ-induced diabetes.

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.

Angiotensin II receptor blocker telmisartan attenuates aortic stiffening and remodelling in STZ-diabetic rats

BACKGROUND

Prevention or attenuation of diabetic vascular complications includes anti-hypertensive treatment with renin-angiotensin system inhibitors on account of their protective effects beyond blood pressure reduction. The present study aimed to investigate the effects of telmisartan, an angiotensin II type 1 receptor blocker (ARB), on blood pressure, aortic stiffening, and aortic remodelling in experimental type 1 diabetes in rats.

METHODS

Diabetes was induced by streptozotocin (STZ) (65 mg/kg) in male Wistar rats. One diabetic group was treated for 10 weeks with telmisartan (10 mg/kg/day p/o). Pressure-independent aortic pulse wave velocity (PWV) was measured under anaesthesia after intravenous infusion of phenylephrine and nitroglycerine. Aortic wall samples were collected for histomorphometrical analysis.

RESULTS

Untreated diabetes imposed differential effects on aortic stiffening, as demonstrated by increased isobaric PWV over a range of high blood pressures, but not at lower blood pressures. This was associated with loss and disruption of elastin fibres and an increase in collagen fibres in the aortic media. Treatment with telmisartan decreased resting blood pressure, reduced aortic stiffness, and partially prevented the degradation of elastin network within the aortic wall.

CONCLUSIONS

Telmisartan improved the structural and functional indices of aortic stiffening induced by untreated STZ-diabetes, demonstrating the importance of ARBs in the therapeutic approach to diabetic vascular complications.

Chicken embryos as a potential new model for early onset type I diabetes

Diabetic retinopathy (DR) is the leading cause of blindness among the American working population. The purpose of this study is to establish a new diabetic animal model using a cone-dominant avian species to address the distorted color vision and altered cone pathway responses in prediabetic and early diabetic patients. Chicken embryos were injected with either streptozotocin (STZ), high concentration of glucose (high-glucose), or vehicle at embryonic day 11. Cataracts occurred in varying degrees in both STZ- and high glucose-induced diabetic chick embryos at E18. Streptozotocin-diabetic chicken embryos had decreased levels of blood insulin, glucose transporter 4 (Glut4), and phosphorylated protein kinase B (pAKT). In STZ-injected E20 embryos, the ERG amplitudes of both a- and b-waves were significantly decreased, the implicit time of the a-wave was delayed, while that of the b-wave was significantly increased. Photoreceptors cultured from STZ-injected E18 embryos had a significant decrease in L-type voltage-gated calcium channel (L-VGCC) currents, which was reflected in the decreased level of L-VGCCα1D subunit in the STZ-diabetic retinas. Through these independent lines of evidence, STZ-injection was able to induce pathological conditions in the chicken embryonic retina, and it is promising to use chickens as a potential new animal model for type I diabetes.

Reduced GABAergic inhibition of kidney-related PVN neurons in streptozotocin-treated type 1 diabetic mouse

Activity of presympathetic neurons in the paraventricular nucleus (PVN) of the hypothalamus is known to play an important role in the regulation of sympathetic outflow. Sympathetic overactivity is associated with many pathophysiological conditions such as diabetes mellitus and hypertension; however, the underlying synaptic mechanisms are poorly understood. In this study, we examined the GABAergic inhibitory synaptic control of kidney-related presympathetic PVN neurons in the streptozotocin-treated type 1 diabetic mouse model, using patch-clamp slice electrophysiology in combination with retrograde labeling. Type 1 diabetes resulted in decreased frequency of miniature inhibitory postsynaptic currents (mIPSCs). Our data also demonstrated a reduction of mIPSC amplitude and mean inhibitory current without alteration of input resistance. Furthermore, our data revealed decreased tonic GABAergic inhibition of kidney-related PVN neurons in diabetic conditions, which was consistent with the observed increased excitability of the presympathetic PVN neurons. In summary, our data demonstrated decreased phasic and tonic inhibitory control of kidney-related presympathetic PVN neurons that suggest altered sympathetic circuitry in type 1 diabetes.

Validation of octanoate breath test for measuring gastric emptying in rats

Background/Aims

Lack of simple and repeatable tests hampers gastric emptying studies in rats. The aim of this study was to adapt the ¹⁴C-octanoate solid gastric emptying breath test for application in rats, and to validate it against radioscintigraphic method.

Methods

After ingestion of a meal containing 3 mCi ^99mTc and 2 µCi ¹⁴C-octanoate, 23 male Wistar rats were placed on a gamma camera in a airflow container. Scintigraphic images were taken at regular intervals. The amount of ¹⁴CO₂ in a regularly replaced hyamine hydroxide solution, capturing CO₂ in the outflow air, was counted using liquid scintillation spectrometry. ^99mTc gastric retention curves and ¹⁴CO₂-excretion curves were fitted to their respective data. Three rats underwent the same procedures after administration of atropine.

Results

Overall T_r10% (time at which 10% of the original amount of ^99mTc remained in the stomach) was 355 ± 64 minutes; T_e90% (time at which 90% of total amount of ¹⁴CO₂ was excreted) was 325 ± 106 minutes. Their correlation coefficient was 0.71, R-square 0.50 and P < 0.005. T_r1/2 (50% of original amount of ^99mTc remained) was 124 ± 28 minutes; T_e1/2 (50% of total amount of ¹⁴CO₂ excreted) 114 ± 32 minutes. Their correlation coefficient was 0.83 with R-square of 0.69 and P < 0.00005. In 12 immobilized animals correlation was even better: correlation coefficient 0.84; R-square 0.71 and P < 0.001 (T_r10% was 388 ± 117 minutes; T_e90% 532 ± 219 minutes; T_r1/2 of 165 ± 54 minutes; T_e1/2 of 175 ± 67 minutes). Atropine significantly lengthened all emptying times: 904 ± 307 and 1461 ± 684 minutes for T_r10% and T_e90%, respectively; and 432 ± 117 minutes for T_r1/2 and 473 ± 190 minutes for T_e1/2.

Conclusions

We adapted and validated the ¹⁴C-octanoate gastric emptying breath test for application in rats.

Rosmarinic acid administration attenuates diabetes-induced vascular dysfunction of the rat aorta

OBJECTIVES

Oxidative stress as well as inflammation processes are engaged in diabetic vascular complications. Rosmarinic acid, a natural phenol antioxidant carboxylic acid, was found to have multiple biological activity, including anti-inflammatory and antitumour effects, which are a consequence of its inhibition of the inflammatory processes and of reactive oxygen species scavenging. The aim of this work was to study effects of rosmarinic acid administration on vascular impairment induced by experimental diabetes in rats.

METHODS

Diabetes was induced by streptozocin (3 × 30 mg/kg daily, i.p.) in Wistar rats. Rosmarinic acid was administered orally (50 mg/kg daily). Ten weeks after streptozocin administration, the aorta was excised for functional studies, evaluation by electron microscopy and real time PCR analysis.

KEY FINDINGS

In the aorta of diabetic rats, decreased endothelium-dependent relaxation was accompanied by overexpression of interleukin-1β, tumour necrosis factor-α, preproendothelin-1 and endothelin converting enzyme-1. Structural alterations in the endothelium, detected by electron microscopy, indicated aortic dysfunction caused by diabetes. The diabetes-induced aortic disorders were prevented by rosmarinic acid administration.

CONCLUSIONS

Rosmarinic acid protected aortic endothelial function and ultrastructure against diabetes-induced damage. Both antioxidant and anti-inflammatory effects of rosmarinic acid seemed to participate in the mechanism of this protection.

Augmented cardiac formation of oxidatively-induced carbonylated proteins accompanies the increased functional severity of post-myocardial infarction heart failure in the setting of type 1 diabetes mellitus

SUMMARY

Heart failure (HF) is a dominant cause for the higher mortality of diabetics after myocardial infarction (MI). In the present investigation, we have discovered that higher levels of oxidative stress (OS)-induced carbonylated proteins accompany worsening post-MI HF in the presence of type 1 diabetes. These findings provide a mechanistic link between amplified OS and exacerbation of post-infarction HF in diabetes.

BACKGROUND

Type 1 diabetes mellitus (DM) patients surviving myocardial infarction (MI) manifest an increased incidence of subsequent heart failure (HF). We have previously shown that after MI, type 1 DM is associated with accentuated myocardial oxidative stress (OS) and concomitant worsening of left ventricular (LV) function. However, the precise mechanisms whereby type 1 DM-enhanced OS adversely affects HF after MI remain obscure. As carbonylation of proteins is an irreversible post-translational modification induced only by OS that often leads to the loss of function, we analyzed protein-bound carbonyls in the surviving LV myocardium of MI and DM+MI rats in relation to residual LV function.

METHODS

Type 1 DM was induced in rats via administration of streptozotocin. Two weeks after induction of type 1 DM, MI was produced in DM and non-DM rats by coronary artery ligation. Residual LV function and remodeling was assessed at 4 weeks post-MI by echocardiography. Myocardial carbonylated proteins were detected through OxyBlot analysis, and identified by mass spectrometry.

RESULTS

Compared with MI rats, DM+MI rats exhibited significantly poorer residual LV systolic function and elevated wet to dry weight ratios of the lungs. Protein carbonyl content in cardiac tissue and isolated heart mitochondria of DM+MI rats was 20% and 48% higher, respectively, versus MI rats. Anti-oxidative enzymes and fatty acid utilization proteins were among the carbonylated protein candidates identified.

CONCLUSIONS

These findings implicate myocardial protein carbonylation as part of the molecular pathophysiology of aggravated HF in the type 1 diabetic post-infarction heart.

Type 1 diabetes mellitus abrogates compensatory augmentation of myocardial neuregulin-1β/ErbB in response to myocardial infarction resulting in worsening heart failure

Background

Diabetes mellitus (DM) patients surviving myocardial infarction (MI) exhibit a substantially higher incidence of subsequent heart failure (HF). Neuregulin (NRG)-1 and erythroblastic leukemia viral oncogene homolog (ErbB) receptors have been shown to play a critical role in maintenance of cardiac function. However, whether myocardial NRG-1/ErbB is altered during post-MI HF associated with DM remains unknown. The aim of this study was to determine the impact of type 1 DM on the myocardial NRG-1/ErbB system following MI in relation to residual left ventricular (LV) function.

Methods

Type 1 DM was induced in rats via administration of streptozotocin (65 mg/kg, i.p.). Control rats were injected with citrate buffer (vehicle) only. Two weeks after induction of type 1 DM, MI was produced in DM and non-DM rats by ligation of the left coronary artery. Sham MI rats underwent the same surgical procedure with the exception that the left coronary artery was not ligated. At 4 weeks after surgery, residual in vivo LV function was assessed via echocardiography. Myocardial protein expression of NRG-1β, ErbB2 and ErbB4 receptors, and MDM2 (a downstream signaling pathway induced by NRG-1 that has been implicated in cell survival) was assessed in the remaining, viable LV myocardium by Western blotting. Changes in ErbB receptor localization in the surviving LV myocardium of diabetic and non-diabetic post-MI rats was determined using immunohistochemistry techniques.

Results

At 4 weeks post-MI, echocardiography revealed that LV fractional shortening (FS) and LV ejection fraction (EF) were significantly lower in the DM + MI group compared to the MI group (LVFS: 17.9 ± 0.7 vs. 25.2 ± 2.2; LVEF: 35.5 ± 1.4 vs. 47.5 ± 3.5, respectively; P < 0.05), indicating an increased functional severity of HF among the DM + MI rats. Up-regulation of NRG-1β and ErbB2 protein expression in the MI group was abrogated in the DM + MI group concurrent with degradation of MDM2, a downstream negative regulator of p53. ErbB2 and ErbB4 receptors re-localized to cardiac myocyte nuclei in failing type 1 diabetic post-MI hearts.

Conclusions

Type 1 DM prevents compensatory up-regulation of myocardial NRG-1/ErbB after MI coincident with an increased severity of HF.

Increased susceptibility to hypertensive renal disease in streptozotocin-treated diabetic rats is not modulated by salt intake

AIMS/HYPOTHESIS

In early type 1 diabetes mellitus, renal salt handling is dysregulated, so that the glomerular filtration rate becomes inversely proportional to salt intake. The salt paradox occurs in both humans and rats and, with low salt intake, results in diabetic hyperfiltration. We tested whether increased salt intake could reduce the susceptibility to injury of non-clipped kidneys in diabetic rats with pre-existing Goldblatt hypertension.

METHODS

Male Long-Evans rats were made hypertensive and half were then made diabetic. Blood glucose was maintained at ~20-25 mmol/l by insulin implants. One half of each received only the salt in normal chow (1% by weight) and the other half received added salt in drinking water to equal 2.7% by weight of food intake. Weekly 24 h blood pressure records were acquired by telemetry during the 4-month experiment.

RESULTS

Systolic blood pressure was not affected by diabetes or increased salt intake, alone or together. Autoregulation was highly efficient in the non-clipped kidney of both intact and diabetic rats. Histological examination showed minor injury in the clipped kidney, which did not differ among groups. The non-clipped kidney showed extensive pressure-dependent glomerular and vascular injury in both intact and diabetic rats.

CONCLUSIONS/INTERPRETATION

The relationship between pressure and injury was shifted toward lower blood pressure in diabetic rats, indicating that diabetes increased the susceptibility of the kidney to injury despite preservation of autoregulation. The increased susceptibility was not affected by high salt intake in the diabetic rats, thus disproving the hypothesis.

Luteolin ameliorates cardiac failure in type I diabetic cardiomyopathy

OBJECTIVE

The study aimed to determine whether luteolin can confer cardioprotective effects against diabetic cardiomyopathy in relation to specific and quantitative markers of oxidative stress.

METHODS

We examined diabetic cardiomyopathy by left ventricular hemodynamic analysis. Myocardial oxidative stress was assessed by measuring the activity of superoxide dismutase (SOD) as well as the level of malondialdehyde (MDA). Hypolipidaemic effects of luteolin were also investigated in STZ-induced diabetic rats. Myocardial Akt/PKB phosphorylation, heme oxygenase-1 (HO-1) and connective tissue growth factor (CTGF) protein levels were measured by Western blot in all rats at the end of the study.

RESULTS

This study showed a significant increase in serum triacylglycerol (TG), total cholesterol (TC), lower density lipoprotein (LDL), MDA content, creatine kinase (CK), lactate dehydrogenase (LDH), and myocardial CTGF and a significant decrease in high density lipoprotein (HDL), SOD and Akt phosphorylation level in the diabetic group compared to the control group. Luteolin treatment induced a significant decrease in serum TG, TC, LDL, MDA, CK, LDH, and myocardial CTGF and a significant increase in HDL, SOD and Akt phosphorylation levels in comparison with the diabetic group.

CONCLUSION

These results show that luteolin protects against the progression of diabetes mellitus-induced cardiac dysfunction by attenuation of myocardial oxidative stress probably through its antioxidant properties.

Alteration of the cardiac sympathetic innervation is modulated by duration of diabetes in female rats

To evaluate the sympathetic innervation of the female diabetic heart, resting heart rate and sympathetic tone were assessed in vivo, and effect of tyramine on spontaneous beating rate, norepinephrine atrial concentrations, uptake, and release were determined in vitro in streptozotocin- (STZ-) treated rats and respective controls aged 3 months to 2 years. Resting bradycardia, decreased sympathetic tone, deceleration of spontaneous beating rate, and slightly declining carrier-mediated, but preserved exocytotic norepinephrine release from the atria were found in younger diabetic rats while the reactivity of the right atria to tyramine was not affected with age and disease duration. Diabetic two-year-old animals displayed symptoms of partial spontaneous recovery including normoglycemia, increased plasma insulin concentrations, fully recovered sympathetic tone, but putative change, in releasable norepinephrine tissue stores. Our data suggested that female diabetic heart exposed to long-lasting diabetic conditions seems to be more resistant to alteration in sympathetic innervation than the male one.

The insulin-melatonin antagonism: studies in the LEW.1AR1-iddm rat (an animal model of human type 1 diabetes mellitus)

AIMS/HYPOTHESIS

It is well documented that melatonin influences insulin secretion mediated by G-protein-coupled melatonin receptor isoforms MT1 and MT2, which are present in rat and human pancreatic islets, as well as in rat insulinoma cells. Recent investigations have proven that hyperinsulinaemic Goto-Kakizaki (GK) rats, which are a rat model of type 2 diabetic rats, and humans have decreased melatonin plasma levels, whereas a streptozotocin-induced rat model of diabetes developed reduced insulin levels combined with increased melatonin levels.

METHODS

Plasma levels of glucose, insulin and melatonin as well as RNA expression of pineal Aanat, Hiomt (also known as Asmt), insulin receptor, adrenoceptor β1 and the clock genes Per1 and Bmal1 (also known as Arntl) were determined in male and female LEW.1AR1-iddm rats as well as in insulin-substituted LEW.1AR1-iddm rats.

RESULTS

Severe hypoinsulinaemia in diabetic LEW.1AR1-iddm rats was associated with decreased body weight and increased melatonin plasma levels combined with mainly elevated expression of Aanat, Hiomt, pineal insulin receptor and adrenoceptor β1. The changes were normalised by insulin substitution. Diurnal profiles of plasma melatonin and of antagonistic clock genes Per1 and Bmal1 were maintained in diabetic and insulin-substituted rats.

CONCLUSIONS/INTERPRETATION

The assumed causal relation between elevated melatonin and reduced insulin levels in LEW.1AR1-iddm rats is supported by the observation that insulin substitution normalised these changes. Further support for this interpretation comes from the observation that in GK rats an increase of plasma insulin was combined with a decrease of plasma noradrenaline (norepinephrine), the most important activator of melatonin synthesis. These relationships between the noradrenergic and insulin pathway support the existence of melatonin-insulin antagonism.

Circadian Variations in Blood Pressure, Heart Rate, and HR-BP Cross-Correlation Coefficient during Progression of Diabetes Mellitus in Rat

Circadian changes in cardiovascular function during the progression of diabetes mellitus in the diabetes prone rat (BBDP) (n = 8) were studied. Age-matched diabetes-resistant rats (BBDR) served as controls. BP was recorded via telemetry in contiguous 4 hr time periods over 24 hours starting with 12 midnight to 4 am as period zero (P0). Prior to onset of diabetes BP was high at P0, peaked at P2, and then fell again at P3; BP and heart rate (HR) then increased gradually at P4 and leveled off at P5, thereby exhibiting a bipodal rhythm. These patterns changed during long-term diabetes. The cross-correlation coefficient of BP and HR was not significantly different across groups at onset, but it fell significantly at 9 months of duration of diabetes (BBDP: 0.39 ± 0.06; BBDR: 0.65 ± 0.03; P < .05). These results show that changes in circadian cardiovascular rhythms in diabetes mellitus became significant at the late stage of the disease.

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.

Hemodynamic and autonomic response to acute hemorrhage in streptozotocin-induced diabetic rats

Background

The various autonomic control systems lead to characteristic changes in heart rate (HR) and blood pressure (BP) during acute hemorrhage. However, cardiovascular autonomic neuropathy due to diabetes mellitus may interfere with the normal compensation for hemorrhage.

Materials and methods

A controlled graded bleeding (6 - 36% loss of estimated total blood volume: ETBV) was performed in streptozotocin-induced diabetic rats (STZ rats) under a conscious state. Hemodynamic and autonomic responses to acute hemorrhage were examined using analysis of BP-HR variability. The effects of dextran treatment after hemorrhage were also examined.

Results

A significant reduction in mean arterial pressure began at 12% ETBV loss in STZ rats and 18% in the control rats, respectively. When blood loss reached 18% of TEBV, the decrease in HR was prominent in STD rats due to the activation of a parasympathetic drive, as indicated by the increase in high frequency (HF; 0.75~3.0 Hz) power in HR variability, while in the control rats this response was not observed. The administration of dextran prevented the activation of the parasympathetic drive in STZ rats during hemorrhaging. In the control rats, the dextran treatment sustained the initial increase in HR with reduced HF power in HR variability.

Conclusion

STZ rats showed different hemodynamic and autonomic responses to acute hemorrhage from the control rats. STZ rats were prone to develop bradycardiac hypotension characterized by marked parasympathetic activation during hemorrhaging. This finding suggests enhancement of the Bezold-Jarisch reflex in STZ rats. Dextran treatment to maintain a normovolemic hemorrhage state inhibits this reflex.

The anatomy and physiology of the sinoatrial node--a contemporary review

The sinoatrial node is the primary pacemaker of the heart. Nodal dysfunction with aging, heart failure, atrial fibrillation, and even endurance athletic training can lead to a wide variety of pathological clinical syndromes. Recent work utilizing molecular markers to map the extent of the node, along with the delineation of a novel paranodal area intermediate in characteristics between the node and the surrounding atrial muscle, has shown that pacemaker tissue is more widely spread in the right atrium than previously appreciated. This can explain the phenomenon of a "wandering pacemaker" and concomitant changes in the P-wave morphology. Extensive knowledge now exists regarding the molecular architecture of the node (in particular, the expression of ion channels) and how this relates to pacemaking. This review is an up-to-date summary of the current state of our appreciation of the above topics.

Attenuated alpha-adrenoceptor-mediated arterial and venous constrictions in rat models of diabetes

Diabetes is associated with metabolic and vascular abnormalities. We investigated if arterial and venous constrictions are impaired in rat models of diabetes. Wistar rats (5 weeks old) were fed a normal or high-fructose diet (60% of caloric intake). On Day 14, half of the animals in each diet regimen were given streptozotocin (60 mg/kg, i.v.). On Day 35, plasma insulin and triglyceride were measured, and on Day 42, insulin sensitivity (via hyperinsulinemic euglycemic clamp), and pressor as well as mean circulatory filling pressure (index of venous tone) responses to noradrenaline were determined. The rats treated with streptozotocin or fructose-streptozotocin were hyperglycemic, hypoinsulinemic and insulin resistant, and they also had reduced potency (increased ED(50)) of pressor response and reduced venoconstriction to noradrenaline compared to the two groups not given streptozotocin. Plasma triglyceride was unchanged in streptozotocin-treated rats, moderately increased in fructose-fed rats, and markedly increased in fructose-streptozotocin-treated rats. Hyperglycemia, insulin resistance and alpha-adrenoceptor-mediated venous contractile dysfunction were more pronounced in the group given fructose-streptozotocin than that given streptozotocin alone. The presence of marked hypertriglyceridemia, insulin resistance and vascular dysfunction makes the fructose-streptozotocin-treated rats a suitable model for study of metabolic and vascular abnormalities in advanced type 2 diabetes.

Salt-resistant blood pressure and salt-sensitive renal autoregulation in chronic streptozotocin diabetes

Hyperfiltration occurs in early type 1 diabetes mellitus in both rats and humans. It results from afferent vasodilation and thus may impair stabilization of glomerular capillary pressure by autoregulation. It is inversely related to dietary salt intake, the "salt paradox." Restoration of normal glomerular filtration rate (GFR) involves increased preglomerular resistance, probably mediated by tubuloglomerular feedback (TGF). To begin to test whether the salt paradox has pathogenic significance, we compared intact vs. diabetic (streptozotocin) Long-Evans rats with normal and increased salt intake, 1 and approximately 3% by weight of food eaten, respectively. Weekly 24-h blood pressure records were acquired by telemetry before and during diabetes. Blood glucose was maintained at approximately 20 mmol/l by insulin implants. GFR was significantly elevated only in diabetic rats on normal salt intake, confirming diabetic hyperfiltration and the salt paradox. Renal blood flow dynamics show strong contributions to autoregulation by both TGF and the myogenic mechanism and were not impaired by diabetes or by increased salt intake. Separately, systolic pressure was not elevated in diabetic rats at any time during 12 wk with normal or high salt intake. Autoregulation was effective in all groups, and the diabetic-normal salt group showed significantly improved autoregulation at low perfusion pressures. Histological examination revealed very minor glomerulosclerosis and modest mesangial expansion, although neither was diagnostic of diabetes. Periodic acid-Schiff-positive droplets found in distal tubules and collecting duct segments were diagnostic of diabetic kidneys. Biologically significant effects attributable to increased salt intake were abrogation of hyperfiltration and of the left shift in autoregulation in diabetic rats.

Diabetes and cardiovascular autonomic dysfunction: application of animal models

When diabetes is associated with cardiovascular autonomic dysfunction, there is a poor prognosis and increased morbidity and mortality. Information on the mechanisms of diabetes-associated autonomic dysfunction has been provided by advanced studies using physiological, pharmacological, anatomical and molecular methods in experimental animal models of insulin deficiency and resistance. This has been augmented by new approaches which combine diabetes induction with genetically modified animal models. The aim of this review is to outline and discuss the animal models used for the study of insulin deficiency and insulin resistance with a focus on autonomic neural interactions. The goal is to better understand the clinical relevance of cardiovascular autonomic dysfunction associated with diabetes.

Increased melatonin synthesis in pineal glands of rats in streptozotocin induced type 1 diabetes

It is well-documented that melatonin influences insulin secretion. The effects are mediated by specific, high-affinity, pertussis-toxin-sensitive, G protein-coupled membrane receptors (MT(1) as well MT(2)), which are present in both the pancreatic tissue and islets of rats and humans, as well as in rat insulinoma cells (INS1). Via the Gi-protein-adenylatecyclase-3',5'-cyclic adenosine monophosphate (cAMP) and, possibly, the guanylatecyclase-cGMP pathways, melatonin decreases insulin secretion, whereas, by activating the Gq-protein-phospholipase C-IP(3) pathway, it has the opposite effect. For further analysis of the interactions between melatonin and insulin, diabetic rats were investigated with respect to melatonin synthesis in the pineal gland and plasma insulin levels. In this context, recent investigations have proven that type 2 diabetic rats and humans display decreased melatonin levels, whereas type 1 diabetic IDDM rats or those with diabetes induced by streptozotocin (STZ) of the present study show increased plasma melatonin levels and elevated AA-NAT-mRNA. Furthermore, the mRNA of pineal insulin receptors and beta1-adrenoceptors, including the clock genes Per1 and Bmal1 and the clock-controlled output gene Dbp, increases in both young and middle-aged STZ rats. The results therefore indicate that the decreased insulin levels in STZ-induced type 1 diabetes are associated with higher melatonin plasma levels. In good agreement with earlier investigations, it was shown that the elevated insulin levels observed in type 2 diabetes, are associated with decreased melatonin levels. The results thus prove that a melatonin-insulin antagonism exists. Astonishingly, notwithstanding the drastic metabolic disturbances in STZ-diabetic rats, the diurnal rhythms of the parameters investigated are maintained.

Chronic central leptin infusion restores cardiac sympathetic-vagal balance and baroreflex sensitivity in diabetic rats

This study tested whether leptin restores sympathetic-vagal balance, heart rate (HR) variability, and cardiac baroreflex sensitivity (BRS) in streptozotocin (STZ)-induced diabetes. Sprague-Dawley rats were instrumented with arterial and venous catheters, and a cannula was placed in the lateral ventricle for intracerebroventricular (ICV) leptin infusion. Blood pressure (BP) and HR were monitored by telemetry. BRS and HR variability were estimated by linear regression between HR and BP responses to phenylephrine or sodium nitroprusside and autoregressive spectral analysis. Measurements were made during control period, 7 days after induction of diabetes, and 7 days after ICV leptin infusion. STZ diabetes was associated with hyperglycemia (422 +/- 17 mg/dl) and bradycardia (-79 +/- 4 beats/min). Leptin decreased glucose levels (165 +/- 16 mg/dl) and raised HR to control values (303 +/- 10 to 389 +/- 10 beats/min). Intrinsic HR (IHR) and chronotropic responses to a full-blocking dose of propranolol and atropine were reduced during diabetes (260 +/- 7 vs. 316 +/- 6, -19 +/- 2 vs. -43 +/- 6, and 39 +/- 3 vs. 68 +/- 8 beats/min), and leptin treatment restored these variables to normal (300 +/- 7, -68 +/- 10, and 71 +/- 8 beats/min). Leptin normalized BRS (bradycardia, -2.6 +/- 0.3, -1.7 +/- 0.2, and -3.0 +/- 0.5; and tachycardia, -3.2 +/- 0.4, -1.9 +/- 0.3, and -3.4 +/- 0.3 beats.min(-1).mmHg(-1) for control, diabetes, and leptin) and HR variability (23 +/- 4 to 11 +/- 1.5 ms2). Chronic glucose infusion to maintain hyperglycemia during leptin infusion did not alter the effect of leptin on IHR but abolished the improved BRS. These results show rapid impairment of autonomic nervous system control of HR after the induction of diabetes and that central nervous system actions of leptin can abolish the hyperglycemia as well as the altered IHR and BRS in STZ-induced diabetes.

Does acute hyperglycemia alter rat aortic depressor nerve function?

Because it is not known where in the reflex arch, i.e., afference, central nervous system or efferences, hyperglycemia affects baroreflex function, the present study examined the effect of short-term (30 min) hyperglycemia on aortic depressor nerve function measured by a mean arterial pressure vs aortic depressor nerve activity curve, fitted by sigmoidal regression, or by cross-spectral analysis between mean arterial pressure and aortic depressor nerve activity. Anesthetized male Wistar rats received an intravenous bolus (0.25 mL) injection, followed by 30 min of infusion (1 mL/h) of 30% glucose (N = 14). Control groups received a bolus injection and infusion of 0.9% saline (N = 14), or 30% mannitol (N = 14). Glucose significantly increased both blood glucose and plasma osmolarity (P < 0.05). Mean arterial pressure did not change after glucose, saline or mannitol infusion. Mean arterial pressure vs nerve activity curves were identical before and 10 and 30 min after the beginning of glucose, saline or mannitol infusion. Slow (0.3 Hz) oscillations of arterial pressure were induced by controlled bleeding, and cross-spectral analysis was applied to arterial pressure and aortic nerve activity. Transfer function magnitude (aortic depressor nerve activity/mean arterial pressure ratio in the frequency domain) was calculated as an index of gain of the aortic depressor nerve. Transfer function magnitude was similar in all groups during induced or spontaneous oscillations of arterial pressure. In conclusion, the present study demonstrates, by means of two different approaches for assessing baroreceptor function, that aortic depressor nerve activity was not altered by short-term (30 min) hyperglycemia.

The effect of experimental diabetes on the twenty-four-hour pattern of the vasodilator responses to acetylcholine and isoprenaline in the rat aorta

The aim of this study was to investigate whether time-dependent variations in the relaxant effect of acetylcholine, an endothelium-dependent vasorelaxant via muscarinic receptors, and isoprenaline, a nonselective beta-adrenoceptor agonist in rat aorta, are influenced by streptozotocin (STZ)-induced experimental diabetes. Adult male rats were divided randomly into two groups: control and STZ-induced (STZ, 55 mg/kg, intraperitoneal) diabetes. The animals were synchronized to a 12:12 h light-dark cycle (lights on 08:00 h) and sacrificed at six different times of day (1, 5, 9, 13, 17, and 21 hours after lights on; HALO) eight weeks after STZ injection. The in vitro responsiveness of thoracic aorta rings obtained from control and diabetic rats to acetylcholine (10(-9)-10(-5) M) and isoprenaline (10(-10)-10(-3) M) was determined in six different times. EC(50) (the concentration inducing half of the maximum response) values and maximum responses were calculated from cumulative concentration-response curves of the agonists and were analyzed with respect to time and STZ treatment. Treatment, time, and interactions between treatment and time were tested by two-way analysis of variance (ANOVA). To analyze differences due to biological time, one-way ANOVA was used. STZ treatment did not significantly change EC(50) values or maximum responses for both agonists. There were statistically significant time-dependent variations in the EC(50) values for isoprenaline and maximum responses for both acetylcholine and isoprenaline in control groups by one-way ANOVA, but significant time-dependent variations disappeared in the aortas isolated from STZ-induced diabetic rats. The vasodilator responses to acetylcholine and isoprenaline failed to show any significant interaction (treatmentxtime of study) between STZ treatment and time of sacrifice in both EC(50) values and maximum responses by two-way ANOVA. These results indicate there is a basic temporal pattern in the responses to acetylcholine and isoprenaline in rat aorta which continues in diabetes. It is shown for the first time that experimental diabetes does not change the 24 h pattern of responses to acetylcholine and isoprenaline, and that time-dependent variations in the responses to these agonists disappear in diabetic animals. Although further studies are required to define the underlying mechanism(s) of these findings, results suggest that experimental diabetes can modify the time-dependent vasorelaxant responses of rat aorta. This may help to understand the circadian rhythms in cardiovascular physiology and pathology or in drug effects in diabetes.

Vagal tone dominates autonomic control of mouse heart rate at thermoneutrality

It is generally accepted that cardiac sympathetic tone dominates the control of heart rate (HR) in mice. However, we have recently challenged this notion given that HR in the mouse is responsive to ambient temperature (Ta) and that the housing Tais typically 21–23°C, well below the thermoneutral zone (∼30°C) of this species. To specifically test the hypothesis that cardiac sympathetic tone is the primary mediator of HR control in the mouse, we first examined the metabolic and cardiovascular responses to rapid changes in Tato demonstrate the sensitivity of the mouse cardiovascular system to Ta. We then determined HR in 1) mice deficient in cardiac sympathetic tone (“β-less” mice), 2) mice deficient in cardiac vagal tone [muscarinic M2receptor ( M2R−/−) mice], and 3) littermate controls. At a Taof 30°C, the HR of β-less mice was identical to that of wild-type mice (351 ± 11 and 363 ± 10 beats/min, respectively). However, the HR of M2R−/−mice was significantly greater (416 ± 7 beats/min), demonstrating that vagal tone predominates over HR control at this Ta. When these mice were calorically restricted to 70% of normal intake, HR fell equally in wild-type, β-less, and M2R−/−mice (ΔHR = 73 ± 9, 76 ± 3, and 73 ± 7 beats/min, respectively), suggesting that the fall in intrinsic HR governs bradycardia of calorically restricted mice. Only when the Tawas relatively cool, at 23°C, did β-less mice exhibit a HR (442 ± 14 beats/min) that was different from that of littermate controls (604 ± 10 beats/min) and M2R−/−mice (602 ± 5 beats/min). These experiments conclusively demonstrate that in the absence of cold stress, regulation of vagal tone and modulation of intrinsic rate are important determinants of HR control in the mouse.