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

Glucose Counteracts Isoprenaline Effects on Ion Channel Functions in Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Recent studies indicate that the disorder of glucose metabolism in myocardial tissue is involved in the development of Takotsubo syndrome (TTS). This study investigated the effects of a high level of glucose on the pathogenesis of TTS, focusing on the electrophysiological mechanisms. Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were treated with toxic concentration of isoprenaline (Iso, 1 mM) and a high level of glucose (22 mM) to mimic the setting of TTS and diabetes mellitus (DM). Iso prolonged action potential duration (APD) through enhancing the late sodium channel current and suppressing the transient outward potassium current (Ito). However, a high level of glucose prevented the APD prolongation and the change in Ito. High-level glucose reduced the expression levels of PI3K/Akt, β1-adrenoceptors, Gs-protein, and PKA, suggesting their involvement in the protective effects of high-level glucose against toxic effects of catecholamine. High glucose level did not influence Iso-induced ROS-generation, suggesting that the protective effects of high-level glucose against Iso did not result from changes in ROS generation. High-level glucose may protect cardiomyocytes from the toxic effects of catecholamine excess through suppressing β1-adrenoceptor-Gs-PKA signaling. DM may reduce the risk for occurrence of arrhythmias due to QT prolongation in TTS patients.

Cerebral-Cardiac Syndrome and Diabetes: Cardiac Damage After Ischemic Stroke in Diabetic State

Cerebral-cardiac syndrome (CCS) refers to cardiac dysfunction following varying brain injuries. Ischemic stroke is strongly evidenced to induce CCS characterizing as arrhythmia, myocardial damage, and heart failure. CCS is attributed to be the second leading cause of death in the post-stroke stage; however, the responsible mechanisms are obscure. Studies indicated the possible mechanisms including insular cortex injury, autonomic imbalance, catecholamine surge, immune response, and systemic inflammation. Of note, the characteristics of the stroke population reveal a common comorbidity with diabetes. The close and causative correlation of diabetes and stroke directs the involvement of diabetes in CCS. Nevertheless, the role of diabetes and its corresponding molecular mechanisms in CCS have not been clarified. Here we conclude the features of CCS and the potential role of diabetes in CCS. Diabetes drives establish a “primed” inflammatory microenvironment and further induces severe systemic inflammation after stroke. The boosted inflammation is suspected to provoke cardiac pathological changes and hence exacerbate CCS. Importantly, as the key element of inflammation, NOD-like receptor pyrin domain containing 3 (NLRP3) inflammasome is indicated to play an important role in diabetes, stroke, and the sequential CCS. Overall, we characterize the corresponding role of diabetes in CCS and speculate a link of NLRP3 inflammasome between them.

Metabolic and Molecular Imaging of the Diabetic Cardiomyopathy

The term diabetic cardiomyopathy is defined as the presence of abnormalities in myocardial structure and function that occur in the absence of, or in addition to, well-established cardiovascular risk factors. A key contributor to this abnormal structural-functional relation is the complex interplay of myocardial metabolic remodeling, defined as the loss the flexibility in myocardial substrate metabolism and its downstream detrimental effects, such as mitochondrial dysfunction, inflammation, and fibrosis. In parallel with the growth in understanding of these biological underpinnings has been developmental advances in imaging tools such as positron emission tomography and magnetic resonance imaging and spectroscopy that permit the detection and in many cases quantification, of the processes that typifies the myocardial metabolic remodeling in diabetic cardiomyopathy. The imaging readouts can be obtained in both preclinical models of diabetes mellitus and patients with diabetes mellitus facilitating the bi-directional movement of information between bench and bedside. Moreover, imaging biomarkers provided by these tools are now being used to enhance discovery and development of therapies designed to reduce the myocardial effects of diabetes mellitus through metabolic modulation. In this review, the use of these imaging tools in the patient with diabetes mellitus from a mechanistic, therapeutic effect, and clinical management perspective will be discussed.

Pharmacological evaluation of metformin and N-benzylbiguanide, a novel analogue of metformin, on the vasopressor responses to adrenergic system stimulation in pithed rats with fructose-induced insulin resistance

Metformin has been associated with cardioprotection, vasorelaxation and normalization of endothelial function during type 2 Diabetes Mellitus. However, few studies have analysed its effects on vascular adrenergic system. Our study has evaluated the vasopressor responses induced by sympathetic stimulation or by i.v. bolus injections of the agonists noradrenaline (α_1/2), methoxamine (α₁) and UK 14,304 (α₂) in rats with fructose-induced insulin resistance chronically pretreated with either metformin or EGL-6M (N-benzylbiguanide), a novel analogue of metformin. Rats were treated with fructose (15%) or tap water (control) during 16 weeks. Next, both groups were treated daily during 4 weeks with: (1) vehicle; (2) metformin (50mg/kg); or (3) EGL-6M (50mg/kg). Blood glucose and plasma insulin were determined before and after administration of glucose during oral glucose tolerance test. Animals treated with fructose showed hyperinsulinemia and insulin resistance, which were decreased by metformin and EGL-6M. In animals treated with fructose, the vasopressor responses induced by: (1) sympathetic stimulation were decreased; (2) noradrenaline were increased; and (3) methoxamine and UK 14,304 remained unaffected compared with control group. In control animals, metformin failed to modify the vasopressor responses analysed, while EGL-6M increased the vasopressor responses to sympathetic stimulation. In rats treated with fructose, metformin decreased vasopressor response to noradrenaline but did not modify the sympathetic stimulation responses. EGL-6M increased the vasopressor responses to sympathetic stimulation without modifying those to noradrenaline, methoxamine or UK 14,304. Collectively, these data suggest that EGL-6M is capable to increase insulin sensitivity and the vasopressor sympathetic outflow in rats.

Cardiac β-Adrenoceptor Expression Is Reduced in Zucker Diabetic Fatty Rats as Type-2 Diabetes Progresses

Objectives

Reduced cardiac β-adrenoceptor (β-AR) expression and cardiovascular dysfunction occur in models of hyperglycemia and hypoinsulinemia. Cardiac β-AR expression in type-2 diabetes models of hyperglycemia and hyperinsulinemia, remain less clear. This study investigates cardiac β-AR expression in type-2 diabetic Zucker diabetic fatty (ZDF) rats.

Methods

Ex vivo biodistribution experiments with [³H]CGP12177 were performed in Zucker lean (ZL) and ZDF rats at 10 and 16 weeks of age as diabetes develops. Blood glucose, body mass, and diet consumption were measured. Western blotting of β-AR subtypes was completed in parallel. Echocardiography was performed at 10 and 16 weeks to assess systolic and diastolic function. Fasted plasma insulin, free fatty acids (FFA), leptin and fed-state insulin were also measured.

Results

At 10 weeks, myocardial [³H]CGP12177 was normal in hyperglycemic ZDF (17±4.1mM) compared to ZL, but reduced 16-25% at 16 weeks of age as diabetes and hyperglycemia (22±2.4mM) progressed. Reduced β-AR expression not apparent at 10 weeks also developed by 16 weeks of age in ZDF brown adipose tissue. In the heart, Western blotting at 10 weeks indicated normal β₁-AR (98±9%), reduced β₂-AR (76±10%), and elevated β₃-AR (108±6). At 16 weeks, β₁-AR expression became reduced (69±16%), β₂-AR expression decreased further (68±14%), and β₃-AR remained elevated, similar to 10 weeks (112±9%). While HR was reduced at 10 and 16 weeks in ZDF rats, no significant changes were observed in diastolic or systolic function.

Conclusions

Cardiac β-AR are reduced over 6 weeks of sustained hyperglycemia in type-2 diabetic ZDF rats. This indicates cardiac [³H]CGP12177 retention and β₁- and β₂-AR expression are inversely correlated with the progression of type-2 diabetes.

Insulin therapy normalizes reduced myocardial β-adrenoceptors at both the onset and after sustained hyperglycemia in diabetic rats

AIMS

Reduced cardiac β-adrenoceptors (β-AR) and cardiovascular (CV) dysfunction occur in diabetes mellitus (DM) and can be normalized by insulin. It is unclear how the duration of untreated hyperglycemia prior to intervention impacts insulin's effects. This study assesses insulin's effect on reduced myocardial β-AR and CV function, comparing insulin therapy at the onset of hyperglycemia and after a sustained period of hyperglycemia in streptozotocin (STZ) rats.

MAIN METHODS

Ex vivo biodistribution experiments with [(3)H]CGP12177 were performed in high-fat fed STZ rats after 8 weeks of hyperglycemia evaluating cardiac β-AR expression. Western blotting of β-AR subtypes was completed in parallel. Serial echocardiography at 0, 6, and 8 weeks post-STZ investigated CV function. Sub-groups of hyperglycemic rats were treated with insulin early, at 1 week post-STZ (InsE) for 7 weeks, or late at 6 weeks post-STZ (InsL) for 2 weeks to observe how the duration of hyperglycemia prior to insulin impacts its effects.

KEY FINDINGS

Reduced myocardial [(3)H]CGP12177 binding occurred 8 weeks post-STZ in hyperglycemics, but was normal in both insulin treatments. Western blotting supported reduced β1-AR in hyperglycemics, but not in either treatment. InsE and InsL treatments improved prolonged mitral valve deceleration (MVD) observed in hyperglycemic animals, but hyperglycemic and InsL still displayed reduced heart rates (HR).

SIGNIFICANCE

This work supports that glycemic control with insulin normalizes cardiac β-AR effectively regardless of prior hyperglycemia but HR may not recover as readily, indirectly supporting the utility of [(11)C]CGP12177 positron emission tomography (PET) in assessing cardiac β-AR and their modulation with glycemic therapy.

Quantification of [(11)C]-meta-hydroxyephedrine uptake in human myocardium

Background

The aims of this study were to determine the optimal tracer kinetic model for [¹¹C]-meta-hydroxyephedrine ([¹¹C]HED) and to evaluate the performance of several simplified methods.

Methods

Thirty patients underwent dynamic 60-min [¹¹C]HED scans with online arterial blood sampling. Single-tissue and both reversible and irreversible two-tissue models were fitted to the data using the metabolite-corrected arterial input function. For each model, reliable fits were defined as those yielding outcome parameters with a coefficient of variation (CoV) <25%. The optimal model was determined using Akaike and Schwarz criteria and the F-test, together with the number of reliable fits. Simulations were performed to study accuracy and precision of each model. Finally, quantitative results obtained using a population-averaged metabolite correction were evaluated, and simplified retention index (RI) and standardized uptake value (SUV) results were compared with quantitative volume of distribution (V_T) data.

Results

The reversible two-tissue model was preferred in 75.8% of all segments, based on the Akaike information criterion. However, V_T derived using the single-tissue model correlated highly with that of the two-tissue model (r² = 0.94, intraclass correlation coefficient (ICC) = 0.96) and showed higher precision (CoV of 24.6% and 89.2% for single- and two-tissue models, respectively, at 20% noise). In addition, the single-tissue model yielded reliable fits in 94.6% of all segments as compared with 77.1% for the reversible two-tissue model. A population-averaged metabolite correction could not be used in approximately 20% of the patients because of large biases in V_T. RI and SUV can provide misleading results because of non-linear relationships with V_T.

Conclusions

Although the reversible two-tissue model provided the best fits, the single-tissue model was more robust and results obtained were similar. Therefore, the single-tissue model was preferred. RI showed a non-linear correlation with V_T, and therefore, care has to be taken when using RI as a quantitative measure.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-014-0052-4) contains supplementary material, which is available to authorized users.

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

BACKGROUND

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

METHODS AND RESULTS

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

CONCLUSION

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