Increased neuronal activation in sympathoregulatory regions of the brain and spinal cord in type 2 diabetic rats

Increased cardiac sympathetic nerve activity in type 2 diabetes mellitus (DM) suggests impaired autonomic control of the heart. However, the central regions that contribute to the autonomic cardiac pathologies in type 2 DM are unknown. Therefore, we tested the hypothesis that neuronal activation would be increased in central sympathoregulatory areas in a pre-clinical type 2 DM animal model. Immunohistochemistry in 20-week-old male Zucker diabetic fatty (ZDF) rats revealed an increased number of neurones expressing ΔFosB (a marker of chronic neuronal activation) in the intermediolateral column (IML) of the spinal cord in DM compared to non-diabetic (non-DM) rats (P < 0.05). Rostral ventrolateral medulla (RVLM) neurones activate IML neurones and receive inputs from the hypothalamic paraventricular nucleus (PVN), as well as the nucleus tractus solitarius (NTS) and area postrema (AP), in the brainstem. We observed more ΔFosB-positive noradrenergic RVLM neurones (P < 0.001) and corticotrophin-releasing hormone PVN neurones (P < 0.05) in DM compared to non-DM rats. More ΔFosB-positive neurones were also observed in the NTS (P < 0.05) and AP (P < 0.01) of DM rats compared to non-DM rats. Finally, because DM ZDF rats are obese, we also expected increased activation of pro-opiomelanocortin (POMC) arcuate nucleus (ARC) neurones in DM rats; however, fewer ΔFosB-positive POMC ARC neurones were observed in DM compared to non-DM rats (P < 0.01). In conclusion, increased neuronal activation in the IML of type 2 DM ZDF rats might be driven by RVLM neurones that are possibly activated by PVN, NTS and AP inputs. Elucidating the contribution of central sympathoexcitatory drive in type 2 DM might improve the effectiveness of pharmacotherapies for diabetic heart disease.

Elevated myocardial fructose and sorbitol levels are associated with diastolic dysfunction in diabetic patients, and cardiomyocyte lipid inclusions in vitro

Diabetes is associated with cardiac metabolic disturbances and increased heart failure risk. Plasma fructose levels are elevated in diabetic patients. A direct role for fructose involvement in diabetic heart pathology has not been investigated. The goals of this study were to clinically evaluate links between myocardial fructose and sorbitol (a polyol pathway fructose precursor) levels with evidence of cardiac dysfunction, and to experimentally assess the cardiomyocyte mechanisms involved in mediating the metabolic effects of elevated fructose. Fructose and sorbitol levels were increased in right atrial appendage tissues of type 2 diabetic patients (2.8- and 1.5-fold increase respectively). Elevated cardiac fructose levels were confirmed in type 2 diabetic rats. Diastolic dysfunction (increased E/e’, echocardiography) was significantly correlated with cardiac sorbitol levels. Elevated myocardial mRNA expression of the fructose-specific transporter, Glut5 (43% increase), and the key fructose-metabolizing enzyme, Fructokinase-A (50% increase) was observed in type 2 diabetic rats (Zucker diabetic fatty rat). In neonatal rat ventricular myocytes, fructose increased glycolytic capacity and cytosolic lipid inclusions (28% increase in lipid droplets/cell). This study provides the first evidence that elevated myocardial fructose and sorbitol are associated with diastolic dysfunction in diabetic patients. Experimental evidence suggests that fructose promotes the formation of cardiomyocyte cytosolic lipid inclusions, and may contribute to lipotoxicity in the diabetic heart.

Carvedilol and metoprolol are both able to preserve myocardial function in type 2 diabetes

PURPOSE

Increasing cohorts of patients present with diabetic cardiomyopathy, and with no targeted options, treatment often rely on generic pharmaceuticals such as β-blockers. β-blocker efficacy is heterogenous, with second generation β-blocker metoprolol selectively inhibiting β₁ -AR, while third generation β-blocker carvedilol has α₁ -AR inhibition, antioxidant, and anti-apoptotic actions alongside nonselective β-AR inhibition. These additional properties have led to the hypothesis that carvedilol may improve cardiac contractility in the diabetic heart to a greater extent than metoprolol. The present study aimed to compare the efficacy of metoprolol and carvedilol on myocardial function in animal models and cardiac tissue from patients with type 2 diabetes and preserved ejection fraction.

METHODS

Echocardiographic examination of cardiac function and assessment of myocardial function in isolated trabeculae was carried out in patients with and without diabetes undergoing coronary artery bypass grafting (CABG) who were prescribed metoprolol or carvedilol. Equivalent measures were undertaken in Zucker Diabetic Fatty (ZDF) rats following 4 weeks treatment with metoprolol or carvedilol.

RESULTS

Patients receiving carvedilol compared to metoprolol had no difference in cardiac function, and no difference was apparent in myocardial function between β-blockers. Both β-blockers similarly improved myocardial function in diabetic ZDF rats treated for 4 weeks, without significantly affecting in vivo cardiac function.

CONCLUSIONS

Metoprolol and carvedilol were found to have no effect on cardiac function in type 2 diabetes with preserved ejection fraction, and were similarly effective in preventing myocardial dysfunction in ZDF rats.

Cardiac β-adrenergic responsiveness of obese Zucker rats: The role of AMPK

NEW FINDINGS

What is the central question of the study? Is the reduced signalling of AMP-activated protein kinase (AMPK), a key regulator of energy homeostasis in the heart, responsible for the reduced β-adrenergic responsiveness of the heart in obesity? What is the main finding and its importance? Inhibition of AMPK in isolated hearts prevented the reduced cardiac β-adrenergic responsiveness of obese rats, which was accompanied by reduced phosphorylation of AMPK, a proxy of AMPK activity. This suggests a direct functional link between β-adrenergic responsiveness and AMPK signalling in the heart, and it suggests that AMPK might be an important target to restore the β-adrenergic responsiveness in the heart in obesity.

ABSTRACT

The obesity epidemic impacts heavily on cardiovascular health, in part owing to changes in cardiac metabolism. AMP-activated protein kinase (AMPK) is a key regulator of energy homeostasis in the heart and is regulated by β-adrenoceptors (β-ARs) in normal conditions. In obesity, chronic sympathetic overactivation leads to impaired cardiac β-AR responsiveness, although it is unclear whether AMPK signalling, downstream of β-ARs, contributes to this dysfunction. Therefore, we aimed to determine whether reduced AMPK signalling is responsible for the reduced β-AR responsiveness in obesity. In isolated hearts of lean and obese Zucker rats, we tested β-AR responsiveness to the β₁ -AR agonist isoprenaline (ISO, 1 × 10^-10 to 5 × 10^-8  m) in the absence and presence of the AMPK inhibitor, compound C (CC, 10 μm). The β₁ -AR expression and AMPK phosphorylation were assessed by Western blot. β-Adrenergic responsiveness was reduced in the hearts of obese rats (logEC₅₀ of ISO-developed pressure dose-response curves: lean -8.53 ± 0.13 × 10^x  m versus obese -8.35 ± 0.10 × 10^x  m ; P < 0.05 lean versus obese, n = 6 per group). This difference was not apparent after AMPK inhibition (logEC₅₀ of ISO-developed pressure curves: lean CC -8.19 ± 0.12 × 10^x  m versus obese CC 8.17 ± 0.13 × 10^x  m, P < 0.05, n = 6 per group). β₁ -Adrenergic receptor expression and AMPK phosphorylation were reduced in hearts of obese rats (AMPK at Thr¹⁷² : lean 1.73 ± 0.17 a.u. versus lean CC 0.81 ± 0.13 a.u., and obese 1.18 ± 0.09 a.u. versus obese CC 0.81 ± 0.16 a.u., P < 0.05, n = 6 per group). Thus, a direct functional link between β-adrenergic responsiveness and AMPK signalling in the heart exists, and AMPK might be an important target to restore the reduced cardiac β-adrenergic responsiveness in obesity.

Effect of type 2 diabetes, surgical incision, and volatile anesthesia on hemodynamics in the rat

Diabetic patients have increased cardiac complications during surgery, possibly due to impaired autonomic regulation. Anesthesia lowers blood pressure and heart rate (HR), whereas surgical intervention has opposing effects. The interaction of anesthesia and surgical intervention on hemodynamics in diabetes is unknown, despite being a potential perioperative risk factor. We aimed to determine the effect of diabetes on the integrative interaction between hemodynamics, anesthesia, and surgical incision. Zucker type 2 diabetic rats (DM) and their nondiabetic littermates (ND) were implanted with an intravenous port for drug delivery, and a radiotelemeter to measure mean arterial blood pressure (MAP) and derive HR (total n = 50). Hemodynamic pharmacological responses were assessed under conscious, isoflurane anesthesia (~2-2.5%), and anesthesia-surgical conditions; the latter performed as a laparotomy. MAP was not different between groups under conscious conditions (ND 120 ± 6 vs. DM 131 ± 4 mmHg, P > 0.05). Anesthesia reduced MAP, but not differently in DM (ND -30 ± 6 vs. DM -38 ± 4 ΔmmHg, P > 0.05). Despite adequate anesthesia, surgical incision increased MAP, which tended to be less in DM (ND +21 ± 4 vs. DM +13 ± 2 ΔmmHg, P = 0.052). Anesthesia disrupted central baroreflex HR responses to sympathetic activation (sodium nitroprusside 10 μg·kg^-1, ND conscious 83 ± 13 vs. anesthetized 16 ± 5 Δbpm; P < 0.05) or to sympathetic withdrawal (phenylephrine 10 μg·kg^-1, ND conscious -168 ± 37 vs. anesthetized -20 ± 6 Δbpm; P < 0.05) with no additional changes observed after surgical incision or during diabetes. During perioperative conditions, type 2 diabetes did not impact on short-term hemodynamic regulation. Anesthesia had the largest hemodynamic impact, whereas surgical effects were limited to modulation of baseline blood pressure.

Sympathetic hyper-excitation in obesity and pulmonary hypertension: physiological relevance to the 'obesity paradox'

BACKGROUND

Within the lung, sympathetic nerve activity (SNA) has an important role in facilitating pulmonary vasodilation. As SNA is elevated in obesity, we aimed to assess the impact of sympathetic hyper-excitation on pulmonary vascular homeostasis in obesity, and its potential role in ameliorating the severity of pulmonary hypertension (PH); the well-documented 'obesity paradox' phenomenon.

METHODS

Zucker obese and lean rats were exposed to normoxia or chronic hypoxia (CH-10% O2) for 2 weeks. Subsequently, pulmonary SNA (pSNA) was recorded (electrophysiology), or the pulmonary microcirculation was visualized using Synchrotron microangiography. Acute hypoxic pulmonary vasoconstriction (HPV) was assessed before and after blockade of β1-adrenergic receptors (ARs) (atenolol, 3 mg kg(-1)) and β1+β2-adrenergic (propranolol, 2 mg kg(-1)).

RESULTS

pSNA of normoxic obese rats was higher than lean counterparts (2.4 and 0.5 μV s, respectively). SNA was enhanced following the development of PH in lean rats, but more so in obese rats (1.7 and 6.8 μV s, respectively). The magnitude of HPV was similar for all groups (for example, ~20% constriction of the 200-300 μm vessels). Although β-blockade did not modify HPV in lean rats, it significantly augmented the HPV in normoxic obese rats (β1 and β2 blockade), and more so in obese rats with PH (β2-blockade alone). Western blots showed, while the expression of pulmonary β1-ARs was similar for all rats, the expression of β2-ARs was downregulated in obesity and PH.

CONCLUSIONS

This study suggests that sympathetic hyper-excitation in obesity may have an important role in constraining the severity of PH and, thus, contribute in part to the 'obesity paradox' in PH.

Increased Efferent Cardiac Sympathetic Nerve Activity and Defective Intrinsic Heart Rate Regulation in Type 2 Diabetes

Elevated sympathetic nerve activity (SNA) coupled with dysregulated β-adrenoceptor (β-AR) signaling is postulated as a major driving force for cardiac dysfunction in patients with type 2 diabetes; however, cardiac SNA has never been assessed directly in diabetes. Our aim was to measure the sympathetic input to and the β-AR responsiveness of the heart in the type 2 diabetic heart. In vivo recording of SNA of the left efferent cardiac sympathetic branch of the stellate ganglion in Zucker diabetic fatty rats revealed an elevated resting cardiac SNA and doubled firing rate compared with nondiabetic rats. Ex vivo, in isolated denervated hearts, the intrinsic heart rate was markedly reduced. Contractile and relaxation responses to β-AR stimulation with dobutamine were compromised in externally paced diabetic hearts, but not in diabetic hearts allowed to regulate their own heart rate. Protein levels of left ventricular β1-AR and Gs (guanine nucleotide binding protein stimulatory) were reduced, whereas left ventricular and right atrial β2-AR and Gi (guanine nucleotide binding protein inhibitory regulatory) levels were increased. The elevated resting cardiac SNA in type 2 diabetes, combined with the reduced cardiac β-AR responsiveness, suggests that the maintenance of normal cardiovascular function requires elevated cardiac sympathetic input to compensate for changes in the intrinsic properties of the diabetic heart.

Increased haemodynamic adrenergic load with isoflurane anaesthesia in type 2 diabetic and obese rats in vivo

Background

Increasing numbers of type 2 diabetic and obese patients with enhanced rates of cardiovascular complications require surgical interventions, however they have a higher incidence of perioperative haemodynamic complications, which has been linked to adrenergic dysfunction. Therefore, we aimed to determine how α- and β-adrenoceptor (AR)-mediated haemodynamic responses are affected by isoflurane anaesthesia in experimental type 2 diabetes and obesity in vivo.

Methods

Sixteen-week old male Zucker type 2 Diabetic Fatty (ZDF) rats, Zucker Obese rats and their lean counterparts (n = 7-9 per group) were instrumented with radio telemeters to record blood pressure and heart rate and with vascular access ports for non-invasive intravenous drug delivery in vivo. Haemodynamic effects of α-AR (phenylephrine; 1-100 μg.kg⁻¹) or β-AR (dobutamine; 2-120 μg.kg⁻¹) stimulation were assessed under conscious and anaesthetised (isoflurane; 2%) conditions.

Results

Vascular α-AR sensitivity was increased in both diabetic (non-diabetic 80 ± 3 vs. diabetic 95 ± 4 ΔmmHg at 100 μg.kg⁻¹; p < 0.05) and obese (lean 65 ± 6 vs. obese 84 ± 6 ΔmmHg at 20 μg.kg⁻¹; p < 0.05) conscious rats. Interestingly, anaesthesia exacerbated and prolonged the increased α-AR function in both diabetic and obese animals (non-diabetic 51 ± 1 vs. diabetic 68 ± 4 ΔmmHg, lean 61 ± 5 vs. obese 84 ± 2 ΔmmHg at 20 μg.kg⁻¹; p < 0.05). Meanwhile, β-AR chronotropic sensitivity was reduced in conscious diabetic and obese rats (non-diabetic 58 ± 7 vs. diabetic 27 ± 8 Δbpm, lean 103 ± 12 vs. obese 61 ± 9 Δbpm at 15 μg.kg⁻¹; p < 0.05). Anaesthesia normalised chronotropic β-AR responses, via either a limited reduction in obese (lean 51 ± 3 vs. obese 66 ± 5 Δbpm; NS at 15 μg.kg⁻¹) or increased responses in diabetic animals (non-diabetic 49 ± 8 vs. diabetic 63 ± 8 Δbpm, at 15 μg.kg⁻¹; NS at 15 μg.kg⁻¹).

Conclusions

Long term metabolic stress, such as during type 2 diabetes and obesity, alters α- and β-AR function, its dynamics and the interaction with isoflurane anaesthesia. During anaesthesia, enhanced α-AR sensitivity and normalised β-AR function may impair cardiovascular function in experimental type 2 diabetes and obesity.

Dual implantation of a radio-telemeter and vascular access port allows repeated hemodynamic and pharmacological measures in conscious lean and obese rats

Expansion of physiological knowledge increasingly requires examination of processes in the normal, conscious state. The current study describes a novel approach combining surgical implantation of radio-telemeters with vascular access ports (VAPs) to allow repeated hemodynamic and pharmacological measures in conscious rats. Dual implantation was conducted on 16-week-old male lean and obese Zucker rats. Continued viability one month after surgery was observed in 67% of lean and 44% of obese animals, giving an overall 54% completion rate. Over the five-week measurement period, reliable and reproducible basal mean arterial pressure and heart rate measures were observed. VAP patency and receptor-independent vascular reactivity were confirmed by consistent hemodynamic responses to sodium nitroprusside (6.25 µg/kg). Acutely, minimal hemodynamic responses to repeated bolus administration of 0.2 mL saline indicated no significant effect of increased blood volume or administration stress, making repeated acute measures viable. Similarly, repeated administration of the β-adrenoceptor agonist dobutamine (30 µg/kg) at 10 min intervals resulted in reproducible hemodynamic changes in both lean and obese animals. Therefore, our study demonstrates that this new approach is viable for the acute and chronic assessment of hemodynamic and pharmacological responses in both lean and obese conscious rats. This technique reduces the demand for animal numbers and allows hemodynamic measures with minimal disruption to animals' welfare, while providing reliable and reproducible results over several weeks. In conclusion, dual implantation of a radio-telemeter and VAP introduces a valuable technique for undertaking comprehensive studies involving repeated pharmacological tests in conscious animals to address important physiological questions.

Adiponectin opposes endothelin-1-mediated vasoconstriction in the perfused rat hindlimb

Recent studies have shown that adiponectin is able to increase nitric oxide (NO) production by the endothelium and relax preconstricted isolated aortic rings, suggesting that adiponectin may act as a vasodilator. Endothelin-1 (ET-1) is a potent vasoconstrictor, elevated levels of which are associated with obesity, type 2 diabetes, hypertension, and cardiovascular disease. We hypothesized that adiponectin has NO-dependent vascular actions opposing the vasoconstrictor actions of ET-1. We studied the vascular and metabolic effects of a physiological concentration of adiponectin (6.5 μg/ml) on hooded Wistar rats in the constant-flow pump-perfused rat hindlimb. Adiponectin alone had no observable vascular activity; however, adiponectin pretreatment and coinfusion inhibited the increase in perfusion pressure and associated metabolic stimulation caused by low-dose (1 nM) ET-1. Adiponectin was not able to oppose vasoconstriction when infusion was commenced after ET-1. This is in contrast to the NO donor sodium nitroprusside, which significantly reduced the pressure due to established ET-1 vasoconstriction, suggesting dissociation of the actions of adiponectin and NO. In addition, adiponectin had no effect on vasoconstriction caused by either high-dose (20 nM) ET-1 or low-dose (50 nM) norepinephrine. Our findings suggest that adiponectin has specific, apparently NO-independent, vascular activity to oppose the vasoconstrictor effects of ET-1. The hemodynamic actions of adiponectin may be an important aspect of its insulin-sensitizing ability by regulating access of insulin and glucose to myocytes. Imbalance in the relationship between adiponectin and ET-1 in obesity may contribute to the development of insulin resistance and cardiovascular disease.