Spontaneous baroreflex sensitivity is attenuated in male UCD-type 2 diabetes mellitus rats: A link between metabolic and autonomic dysfunction

Abnormal cardiovascular control during exercise: Role of insulin resistance in the brain

During exercise circulatory adjustments to meet oxygen demands are mediated by multiple autonomic mechanisms, the skeletal muscle exercise pressor reflex (EPR), the baroreflex (BR), and by feedforward signals from central command neurons in higher brain centers. Insulin resistance in peripheral tissues includes sensitization of skeletal muscle afferents by hyperinsulinemia which is in part responsible for the abnormally heightened EPR function observed in diabetic animal models and patients. However, the role of insulin signaling within the central nervous system (CNS) is receiving increased attention as a potential therapeutic intervention in diseases with underlying insulin resistance. This review will highlight recent advances in our understanding of how insulin resistance induces changes in central signaling. The alterations in central insulin signaling produce aberrant cardiovascular responses to exercise. In particular, we will discuss the role of insulin signaling within the medullary cardiovascular control nuclei. The nucleus tractus solitarius (NTS) and rostral ventrolateral medulla (RVLM) are key nuclei where insulin has been demonstrated to modulate cardiovascular reflexes. The first locus of integration for the EPR, BR and central command is the NTS which is high in neurons expressing insulin receptors (IRs). The IRs on these neurons are well positioned to modulate cardiovascular responses to exercise. Additionally, the differences in IR density and presence of receptor isoforms enable specificity and diversity of insulin actions within the CNS. Therefore, non-invasive delivery of insulin into the CNS may be an effective means of normalizing cardiovascular responses to exercise in patients with insulin resistance.

Altered leptin signaling and attenuated cardiac vagal activity in rats with type 2 diabetes

Introduction

The leading cause of death in type 2 diabetes mellitus (T2DM) patients is cardiovascular-related events, including myocardial infraction-induced ventricular arrhythmia. Previous studies have shown that T2DM-induced functional remodeling of cardiac vagal postganglionic (CVP) neurons contributes to ventricular arrhythmogenesis. As leptin resistance is common in T2DM patients, and CVP neurons are located in epicardial adipose pads, a tissue that secretes leptin, in this study we aimed to elucidate a correlation between leptin resistance and CVP neuronal dysfunction in T2DM.

Methods

A high fat-diet/low dose streptozotocin-induced T2DM rat model was used in this study to characterize T2DM-induced alterations in cardiac parasympathetic tone, determined by changes in baroreflex sensitivity and CVP neuronal excitability. The impact of leptin resistance on CVP neurons was also studied by examining the expression of leptin in epicardial adipose pads, and leptin receptors and uncoupling protein 2 (UCP2) in CVP neurons.

Results

T2DM rats exhibited diminished baroreflex sensitivity, and decreased CVP neuronal excitability, demonstrated by a reduced frequency of action potentials, diminished nAChR currents, and an attenuated response to nicotine stimulation. Additionally, compared to sham animals, the expression of leptin receptors and UCP2 in CVP neurons was reduced as early as 4 weeks post-T2DM although the leptin levels in epicardial adipose pads was increased during the progression of T2DM, which demonstrated the occurrence of leptin resistance in T2DM CVP neurons.

Conclusion

Cardiac parasympathetic dysfunction in T2DM rats is due, in part, to functional remodeling of CVP neurons. As leptin resistance develops as early as 4 weeks post-T2DM induction, diminished leptin receptors-UCP2 signaling may contribute to CVP neuronal dysregulation.

Interleukin-1 type 1 receptor blockade attenuates the exaggerated exercise pressor reflex in male UC Davis type 2 diabetic mellitus rats

An exaggerated exercise pressor reflex and peripheral neuropathy are both evoked by the same type of thinly myelinated afferents and are present in patients with type 2 diabetes mellitus (T2DM). Although it is known that the pro-inflammatory cytokine interleukin-1β (IL-1β) contributes to peripheral neuropathy, the effects of IL-1β on the exercise pressor reflex in T2DM are not known. Therefore, we aimed to determine the effect of IL-1 receptors on the exercise pressor reflex in T2DM. We compared changes in peak pressor (mean arterial pressure; ΔMAP), blood pressure index (ΔBPi), heart rate (ΔHR) and heart rate index (ΔHRi) responses to static and intermittent contractions and tendon stretch before and after peripheral IL-1 type 1 receptor blockade (anakinra, Kineret®) in T2DM and healthy male rats and IL-1 receptor activation (IL-1β) in healthy rats. Blocking IL-1 receptors significantly attenuated the ΔMAP and ΔBPi to static contraction in T2DM rats. Furthermore, blocking IL-1 receptors significantly attenuated the ΔMAP, ΔBPi and ΔHRi to intermittent contraction, and ΔMAP to tendon stretch in T2DM rats (all P < 0.05). In addition, IL-1 receptor activation significantly exaggerated the ΔMAP and ΔBPi to static contraction and ΔMAP, ΔBPi and ΔHR to intermittent contraction in healthy rats, all P < 0.05. Furthermore, circulating IL-1β serum concentrations were significantly greater in T2DM rats than in healthy rats (P < 0.05). We conclude that IL-1 signalling contributes to the exaggerated exercise pressor reflex in T2DM, suggesting for the first time that inflammatory cytokines play a critical role in exaggerated blood pressure responses to exercise in those with T2DM. KEY POINTS: Chronic inflammation, a complication of type 2 diabetes mellitus (T2DM), causes increased production of pro-inflammatory cytokines, such as interleukin (IL)-1β, IL-6, and tumour necrosis factor-α. IL-1β has been shown to sensitize muscle afferents that conduct the exercise pressor reflex. We found blocking of IL-1 receptors by anakinra (Kineret®), an IL-1 type 1 receptor antagonist, significantly attenuated the exaggerated exercise pressor reflex in T2DM rats, but not in healthy rats. In addition, activating IL-1 receptors with IL-1β significantly augmented the exercise pressor reflex in healthy rats. Our findings suggest that IL-1 receptors, by mediating IL-1β signalling, play a role in exaggerating the exercise pressor reflex in T2DM. These results highlight the complex interplay between inflammation and the autonomic nervous system in regulating cardiovascular function, and the potential for using an FDA-approved IL-1 receptor antagonist, Kineret®, as a therapeutic approach to reduce adverse cardiovascular events during physical activity in those with T2DM.

Blood Pressure Variability is Better Associated with Acute Relative Hyperglycemia Than the Heart Rate Variability in Healthy Young Adults

BACKGROUND

Deranged cardiovascular autonomic functions are well-reported complications of diabetes mellitus, where chronic hyperglycemia is an important factor. The role of acute relative hyperglycemia on cardiovascular autonomic functions, particularly on blood pressure variability in healthy subjects, has been rarely explored. Therefore, this study aimed to examine the effect of acute relative hyperglycemia on cardiovascular autonomic functions in healthy young adults.

METHODS

Beat-to-beat blood pressure and electrocardiogram were recorded to assess the heart rate variability and blood pressure variability in 42 young, healthy subjects during fasting and relative hyperglycemic states. Recorded cardiovascular parameters were analyzed in time and frequency domains. Correlations among analyzed parameters of cardiovascular autonomic variabilities were explored during fasting and relative hyperglycemic state.

RESULTS

A few of the systolic, mean, and diastolic blood-pressure-variability parameters were significantly altered during acute relative hyperglycemia when compared to the fasting state. However, no significant changes were observed in any of the heart-rate-variability parameters. Also, novel significant correlations were found among many of the parameters of cardiovascular autonomic variabilities during fasting and relative hyperglycemic states.

CONCLUSIONS

The blood pressure variability is affected significantly during acute relative hyperglycemia in healthy young adults; however, the heart rate variability does not show such changes. Also, many blood pressure variability parameters show significant correlations with heart rate variability and baroreflex sensitivity. It may be hypothesized that although the variabilities in heart rate and blood pressure assess cardiovascular autonomic functions, blood pressure variability is a better indicator of cardiovascular autonomic effects of acute relative hyperglycemia.

Cyclooxygenase products contribute to the exaggerated exercise pressor reflex evoked by static muscle contraction in male UCD-type 2 diabetes mellitus rats

Cyclooxygenase (COX) products of arachidonic acid metabolism, specifically prostaglandins, play a role in evoking and transmitting the exercise pressor reflex in health and disease. Individuals with type 2 diabetes mellitus (T2DM) have an exaggerated exercise pressor reflex; however, the mechanisms for this exaggerated reflex are not fully understood. We aimed to determine the role played by COX products in the exaggerated exercise pressor reflex in T2DM rats. The exercise pressor reflex was evoked by static muscle contraction in unanesthetized, decerebrate, male, adult University of California Davis (UCD)-T2DM (n = 8) and healthy Sprague-Dawley (n = 8) rats. Changes (Δ) in peak mean arterial pressure (MAP) and heart rate (HR) during muscle contraction were compared before and after intra-arterial injection of indomethacin (1 mg/kg) into the contracting hindlimb. Data are presented as means ± SD. Inhibition of COX activity attenuated the exaggerated peak MAP (Before: Δ32 ± 13 mmHg and After: Δ18 ± 8 mmHg; P = 0.004) and blood pressor index (BPi) (Before: Δ683 ± 324 mmHg·s and After: Δ361 ± 222 mmHg·s; P = 0.006), but not HR (Before: Δ23 ± 8 beats/min and After Δ19 ± 10 beats/min; P = 0.452) responses to muscle contraction in T2DM rats. In healthy rats, COX activity inhibition did not affect MAP, HR, or BPi responses to muscle contraction. Inhibition of COX activity significantly reduced local production of prostaglandin E2 in T2DM and healthy rats. We conclude that peripheral inhibition of COX activity attenuates the pressor response to muscle contraction in T2DM rats, suggesting that COX products partially contribute to the exaggerated exercise pressor reflex in those with T2DM.NEW & NOTEWORTHY We compared the pressor and cardioaccelerator responses to static muscle contraction before and after inhibition of cyclooxygenase (COX) activity within the contracting hindlimb in decerebrate, unanesthetized type 2 diabetic mellitus (T2DM) and healthy rats. The pressor responses to muscle contraction were attenuated after peripheral inhibition of COX activity in T2DM but not in healthy rats. We concluded that COX products partially contribute to the exaggerated pressor reflex in those with T2DM.