Evidence for glucagon-like peptide-1 receptor signaling to activate ATP-sensitive potassium channels in pancreatic beta cells

Diabetes and hypertension: Pivotal involvement of purinergic signaling

Diabetes mellitus (DM) and hypertension are highly prevalent worldwide health problems and frequently associated with severe clinical complications, such as diabetic cardiomyopathy, nephropathy, retinopathy, neuropathy, stroke, and cardiac arrhythmia, among others. Despite all existing research results and reasonable speculations, knowledge about the role of purinergic system in individuals with DM and hypertension remains restricted. Purinergic signaling accounts for a complex network of receptors and extracellular enzymes responsible for the recognition and degradation of extracellular nucleotides and adenosine. The main components of this system that will be presented in this review are: P1 and P2 receptors and the enzymatic cascade composed by CD39 (NTPDase; with ATP and ADP as a substrate), CD73 (5′-nucleotidase; with AMP as a substrate), and adenosine deaminase (ADA; with adenosine as a substrate). The purinergic system has recently emerged as a central player in several physiopathological conditions, particularly those linked to inflammatory responses such as diabetes and hypertension. Therefore, the present review focuses on changes in both purinergic P1 and P2 receptor expression as well as the activities of CD39, CD73, and ADA in diabetes and hypertension conditions. It can be postulated that the manipulation of the purinergic axis at different levels can prevent or exacerbate the insurgency and evolution of diabetes and hypertension working as a compensatory mechanism.

Glucagon-Like Peptide-1 Analog Liraglutide Attenuates Pressure-Overload Induced Cardiac Hypertrophy and Apoptosis through Activating ATP Sensitive Potassium Channels

PURPOSE

This study aimed to investigate whether inhibition of glucagon-like peptide-1 (GLP-1) on pressure overload induced cardiac hypertrophy and apoptosis is related to activation of ATP sensitive potassium (KATP) channels.

METHODS

Male SD rats were randomly divided into five groups: sham, control (abdominal aortic constriction), GLP-1 analog liraglutide (0.3 mg/kg/twice day), KATP channel blocker glibenclamide (5 mg/kg/day), and liraglutide plus glibenclamide.

RESULTS

Relative to the control on week 16, liraglutide upregulated protein and mRNA levels of KATP channel subunits Kir6.2/SUR2 and their expression in the myocardium, vascular smooth muscle, aortic endothelium, and cardiac microvasculature. Consistent with a reduction in aortic wall thickness (61.4 ± 7.6 vs. 75.0 ± 7.6 μm, p < 0.05), liraglutide enhanced maximal aortic endothelium-dependent relaxation in response to acetylcholine (71.9 ± 8.7 vs. 38.6 ± 4.8%, p < 0.05). Along with a reduction in heart to body weight ratio (2.6 ± 0.1 vs. 3.4 ± 0.4, mg/g, p < 0.05) by liraglutide, hypertrophied cardiomyocytes (371.0 ± 34.4 vs. 933.6 ± 156.6 μm², p < 0.05) and apoptotic cells (17.5 ± 8.2 vs. 44.7 ± 7.9%, p < 0.05) were reduced. Expression of anti-apoptotic protein BCL-2 and contents of myocardial ATP were augmented, and expression of cleaved-caspase 3 and levels of serum Tn-I/-T were reduced. Echocardiography and hemodynamic measurement showed that cardiac systolic function was enhanced as evidenced by increased ejection fraction (88.4 ± 4.8 vs. 73.8 ± 5.1%, p < 0.05) and left ventricular systolic pressure (105.2 ± 10.8 vs. 82.7 ± 7.9 mmHg, p < 0.05), and diastolic function was preserved as shown by a reduction of ventricular end-diastolic pressure (-3.1 ± 2.9 vs. 6.7 ± 2.8 mmHg, p < 0.05). Furthermore, left ventricular internal diameter at end-diastole (5.8 ± 0.5 vs. 7.7 ± 0.6 mm, p < 0.05) and left ventricular internal diameter at end-systole (3.0 ± 0.6 vs. 4.7 ± 0.4 mm, p < 0.05) were improved. Dietary administration of glibenclamide alone did not alter all the parameters measured but significantly blocked liraglutide-exerted cardioprotection.

CONCLUSION

Liraglutide ameliorates cardiac hypertrophy and apoptosis, potentially via activating KATP channel-mediated signaling pathway. These data suggest that liraglutide might be considered as an adjuvant therapy to treat patients with heart failure.

Direct and indirect effects of liraglutide on hypothalamic POMC and NPY/AgRP neurons - Implications for energy balance and glucose control

OBJECTIVE

The long-acting glucagon-like peptide-1 receptor (GLP-1R) agonist, liraglutide, stimulates insulin secretion and efficiently suppresses food intake to reduce body weight. As such, liraglutide is growing in popularity in the treatment of diabetes and chronic weight management. Within the brain, liraglutide has been shown to alter the activity of hypothalamic proopiomelanocortin (POMC) and Neuropeptide Y/Agouti-related peptide (NPY/AgRP) neurons. Moreover, the acute activities of POMC and NPY neurons have been directly linked to feeding behavior, body weight, and glucose metabolism. Despite the increased usage of liraglutide and other GLP-1 analogues as diabetic and obesity interventions, the cellular mechanisms by which liraglutide alters the activity of metabolically relevant neuronal populations are poorly understood.

METHODS

In order to resolve this issue, we utilized neuron-specific transgenic mouse models to identify POMC and NPY neurons for patch-clamp electrophysiology experiments.

RESULTS

We found that liraglutide directly activated arcuate POMC neurons via TrpC5 channels, sharing a similar mechanistic pathway to the adipose-derived peptide leptin. Liraglutide also indirectly increases excitatory tone to POMC neurons. In contrast, liraglutide inhibited NPY/AgRP neurons through post-synaptic GABAA receptors and enhanced activity of pre-synaptic GABAergic neurons, which required both TrpC5 subunits and K-ATP channels. In support of an additive role of leptin and liraglutide in suppressing food intake, leptin potentiated the acute effects of liraglutide to activate POMC neurons. TrpC5 subunits in POMC neurons were also required for the intact pharmacological effects of liraglutide on food intake and body weight. Thus, the current study adds to recent work from our group and others, which highlight potential mechanisms to amplify the effects of GLP-1 agonists in vivo. Moreover, these data highlight multiple sites of action (both pre- and post-synaptic) for GLP-1 agonists on this circuit.

CONCLUSIONS

Taken together, our results identify critical molecular mechanisms linking GLP-1 analogues in arcuate POMC and NPY/AgRP neurons with metabolism.

Stevia Nonsweetener Fraction Displays an Insulinotropic Effect Involving Neurotransmission in Pancreatic Islets

Stevia rebaudiana (Bert.) Bertoni besides being a source of noncaloric sweeteners is also an important source of bioactive molecules. Many plant extracts, mostly obtained with ethyl acetate solvent, are rich in polyphenol compounds that present insulinotropic effects. To investigate whether the nonsweetener fraction, which is rich in phenolic compounds isolated from Stevia rebaudiana with the solvent ethyl acetate (EAF), has an insulinotropic effect, including interference at the terminals of the autonomic nervous system of the pancreatic islets of rats. Pancreatic islets were isolated from Wistar rats and incubated with EAF and inhibitory or stimulatory substances of insulin secretion, including cholinergic and adrenergic agonists and antagonists. EAF potentiates glucose-stimulated insulin secretion (GSIS) only in the presence of high glucose and calcium-dependent concentrations. EAF increased muscarinic insulinotropic effects in pancreatic islets, interfering with the muscarinic receptor subfamily M3. Adrenergic inhibitory effects on GSIS were attenuated in the presence of EAF, which interfered with the adrenergic α2 receptor. Results suggest that EAF isolated from stevia leaves is a potential therapy for treating type 2 diabetes mellitus by stimulating insulin secretion only in high glucose concentrations, enhancing parasympathetic signal transduction and inhibiting sympathetic signal transduction in beta cells.

Influence of Flavonoids on Mechanism of Modulation of Insulin Secretion

BACKGROUND

The development of alternatives for insulin secretion control in vivo or in vitro represents an important aspect to be investigated. In this direction, natural products have been progressively explored with this aim. In particular, flavonoids are potential candidates to act as insulin secretagogue.

OBJECTIVE

To study the influence of flavonoid on overall modulation mechanisms of insulin secretion.

METHODS

The research was conducted in the following databases and platforms: PubMed, Scopus, ISI Web of Knowledge, SciELO, LILACS, and ScienceDirect, and the MeSH terms used for the search were flavonoids, flavones, islets of Langerhans, and insulin-secreting cells.

RESULTS

Twelve articles were included and represent the basis of discussion on mechanisms of insulin secretion of flavonoids. Papers in ISI Web of Knowledge were in number of 1, Scopus 44, PubMed 264, ScienceDirect 511, and no papers from LILACS and SciELO databases.

CONCLUSION

According to the literature, the majority of flavonoid subclasses can modulate insulin secretion through several pathways, in an indication that corresponding molecule is a potential candidate for active materials to be applied in the treatment of diabetes.

SUMMARY

The action of natural products on insulin secretion represents an important investigation topic due to their importance in the diabetes controlIn addition to their typical antioxidant properties, flavonoids contribute to the insulin secretionThe modulation of insulin secretion is induced by flavonoids according to different mechanisms. Abbreviations used: KATP channels: ATP-sensitive K+ channels, GLUT4: Glucose transporter 4, ERK1/2: Extracellular signal-regulated protein kinases 1 and 2, L-VDCCs: L-type voltage-dependent Ca+2 channels, GLUT1: Glucose transporter 1, AMPK: Adenosine monophosphate-activated protein kinase, PTP1B: Protein tyrosine phosphatase 1B, GLUT2: Glucose transporter 2, cAMP: Cyclic adenosine monophosphate, PKA: Protein kinase A, PTK: Protein tyrosine kinase, CaMK II: Ca2+/calmodulin-dependent protein kinase II, GSIS: Glucose-stimulated insulin secretion, Insig-1: Insulin-induced gene 1, IRS-2: Insulin receptor substrate 2, PDX-1: Pancreatic and duodenal homeobox 1, SREBP-1c: Sterol regulatory element binding protein-1c, DMC: Dihydroxy-6'-methoxy-3',5'-dimethylchalcone, GLP-1: Glucagon-like peptide-1, GLP-1R: Glucagon-like peptide 1 receptor.

Glucose and GTP-binding protein-coupled receptor cooperatively regulate transient receptor potential-channels to stimulate insulin secretion [Review]

In pancreatic β-cells, glucose-induced closure of the ATP-sensitive K⁺ (KATP) channel is an initial process triggering glucose-stimulated insulin secretion (GSIS). This KATP-channel dependent pathway has been believed to be a central mechanism for GSIS. However, since the resting membrane potential of cells is determined by the balance of the net result of current amplitudes in outward and inward directions, it must be taken into consideration that not only KATP channel inhibition but also inward current via the basal opening of non-selective cation channels (NSCCs) plays a crucial role in membrane potential regulation. The basal activity of NSCCs is essential to effectively evoke depolarization in concert with KATP channel closure that is dependent on glucose metabolism. The present study summarizes recent findings regarding the roles of NSCCs in GSIS and GTP-binding protein coupled receptor-(GPCR) operated potentiation of GSIS.

Cytomembrane ATP-sensitive K+ channels in neurovascular unit targets of ischemic stroke in the recovery period

The present study was to analyze the mechanism of cytomembrane ATP-sensitive K⁺ channels (K_ATP) in the neurovascular unit treatment of ischemic stroke in the recovery period. A total of 24 healthy adult male Wistar rats of 5–8 weeks age, weighing 160–200 g were randomly divided into the control (sham-operation group), model, K_ATP blocker and K_ATP opener groups (n=6 rats per group). Nylon cerebral artery occlusion was conducted using nylon monofilament coated with Poly-L-lysine, which was used to produce a cerebral infarction model. After feeding normally for 3 days, 5-hydroxydecanoate (40 mg/Kg), and diazoxide (40 mg/Kg) were injected to the abdominal cavity in the blocker, and opener groups, respectively. The control received an equivalent normal saline that was injected into the sham-operation and model groups. The animals were mutilated and samples were collected after 3 days. RT-PCR was used to detect the expression levels of the three subunits of K_ATP, i.e., kir6.1, and sulfonylurea receptor (SUR) 1 and SUR2 mRNA, as well as to calculate infarct size in tetrazolium chloride staining. The expression level of mRNA in the opener group were significantly higher, followed by the model and blocker groups, with the control group being the lowest (P<0.05). Infarct size in the opener group was markedly smaller than the model and blocker groups, and infarct size in the blocker group was significantly larger (P<0.05). Thus, the target treatment on K_ATP may improve the prognosis of ischemic stroke during the recovery period.