Physiological aspects of the combination of insulin and GLP-1 in the regulation of blood glucose control

The influence of insulin on diabetic retinopathy and retinal vessel parameters in diabetes

AIM

To investigate the associations between insulin use and diabetic retinopathy (DR), and retinal vascular parameters in type 2 diabetes (T2DM).

METHODS

A total of 6,374 T2DM patients, consisting of 2,231 patients receiving insulin alone and 4143 patients without any hypoglycemic medication, were included in cross-sectional analyses. Among those without DR at baseline, 791 patients were followed for three years in longitudinal analyses. Fundus photography was taken to diagnose DR and calculate central retinal arteriolar equivalent (CRAE), central retinal venular equivalent (CRVE), arteriolar-to-venular ratio (AVR), and vascular tortuosity. Inverse probability treatment-weighted analyses were performed.

RESULTS

After adjusting for gender, age, body mass index, blood pressure, blood glucose, T2DM duration, smoking, and alcohol use, insulin users showed a higher risk of DR (odds ratio (OR) = 2.27, 95% confidence interval (95%CI) = 2.08-2.48, P < 0.001), larger CRVE (β = 3.92, 95%CI = 2.46-5.37, P < 0.001), smaller AVR (β=-0.0083, 95%CI=-0.0121- -0.0046, P < 0.001), and larger vascular curvature (β = 0.19, 95%CI = 0.05-0.33, P = 0.008). After 3 years, insulin users had a higher risk of developing DR (OR = 1.94; 95% CI = 1.37-2.73, P = 0.002), and greater change in CRVE (β = 3.92, 95%CI = 0.96-6.88, P = 0.009).

CONCLUSIONS

The impact of insulin on the retinal microvasculature provides support for linking insulin to the increased risk of DR, as well as cardiovascular events in T2DM.

Glucagon-like peptide-1 receptor agonists for type 2 diabetes: A rational drug development

Today, glucagon-like peptide-1 (GLP-1) receptor agonists are established glucose-lowering drugs used in the management of type 2 diabetes. Their development emerged from the understanding that a combined islet dysfunction comprising of impaired insulin secretion and exaggerated glucagon secretion is the key defect of hyperglycemia. GLP-1 was shown to target these defects, and after the discovery that dipeptidyl peptidase-4 inactivates native GLP-1, several different dipeptidyl peptidase-4-resistant GLP-1 receptor agonists have been developed. They are administered subcutaneously, but show differences in molecular structure, molecular size and pharmacokinetics, the latter allowing twice-daily, once-daily or once-weekly administration. They have been shown to be efficient in reducing both glycated hemoglobin and bodyweight, and to be safe and highly tolerable. Cardiovascular outcomes trials have shown them to be neutral or beneficial. GLP-1 receptor agonists are positioned as add-ons to metformin alone or in combination with oral agents in the clinical paradigm. They are also efficient when combined with insulin, and fixed dose combinations with long-acting insulin have been developed. Recent development includes a very long administration schedule and oral availability. The research from the first demonstration of the antidiabetic action of GLP-1 in the early 1990s to the enormously accumulated data today represents a successful and rational development, which has been characterized by focused perseverance to establish this therapy in the management of type 2 diabetes.

Bidirectional Relationship between Gastric Emptying and Plasma Glucose Control in Normoglycemic Individuals and Diabetic Patients

Gastric emptying and glycemic control pathways are closely interrelated processes. Gastric chyme is transferred into the duodenum with velocities depending on its solid or liquid state, as well as on its caloric and nutritional composition. Once nutrients enter the intestine, the secretion of incretins (hormonal products of intestinal cells) is stimulated. Among incretins, glucagon-like peptide-1 (GLP-1) has multiple glycemic-regulatory effects that include delayed gastric emptying, thus triggering a feedback loop lowering postprandial serum glucose levels. Glycemic values also influence gastric emptying; hyperglycemia slows it down, and hypoglycemia accelerates it, both limiting glycemic fluctuations. Disordered gastric emptying in diabetes mellitus is understood today as a complex pathophysiological condition, with both irreversible and reversible components and high intra- and interindividual variability of time span and clinical features. While limited delays may be useful for reducing postprandial hyperglycemias, severely hindered gastric emptying may be associated with higher glycemic variability and worsened long-term glycemic control. Therapeutic approaches for both gastric emptying and glycemic control include dietary modifications of meal structure or content and drugs acting as GLP-1 receptor agonists. In the foreseeable future, we will probably witness a wider range of dietary interventions and more incretin-based medications used for restoring both gastric emptying and glycemic levels to nearly physiological levels.

Glucagon-like peptide-1 and insulin synergistically activate vagal afferent neurons

Intestinal glucagon-like peptide-1 (GLP-1) and pancreatic insulin, released postprandially, commonly regulate glucose metabolism. Recent clinical experience indicates that the GLP-1R agonist and insulin in combination, compared to insulin alone, results in better glycemic and weight controls in type 2 diabetic patients. These observations suggest possible interactive effect of these hormones. These hormones, in addition to peripherally controlling glycemia, exert central regulation of food intake and glucose metabolism, the effect at least partly mediated by signaling to the brain via the vagal afferents. However, whether the vagal afferents are involved in the interactive effects of GLP-1 and insulin remains unknown. The present study explored possible cooperative effect of GLP-1 and insulin on vagal afferent neurons isolated from nodose ganglion (NG) of mice, while monitoring the neuronal activity by measuring cytosolic Ca²⁺ concentration ([Ca²⁺]_i) with fura-2. GLP-1 at 10^-8M increased [Ca²⁺]_i in 8-11% of single NG neurons. GLP-1-induced [Ca²⁺]_i increases were inhibited by GLP-1 receptor antagonist exendin (9-39). Majority (92%) of GLP-1-responseive NG neurons also responded to 10^-7M insulin with [Ca²⁺]_i increases. Both GLP-1 and insulin at lower concentration of 10^-9M induced [Ca²⁺]_i increases with smaller amplitude in lesser NG neuron population (4-7%). These hormones at 10^-9M in combination recruited the unresponsive neurons to [Ca²⁺]_i increases, and induced [Ca²⁺]_i increases with greater amplitude in the responsive neurons. The results demonstrate that GLP-1 and insulin synergistically and additively activate vagal afferent neurons. This interaction may be linked to the postprandial functions mediated commonly by GLP-1 and insulin and in the beneficial outcome of the therapy with GLP-1 receptor agonist and insulin in combination.

Effect of glucagon-like peptide-1 on major cardiovascular outcomes in patients with type 2 diabetes mellitus: A meta-analysis of randomized controlled trials

The effect of glucagon-like peptide-1 (GLP-1) treatment in patients with type 2 diabetes mellitus (T2DM) remains controversial. The purpose of this study was to compare the effect of GLP-1 and placebo/conventional antidiabetic agents on cardiovascular risk in T2DM patients. PubMed, EmBase and the Cochrane Library were searched to identify its eligible studies as well as manual searches for the reliability of this study. All eligible trials were performed in T2DM patients who received GLP-1 therapy or placebo/conventional antidiabetic agents. The reported outcomes included major cardiovascular events (MACE), and total mortality. Of 490 identified studies, we included 13 trials reporting data on 11,943 T2DM patients. Overall, the pooled results suggested that GLP-1 therapy has no or little effect on MACE (RR: 0.99; 95% CI: 0.88-1.12; P=0.872) and total mortality (RR: 0.90; 95% CI: 0.70-1.15; P=0.399). Furthermore, sensitivity analysis indicated that GLP-1 was associated with lower incidence of total mortality (RR: 0.28; 95% CI: 0.08-0.93; P=0.037). We concluded that GLP-1 therapy was not associated with MACE and total mortality compared with placebo or antidiabetic agents.