Glucagon-Like Peptide-1 (GLP-1): Full Medical Overview

Biochemistry and Molecular Structure

GLP-1 is encoded by the proglucagon gene (GCG) located on chromosome 2. Proglucagon is processed differently in different tissues:

• Pancreatic alpha cells → glucagon (the blood sugar-raising hormone)
• Intestinal L-cells → GLP-1, GLP-2, glicentin, and oxyntomodulin

GLP-1 exists in two active forms: GLP-1(7-37) and GLP-1(7-36) amide. The latter is the predominant circulating form. Both activate the GLP-1 receptor (GLP1R), a class B G-protein-coupled receptor.

Under physiological conditions, plasma GLP-1 concentrations rise from approximately 5–10 pmol/L in the fasted state to 15–50 pmol/L postprandially — a modest increase that is rapidly terminated by degradation via dipeptidyl peptidase-4 (DPP-4). Plasma half-life is approximately 1–2 minutes.

Physiological Actions

GLP-1 receptors are expressed throughout the body, including in the pancreas, brain, heart, kidney, lung, and gastrointestinal tract. The primary physiological effects include:

Pancreatic effects:

• Stimulates insulin secretion from beta cells in a glucose-dependent manner
• Suppresses glucagon secretion from alpha cells
• Promotes beta-cell proliferation and reduces beta-cell apoptosis in animal models

Gastrointestinal effects:

• Slows gastric emptying (enterogastric reflex)
• Inhibits gastric acid secretion
• Reduces intestinal motility

Central nervous system effects:

• Activates satiety-signaling neurons in the hypothalamic arcuate nucleus
• Reduces activation of hunger-promoting AgRP/NPY neurons
• Modulates dopamine signaling in the reward circuit

Cardiovascular effects:

• Improves heart rate variability
• Modest positive inotropic and chronotropic effects
• Cardioprotective effects observed in clinical trials (SELECT 2023)

From Natural Hormone to Drug Class

The therapeutic potential of GLP-1 was first recognized in the 1980s when researchers noted that intravenous GLP-1 infusion produced potent insulin secretion in patients with type 2 diabetes. The challenge was its 2-minute half-life, which made it impractical as a drug.

Two pharmacological strategies were developed to overcome this:

1. DPP-4 inhibitors — block the enzyme that degrades GLP-1, modestly raising endogenous levels. These include sitagliptin (Januvia), saxagliptin, and alogliptin.

2. GLP-1 receptor agonists — synthetic peptides engineered to resist DPP-4 and bind the GLP-1 receptor with equal or superior potency to native GLP-1. These include exenatide, liraglutide, semaglutide, and tirzepatide (dual GLP-1/GIP).

GLP-1 receptor agonists are significantly more effective for weight loss because they produce sustained, pharmacologic-level receptor activation — not just a modest increase in natural GLP-1.

Approved GLP-1 Receptor Agonists

Clinical Applications Beyond Weight and Diabetes

Emerging evidence suggests GLP-1 receptor agonists may have therapeutic applications in:

• Cardiovascular disease — SELECT trial (2023) demonstrated 20% reduction in MACE in people with obesity and established CVD
• Non-alcoholic steatohepatitis (NASH) — semaglutide trials show significant hepatic fat reduction
• Addiction — preliminary evidence for reduction in alcohol, nicotine, and opioid use via reward circuit modulation
• Alzheimer's disease — GLP-1 receptors in the brain; trials underway (semaglutide in the EVOKE trial)
• Obstructive sleep apnea — tirzepatide showed 24–25% reduction in apnea-hypopnea index in SURMOUNT-OSA

Bottom Line

Glucagon-like peptide-1 is a multi-system hormone with broad physiological roles in metabolism, appetite, and cardiovascular function. The development of long-acting GLP-1 receptor agonists represents one of the most significant therapeutic advances in endocrinology and obesity medicine in decades.

Sources

• Holst JJ, "The physiology of glucagon-like peptide 1," Physiological Reviews, 2007
• Drucker DJ, "Mechanisms of Action and Therapeutic Application of Glucagon-like Peptide-1," Cell Metabolism, 2018
• Lincoff AM et al., "Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes," NEJM, 2023