The Hormone That Discovered the Second Messenger: Glucagon, cAMP and an Unexpected Place in Triple-Agonist Research

If you were designing a compound to study weight and metabolism, glucagon would seem like an odd ingredient. It is, after all, the hormone that raises blood sugar. It is the counterpart of insulin. Yet the newest multi-receptor peptides in the research literature include glucagon receptor activity by design. To see why, it helps to go back to how glucagon was found, and how it opened an entire chapter of cell biology along the way.

A contaminant in the insulin vials

The story begins in the early 1920s, right after the isolation of insulin by Banting and Best. Researchers were preparing insulin extracts from animal pancreas and testing them on animals. Some batches caused an unexpected rise in blood sugar before the expected fall. In 1923, Charles Kimball and John Murlin at the University of Rochester investigated and concluded that the pancreatic extract contained a second, hyperglycemic substance. They named it glucagon, a contraction of “glucose agonist.”

For decades afterward, glucagon was a laboratory curiosity. Its structure, a single chain of 29 amino acids, was worked out in the 1950s, and a reliable assay for it was developed in the 1960s, notably by Roger Unger, who showed that it is secreted by the alpha cells of the pancreatic islets and rises in fasting and in diabetes.

The detour that changed biology

Here the glucagon story branches into one of the great episodes of biochemistry. In the 1950s, Earl Sutherland and his colleagues at Case Western Reserve University were investigating how the hormones epinephrine and glucagon cause the liver to release glucose. They knew that both hormones activate an enzyme called phosphorylase, which breaks down stored glycogen. But when they added the hormones to broken liver cells, the activation disappeared. The effect required intact cells.

Working through that puzzle, Sutherland and Theodore Rall found that when hormone-treated cell membranes were exposed to ATP, they produced a heat-stable small molecule that, when added to a broken-cell preparation, did activate phosphorylase. In 1958, they identified it as cyclic adenosine monophosphate, cyclic AMP. The hormone, they proposed, was a “first messenger” that did not enter the cell; instead it triggered the production of cAMP inside, a “second messenger” that carried the signal onward.

The concept of the second messenger transformed the field. It explained how a hormone at a membrane surface could change enzyme activity in the cytoplasm, how signals could be amplified, and how different hormones could converge on shared machinery. Sutherland received the 1971 Nobel Prize in Physiology or Medicine. And glucagon, the hormone that raised blood sugar, had been one of the two tools that made the discovery possible. The receptor system that ties hormone binding to cAMP production later turned out to be the family of G protein-coupled receptors, one of the most important classes of drug targets in medicine.

The bihormonal idea

In the 1970s, Roger Unger articulated what became known as the bihormonal hypothesis of diabetes: that the disease reflects not only a lack of insulin but an excess of glucagon. In patients with diabetes, glucagon fails to fall after meals, contributing to high blood sugar. The idea reframed glucagon as a target for suppression, and spurred efforts to develop glucagon receptor antagonists.

That is the point at which the plot seems to point the other way from triple agonists. But glucagon has other effects, and they became harder to ignore.

What else glucagon does

Studies in rodents and humans, accumulating over decades, show that glucagon receptor activation:

• Increases energy expenditure, apparently by raising metabolic rate and, in animal studies, by engaging brown adipose tissue.
• Reduces food intake, with a satiety effect observed in several experimental settings.
• Stimulates fat oxidation in the liver, reducing liver fat accumulation in animal models.
• Lowers circulating amino acids and lipids, through effects on hepatic metabolism.

In other words, the hormone that tells the body to release stored fuel also drives pathways that could, in principle, favor negative energy balance.

The design logic behind combining receptors

If glucagon receptor activation has desirable metabolic effects but also raises blood glucose, then a molecule that combined it with activation of the GLP-1 receptor and the GIP receptor, both of which promote insulin secretion in a glucose-dependent way, could in theory offset the hyperglycemic effect while retaining the energy-expenditure effect. This is the rationale behind the triple agonist concept.

The 2009 paper from Richard DiMarchi and Matthias Tschöp, who showed in rodents that a glucagon/GLP-1 co-agonist could outperform a single GLP-1 agonist, was an early proof of principle. Retatrutide, developed by Eli Lilly and discussed in the research literature as a single peptide with activity at the GIP, GLP-1 and glucagon receptors, extends that logic to three receptors. Results from its published phase 2 program have drawn attention. Those are clinical findings with a specific pharmaceutical product, and they say nothing about what a research-grade reagent might do in any other setting.

What remains unknown

• The relative contribution of each receptor in a triple agonist is hard to dissect, because comparisons between different molecules differ in more than their receptor profile.
• Glucagon receptor effects on the liver, heart and kidney need long-term assessment, and the field is cautious about off-target consequences.
• Receptor pharmacology details, such as the degree of bias and the potency ratios at each receptor, differ between compounds and influence outcomes.

A lesson in context

Glucagon illustrates a recurring theme in physiology: a molecule’s “role” depends on which tissue and which timescale you look at. Calling it the “opposite of insulin” is accurate for blood glucose and misleading for energy balance.

For researchers working on multi-receptor peptide pharmacology, specifications and batch documentation are on the GLP3RT (Retatrutide) 20mg product page.

All Lucid Aminos products are sold strictly for laboratory research use. They are not drugs, dietary supplements or food, and they are not for human or veterinary use. This article is a historical and educational account of published research and is not medical advice.