The Glycemic Integral Controller

Frederick Banting, Charles Best, and the Pancreatic Bihormonal Loop in Toronto

At any moment, the entire five-liter blood volume of an adult human contains just four grams of dissolved free glucose. That single level teaspoon of sugar is held at a concentration near $90\text{ mg/dL}$ ($5.0\text{ mmol/L}$). Yet the adult brain burns 120 grams of glucose every twenty-four hours and cannot oxidize circulating fatty acids. If arterial glucose falls below $45\text{ mg/dL}$ ($2.5\text{ mmol/L}$) for even thirty minutes, cortical neurons starve for ATP, triggering seizures and hypoglycemic coma. Conversely, if blood glucose remains chronically above $180\text{ mg/dL}$ ($10\text{ mmol/L}$), glucose molecules covalently glycate vascular proteins. Excess sugar also exceeds the renal sodium-glucose cotransporter ($\text{SGLT2}$) threshold and drags liters of osmotic water into the urine.

During the summer of 1921 at the University of Toronto, orthopedic surgeon Frederick Banting and medical student Charles Best worked inside an attic laboratory granted by physiology professor John James Rickard Macleod. Their goal was to isolate the missing feedback hormone of this loop. In 1889 in Strasbourg, Oskar Minkowski and Joseph von Mering had removed the pancreas from a dog and observed severe diabetes mellitus. Over the next three decades, dozens of researchers tried and failed to extract the internal secretion of the pancreatic islets of Langerhans. Every attempt failed because digestive trypsin enzymes from the surrounding exocrine pancreas destroyed the hormone during grinding. Reading a pathology report on pancreatic stones, Banting ligated the pancreatic ducts of dogs for seven weeks until the trypsin-producing acinar cells degenerated, leaving the microscopic endocrine islets intact.

On July 30, 1921, Banting and Best injected a saline extract of those degenerated islets into a depancreatized diabetic dog named Marjorie, dropping her blood glucose by forty percent within two hours. Biochemist James Collip then purified the peptide hormone using fractional alcohol precipitation in December 1921. Clinical injections into a juvenile diabetic patient named Leonard Thompson at Toronto General Hospital in January 1922 cleared his ketonuria and reduced his blood sugar from $520\text{ mg/dL}$ to $120\text{ mg/dL}$.

Within the intact human pancreas, roughly one million islets of Langerhans function as self-contained bihormonal micro-controllers, pairing insulin-secreting beta cells against glucagon-secreting alpha cells in a push-pull feedback loop:

• * Glucose-Sensing Transduction (Glucokinase): Unlike skeletal muscle hexokinase, which saturates at sub-millimolar glucose levels, pancreatic beta cells express glucokinase (hexokinase IV), an enzyme with a low binding affinity ($K_m \approx 8\text{ mmol/L}$) and positive cooperativity right across the physiological $4 \text{ to } 10\text{ mmol/L}$ blood range. Thus, glycolytic ATP production inside the beta cell scales linearly with arterial blood sugar.
• * The $K_{\text{ATP}}$ Electrical Trigger: As the intracellular $\text{ATP/ADP}$ ratio rises after a carbohydrate meal, ATP binds to and closes ATP-sensitive potassium channels ($\text{Kir6.2/SUR1}$) in the beta-cell membrane. Blocking potassium efflux depolarizes the cell, opening voltage-gated L-type calcium channels and driving $\text{Ca}^{2+}$-triggered exocytosis of insulin granules into the portal vein.
• * Counter-Regulatory Glucagon Clamp: When fasting blood sugar dips below $80\text{ mg/dL}$, rising beta-cell inhibition and falling intra-islet paracrine zinc/GABA signals disinhibit neighboring alpha cells. Secreted glucagon binds to hepatic G-protein receptors, activating glycogen phosphorylase and gluconeogenesis so the liver releases glucose at the exact milligram-per-minute rate consumed by the brain.

In 1979, biomedical engineer Richard Bergman and physiologist Claudio Cobelli formalized this feedback dynamics in the Minimal Model of glucose kinetics:

Here, $G(t)$ is plasma glucose concentration above the basal setpoint $G_b$, $S_G$ is glucose effectiveness, and $X(t)$ is remote interstitial insulin action. The pancreatic islets pair fast first-phase insulin granule release (proportional-derivative control) with sustained second-phase synthesis (integral control). Through this combined controller, the pancreas absorbs a 100-gram oral glucose load—twenty-five times the total free sugar circulating in the blood. Within ninety minutes, arterial plasma glucose returns to its $90\text{ mg/dL}$ baseline with zero steady-state error.