Glucose homeostasis: insulin and glucagon action, the fed versus fasted state, and diabetes mechanisms

Intuition Beginner

Blood glucose must stay within a narrow range (about 70--110 mg/dL). After a meal, the pancreas releases insulin, which helps cells take in glucose and stores the excess as glycogen and fat. Between meals, the pancreas releases glucagon, which signals the liver to break down stored glycogen and release glucose back into the blood.

Diabetes occurs when this system fails. In type 1 diabetes, the pancreas cannot produce insulin because immune cells have destroyed the beta cells. In type 2 diabetes, cells stop responding properly to insulin (insulin resistance), and eventually the pancreas cannot keep up.

Visual Beginner

The pancreas contains clusters of cells called islets of Langerhans. Within each islet, beta cells produce insulin and alpha cells produce glucagon. These two cell types sense blood glucose directly and adjust their hormone secretion in opposite directions: beta cells increase insulin release when glucose rises; alpha cells increase glucagon release when glucose falls.

Worked example Beginner

Trace what happens to blood glucose after eating a meal:

1. Digestion breaks carbohydrates down into glucose, which is absorbed into the blood.
2. Blood glucose rises above the resting level.
3. Pancreatic beta cells detect the rise and release insulin into the blood.
4. Insulin acts on muscle, liver, and fat cells, causing them to take up glucose.
5. The liver stores excess glucose as glycogen; fat cells store it as triglycerides.
6. Blood glucose falls back toward the normal range.
7. Beta cells detect the fall and reduce insulin secretion.

Now trace what happens between meals:

1. Blood glucose begins to fall as cells consume glucose for energy.
2. Pancreatic alpha cells detect the fall and release glucagon.
3. Glucagon acts on the liver to break glycogen back down into glucose.
4. The liver releases glucose into the blood.
5. Blood glucose rises back toward the normal range.

Check your understanding Beginner

Formal definition Intermediate+

Pancreatic islet anatomy and cell types

The islets of Langerhans constitute approximately 1--2% of pancreatic mass but receive 10--15% of pancreatic blood flow, reflecting their high metabolic activity. Each islet contains four principal endocrine cell types:

• Beta cells (60--70% of islet cells): produce insulin and C-peptide. Beta cells function as glucose sensors, coupling blood glucose concentration directly to insulin secretion rate through a metabolic-to-electrical transduction mechanism.
• Alpha cells (25--30%): produce glucagon. Alpha cells are also glucose-sensitive but respond in the opposite direction: low glucose stimulates glucagon release.
• Delta cells (5--10%): produce somatostatin, which has paracrine effects within the islet, inhibiting both insulin and glucagon secretion.
• PP cells (F cells) (trace): produce pancreatic polypeptide, which regulates exocrine pancreatic secretion and gastrointestinal motility.

The spatial arrangement within the islet is functionally significant. Beta cells occupy the core of the islet, while alpha and delta cells form a mantle around the periphery. Blood flows from the core outward, so that insulin-rich blood reaches the alpha cells before entering the systemic circulation. This intra-islet portal system allows insulin to exert a paracrine inhibitory effect on alpha-cell glucagon secretion -- a regulatory relationship that becomes pathological in diabetes, where alpha cells lose their suppression and secrete inappropriately high glucagon despite hyperglycaemia.

The beta cell as a glucose sensor

Beta cells convert blood glucose concentration into insulin secretion through a well-characterised sequence:

1. Glucose enters the beta cell via GLUT2 transporters (non-insulin-dependent, high-capacity, Km approximately 15--20 mM).
2. Intracellular glucose is metabolised through glycolysis and oxidative phosphorylation, increasing the intracellular ATP-to-ADP ratio.
3. The rise in ATP closes ATP-sensitive potassium channels (K-ATP channels) on the beta-cell membrane.
4. K-ATP channel closure depolarises the beta cell (membrane potential shifts from approximately --70 mV toward --40 mV).
5. Depolarisation opens voltage-gated calcium channels, allowing calcium influx.
6. The rise in intracellular calcium triggers exocytosis of insulin-containing secretory granules.

This mechanism makes insulin secretion proportional to glucose concentration over the physiological range (approximately 5--25 mM). The K-ATP channel is the pharmacological target of sulfonylurea drugs (e.g., glibenclamide, glipizide), which bind the SUR1 subunit and force channel closure, stimulating insulin secretion regardless of glucose level.

Beta cells also release insulin in a biphasic pattern in response to a glucose stimulus. The first phase (peaking within 2--5 minutes) represents release of pre-docked granules and is lost early in type 2 diabetes. The second phase (sustained release over 30--60 minutes) requires mobilisation and priming of reserve granules.

Insulin signalling cascade

Insulin signals through the insulin receptor, a receptor tyrosine kinase (RTK) composed of two extracellular alpha subunits (hormone binding) and two transmembrane beta subunits (kinase activity). The signalling cascade proceeds through two main branches:

The PI3K/Akt pathway (metabolic effects):

1. Insulin binds the alpha subunits, inducing a conformational change that activates the beta-subunit kinase domains.
2. The activated receptor autophosphorylates tyrosine residues on the beta subunits, creating docking sites.
3. IRS (insulin receptor substrate) proteins, particularly IRS-1 and IRS-2, bind to the phosphotyrosine sites and are themselves phosphorylated on multiple tyrosine residues.
4. Phosphorylated IRS recruits PI3K (phosphatidylinositol 3-kinase), which converts PIP2 to PIP3.
5. PIP3 recruits and activates PDK1, which phosphorylates and activates Akt (protein kinase B).
6. Akt mediates the downstream metabolic effects of insulin:
   • GLUT4 translocation: Akt promotes translocation of GLUT4-containing vesicles from intracellular storage compartments to the plasma membrane, enabling insulin-dependent glucose uptake into muscle and adipose tissue.
   • Glycogen synthesis: Akt inhibits GSK-3 (glycogen synthase kinase-3), relieving inhibition of glycogen synthase and promoting glycogen storage.
   • Protein synthesis: Akt activates mTOR signalling, stimulating translation and cell growth.
   • Lipogenesis: Akt activates SREBP-1c (sterol regulatory element-binding protein), upregulating fatty acid and triglyceride synthesis genes in the liver.
   • Inhibition of lipolysis: Akt phosphorylates and inactivates hormone-sensitive lipase in adipose tissue, reducing triglyceride breakdown and free fatty acid release.

The MAPK/ERK pathway (mitogenic effects):

The insulin receptor also recruits Grb2-SOS, activating Ras, which initiates the Raf-MEK-ERK kinase cascade. This pathway mediates the growth-promoting and mitogenic effects of insulin and IGF-1, including gene expression changes and cell proliferation.

The clinical significance of the two-branch architecture is that insulin resistance in type 2 diabetes selectively impairs the PI3K/Akt pathway while the MAPK pathway remains relatively intact. The consequence is simultaneous hyperglycaemia (loss of metabolic signalling) and accelerated atherosclerosis (persistent mitogenic signalling on vascular smooth muscle and endothelial cells).

Glucagon signalling

Glucagon acts through a G-protein-coupled receptor (GPCR) on hepatocytes, coupling to G_s and activating the cAMP-PKA cascade:

1. Glucagon binds its receptor, activating G_s.
2. G_s stimulates adenylate cyclase, producing cAMP from ATP.
3. cAMP activates protein kinase A (PKA).
4. PKA phosphorylates key metabolic enzymes:
   • Phosphorylase kinase, which activates glycogen phosphorylase, catalysing glycogenolysis (glycogen breakdown to glucose-1-phosphate).
   • Fructose-2,6-bisphosphatase, lowering fructose-2,6-bisphosphate levels, which relieves inhibition of fructose-1,6-bisphosphatase and promotes gluconeogenesis while simultaneously inhibiting phosphofructokinase-1 (reducing glycolysis).
   • In prolonged fasting, cAMP-responsive transcription factors (CREB) upregulate phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase, increasing gluconeogenic capacity.

Glucagon also promotes ketogenesis in the liver during prolonged fasting. By increasing fatty acid oxidation (via PKA-mediated inhibition of acetyl-CoA carboxylase, which lowers malonyl-CoA and disinhibits carnitine palmitoyltransferase I), glucagon drives acetyl-CoA production from fatty acids. When carbohydrate-derived oxaloacetate is diverted to gluconeogenesis, acetyl-CoA cannot enter the TCA cycle and is instead converted to ketone bodies (acetoacetate and beta-hydroxybutyrate) in the liver mitochondria.

The fed versus fasted state

Metabolic regulation is organised around two opposing states defined by the insulin-to-glucagon ratio:

Fed state (high insulin, low glucagon):

• Liver: glycogen synthesis activated (glycogen synthase dephosphorylated by protein phosphatase-1); glycolysis and de novo lipogenesis stimulated; gluconeogenesis suppressed; beta-oxidation inhibited (acetyl-CoA carboxylase active, malonyl-CoA inhibits CPT1).
• Muscle: GLUT4 translocation enables glucose uptake; glycogen synthesis replenishes stores; amino acid uptake and protein synthesis stimulated.
• Adipose tissue: GLUT4 translocation enables glucose uptake; lipoprotein lipase (activated by insulin) hydrolyses circulating triglycerides for fatty acid uptake; hormone-sensitive lipase is phosphorylated and inactivated by insulin, suppressing lipolysis; glycerol and fatty acid release into blood is minimal.

Fasted state (low insulin, high glucagon):

• Liver: glycogenolysis activated (phosphorylase kinase and glycogen phosphorylase phosphorylated); gluconeogenesis activated (PEPCK, fructose-1,6-bisphosphatase, glucose-6-phosphatase upregulated); ketogenesis activated in prolonged fasting; beta-oxidation disinhibited (malonyl-CoA falls, CPT1 active).
• Muscle: reduced glucose uptake (GLUT4 internalised); fatty acid oxidation becomes the primary fuel; branched-chain amino acid oxidation increases.
• Adipose tissue: hormone-sensitive lipase is dephosphorylated and active (stimulated by low insulin and by catecholamines acting on beta-adrenergic receptors), releasing glycerol (a gluconeogenic substrate) and free fatty acids (a fuel for muscle and liver) into the blood.

Counterregulatory hormones

Glucagon is the primary counterregulatory hormone for acute hypoglycaemia, but several other hormones raise blood glucose through distinct mechanisms:

• Epinephrine (adrenaline): released from the adrenal medulla during sympathetic activation (the "fight or flight" response and acute hypoglycaemia). Stimulates hepatic glycogenolysis (beta-2 adrenergic receptors, cAMP), gluconeogenesis, and lipolysis in adipose tissue (beta-3 adrenergic receptors). Epinephrine also inhibits insulin secretion via alpha-2 adrenergic receptors on beta cells. It provides the first rapid defence against hypoglycaemia when glucagon is impaired.
• Cortisol: stimulates gluconeogenesis (upregulates PEPCK and glucose-6-phosphatase transcription), promotes protein catabolism in muscle to provide amino acid substrates for gluconeogenesis, and causes insulin resistance in peripheral tissues. Cortisol acts over hours and is important for sustained defence against hypoglycaemia.
• Growth hormone (GH): induces insulin resistance in muscle and adipose tissue, reducing glucose uptake and raising blood glucose. GH also promotes lipolysis, providing fatty acids as an alternative fuel.

Incretin hormones and DPP-4

The incretin effect refers to the observation that oral glucose elicits a greater insulin response than intravenous glucose producing the same blood glucose level. This enhancement is mediated by gut-derived hormones released during nutrient ingestion:

• GLP-1 (glucagon-like peptide-1): secreted by L-cells in the distal ileum and colon in response to luminal nutrients. GLP-1 potentiates glucose-stimulated insulin secretion (the "incretin effect"), suppresses glucagon secretion, slows gastric emptying, and promotes satiety through central actions. GLP-1 has a short half-life (approximately 2 minutes) because it is rapidly degraded by DPP-4 (dipeptidyl peptidase-4).
• GIP (glucose-dependent insulinotropic polypeptide): secreted by K-cells in the duodenum and proximal jejunum. GIP potentiates insulin secretion in a glucose-dependent manner but, unlike GLP-1, does not suppress glucagon.

The incretin effect is diminished in type 2 diabetes, contributing to the impaired insulin secretion. This observation led to two major drug classes: GLP-1 receptor agonists (e.g., exenatide, liraglutide, semaglutide), which are DPP-4-resistant analogues, and DPP-4 inhibitors (e.g., sitagliptin, linagliptin), which prolong the action of endogenous GLP-1.

Key theorem with proof Intermediate+

Theorem (Minimal model of glucose-insulin dynamics). After an intravenous glucose bolus, the plasma glucose concentration $G(t)$ and insulin concentration $I(t)$ obey the coupled system

$$\frac{dG}{dt}=-(S_G+X(t))G(t)+S_G{G}_b,$$ $$\frac{dX}{dt}=p_3(I(t)-I_b)-p_{2}X(t),$$

where $X(t)$ is the insulin action variable (remote insulin effect on glucose disposal), $S_G$ is glucose effectiveness (glucose's own ability to promote its own disposal at basal insulin), $G_b$ and $I_b$ are basal glucose and insulin, $p_3$ relates plasma insulin above basal to the rate of insulin action increase, and $p_2$ is the decay rate of insulin action. The insulin sensitivity index is $S_I=p_3/p_2$.

Proof. This is the Bergman minimal model, derived by Richard Bergman and colleagues (1979) from analysis of the intravenous glucose tolerance test (IVGTT). The model is constructed by partitioning glucose disposal into two components:

1. Glucose effectiveness ($S_G$): the ability of glucose itself to promote its own net disappearance from plasma, independent of a dynamic insulin response. This represents the mass-action effect of glucose concentration on uptake and the basal suppression of hepatic output.
2. Insulin action ($X(t)$): the time-dependent effect of insulin on accelerating glucose disposal, modelled as a remote compartment because insulin's effect on glucose uptake is not instantaneous -- insulin must first activate the PI3K/Akt cascade and promote GLUT4 translocation, introducing a delay between plasma insulin and its metabolic effect.

The first equation states that the rate of change of glucose equals the net disposal due to both glucose effectiveness and insulin action, plus a return term toward basal glucose. The second equation states that insulin action rises in proportion to plasma insulin above basal and decays exponentially with rate constant $p_2$. The parameter $p_3$ scales how much each unit of insulin above basal contributes to insulin action.

At steady state ($dX/dt=0$): $X_{ss}=(p_3/p_2)(I-I_b)$. Substituting into the glucose equation at steady state ($dG/dt=0$): $G_{ss}=S_G{G}_b/(S_G+X_{ss})$. When insulin rises above basal, $X_{ss}$ increases, the effective disposal rate increases, and $G_{ss}$ falls toward $G_b$.

The insulin sensitivity index $S_I=p_3/p_2$ quantifies the steady-state insulin action per unit increment of plasma insulin above basal. In insulin-resistant states (type 2 diabetes, obesity, metabolic syndrome), $S_I$ is reduced, meaning more insulin is required to achieve the same glucose-lowering effect. $\square$

Clinical interpretation. The minimal model parameters are estimated by fitting $G(t)$ and $I(t)$ data from an IVGTT. $S_I$ is reduced in insulin resistance and is used as a research tool to quantify insulin sensitivity. The model predicts that if $S_I$ falls (insulin resistance), the steady-state insulin required to maintain normal glucose is $I_{ss}=I_b+(S_G/S_I)(G_b/G_{ss}-1)G_{ss}^{-1}$ -- higher insulin is needed, which is exactly the compensatory hyperinsulinaemia observed in early type 2 diabetes.

Exercises Intermediate+

Diabetes mellitus: mechanisms, classification, and management Master

Type 1 diabetes mellitus

Type 1 diabetes results from autoimmune destruction of pancreatic beta cells, producing absolute insulin deficiency. The disease accounts for approximately 5--10% of all diabetes cases and typically presents in childhood or adolescence, although latent autoimmune diabetes in adults (LADA) represents a slower-onset variant.

Genetic susceptibility is strongly associated with specific HLA class II alleles. The HLA-DR3-DQ2 and HLA-DR4-DQ8 haplotypes confer the highest risk, with heterozygotes carrying both haplotypes at greatest risk. These HLA molecules present beta-cell antigens to CD4+ T-cells, initiating the autoimmune cascade. Concordance in identical twins is 30--50%, indicating that environmental triggers (viral infections, particularly coxsackievirus, are hypothesised) are also required.

Autoantibodies serve as markers of the autoimmune process and can be detected years before clinical onset:

• Anti-GAD65 (glutamic acid decarboxylase): the most common antibody in adult-onset type 1 and LADA.
• Anti-IA-2 (insulinoma-associated protein 2): highly specific for type 1 diabetes.
• Anti-ZnT8 (zinc transporter 8): present in approximately 60--80% of new-onset type 1.
• Insulin autoantibodies (IAA): most common in young children at diagnosis.
• Islet cell antibodies (ICA): the original antibody described; detected by immunofluorescence.

The presence of two or more autoantibodies confers a high risk of progression to clinical diabetes within 5--10 years, although the precise trigger for progression from autoimmunity to overt beta-cell failure remains incompletely understood.

Diabetic ketoacidosis (DKA) is the acute, life-threatening complication of absolute insulin deficiency. The pathogenesis involves:

1. No insulin $\to$ glucose cannot enter muscle and adipose tissue $\to$ hyperglycaemia (often > 300 mg/dL).
2. No insulin $\to$ hormone-sensitive lipase is uninhibited $\to$ massive lipolysis $\to$ flood of free fatty acids to the liver.
3. Low insulin-to-glucagon ratio $\to$ hepatic mitochondria convert fatty-acid-derived acetyl-CoA to ketone bodies (acetoacetate, beta-hydroxybutyrate) via HMG-CoA synthase and lyase.
4. Ketone bodies are weak acids; accumulation overwhelms bicarbonate buffering $\to$ metabolic acidosis (pH < 7.3, bicarbonate < 18 mEq/L, anion gap elevated).
5. Hyperglycaemia exceeds renal threshold $\to$ osmotic diuresis $\to$ volume depletion, electrolyte loss (potassium, sodium, phosphate), and dehydration.
6. Progressive acidosis and volume depletion $\to$ cerebral depression, coma, and death if untreated.

DKA management requires simultaneous insulin replacement (continuous IV infusion), aggressive fluid resuscitation, potassium replacement (total body potassium is depleted despite normal or elevated serum potassium due to acidosis-driven extracellular shift), and monitoring for cerebral oedema (a feared complication in children, caused by too-rapid osmolar correction).

Type 2 diabetes mellitus

Type 2 diabetes is characterised by insulin resistance followed by progressive beta-cell failure. It accounts for 90--95% of diabetes cases and is strongly associated with obesity, sedentary lifestyle, and genetic predisposition (concordance in identical twins approaches 70--90%, higher than type 1).

Insulin resistance is defined as a reduced biological response to a given insulin concentration. At the cellular level, it involves:

• Receptor-level defects: reduced insulin receptor number (downregulation from chronic hyperinsulinaemia) and reduced receptor tyrosine kinase activity.
• Post-receptor defects: impaired IRS-1/IRS-2 phosphorylation (serine phosphorylation by inflammatory kinases such as JNK and IKK-beta, activated by nutrient excess and adipose tissue inflammation); reduced PI3K activation; impaired Akt signalling.
• Inflammatory mediators: visceral adipose tissue releases pro-inflammatory cytokines (TNF-alpha, IL-6, MCP-1) and resistin, which activate intracellular serine kinases that phosphorylate IRS on inhibitory serine residues, blocking insulin signalling.
• Lipotoxicity: elevated circulating free fatty acids and ectopic lipid accumulation in liver and muscle activate PKC-theta and ceramide pathways that interfere with insulin signalling.

Metabolic syndrome (insulin resistance syndrome) describes the clustering of abdominal obesity, hypertension, dyslipidaemia (elevated triglycerides, low HDL), impaired fasting glucose, and prothrombotic and pro-inflammatory states. It represents the systemic manifestation of insulin resistance and is a major risk factor for type 2 diabetes and cardiovascular disease.

Beta-cell exhaustion: in early insulin resistance, beta cells compensate by increasing insulin secretion (hyperinsulinaemia), maintaining normal glucose tolerance. Over time, chronic hyperglycaemia (glucotoxicity) and elevated free fatty acids (lipotoxicity) impair beta-cell function through oxidative stress, ER stress, and amyloid deposition (islet amyloid polypeptide, IAPP, co-secreted with insulin, aggregates in the islet in type 2 diabetes). Beta-cell mass declines through apoptosis, and the compensatory hyperinsulinaemia gives way to relative insulin deficiency. The loss of first-phase insulin secretion is an early marker of beta-cell dysfunction.

HbA1c (glycated haemoglobin) reflects average blood glucose over the preceding 2--3 months (the lifespan of red blood cells). The relationship is approximately: average glucose (mg/dL) = 28.7 x HbA1c - 46.7. Diagnostic thresholds: HbA1c >= 6.5% confirms diabetes; 5.7--6.4% indicates prediabetes. HbA1c is the standard measure of long-term glycaemic control, with treatment targets typically < 7.0% for most adults.

Diabetic complications

Chronic hyperglycaemia produces tissue damage through four major pathways:

1. Polyol pathway: aldose reductase converts glucose to sorbitol, consuming NADPH. NADPH depletion reduces glutathione regeneration, increasing oxidative stress. Sorbitol accumulation causes osmotic damage, particularly in the lens (cataract) and peripheral nerves.

2. Advanced glycation end-products (AGEs): non-enzymatic glycation of proteins produces AGEs (e.g., HbA1c is an AGE). AGEs cross-link extracellular matrix proteins, reducing vascular compliance. AGEs also bind RAGE (receptor for AGEs) on endothelial cells, activating NF-kappaB and promoting inflammation, endothelial dysfunction, and atherogenesis.

3. Protein kinase C (PKC) activation: hyperglycaemia increases diacylglycerol (DAG) synthesis, activating PKC-beta in vascular cells. PKC activation increases vascular permeability, promotes angiogenesis (pathological, as in proliferative retinopathy), and increases expression of pro-inflammatory and pro-thrombotic factors.

4. Hexosamine pathway: excess fructose-6-phosphate is diverted to the hexosamine pathway, producing N-acetylglucosamine that O-GlcNAcylates transcription factors, altering expression of pro-fibrotic and pro-inflammatory genes.

Microvascular complications (specific to diabetes):

• Diabetic nephropathy: begins with glomerular hyperfiltration (GFR increase), followed by microalbuminuria (30--300 mg/day), progressing to macroalbuminuria (> 300 mg/day) and declining GFR. The pathological hallmark is glomerular basement membrane thickening and mesangial expansion from AGE deposition and TGF-beta-mediated fibrosis. Diabetic nephropathy is the leading cause of end-stage renal disease in developed countries. ACE inhibitors and ARBs slow progression by reducing intraglomerular pressure.

• Diabetic retinopathy: the leading cause of blindness in working-age adults. Non-proliferative retinopathy features microaneurysms, haemorrhages, and hard exudates. Proliferative retinopathy features neovascularisation driven by VEGF in response to retinal ischaemia; the fragile new vessels bleed (vitreous haemorrhage) and contract (retinal detachment). Pan-retinal photocoagulation and anti-VEGF intravitreal injections are mainstay treatments.

• Diabetic neuropathy: symmetric distal sensorimotor polyneuropathy ("stocking-glove" pattern) is most common. Mononeuropathies (e.g., cranial nerve palsies) and autonomic neuropathy (gastroparesis, orthostatic hypotension, erectile dysfunction, loss of hypoglycaemia awareness) also occur. The mechanism involves microvascular nerve damage, sorbitol accumulation, and oxidative stress.

Macrovascular complications: diabetes is a coronary artery disease equivalent, conferring the same cardiovascular risk as established coronary disease. Accelerated atherosclerosis results from endothelial dysfunction, AGE-mediated vessel stiffness, chronic inflammation, and the pro-thrombotic state. Stroke, peripheral arterial disease, and coronary heart disease are the major macrovascular complications and the leading causes of death in diabetes.

Insulin therapy

Exogenous insulin replacement is required in type 1 diabetes and is used in advanced type 2 diabetes when oral agents fail to maintain glycaemic targets.

Basal-bolus regimen (multiple daily injections, MDI): aims to mimic physiological insulin secretion. Basal insulin (long-acting: glargine, detemir, degludec) provides a steady background level suppressing hepatic glucose output between meals and overnight. Bolus insulin (rapid-acting: lispro, aspart, glulisine) is given before meals to cover postprandial glucose excursions. The total daily dose is typically divided as approximately 50% basal and 50% bolus.

Insulin pumps (continuous subcutaneous insulin infusion, CSII): deliver a programmable basal rate (which can vary by time of day) and patient-initiated boluses for meals. Pump therapy provides more flexible basal dosing and eliminates the need for multiple daily injections but requires frequent self-monitoring and carries the risk of diabetic ketoacidosis if the infusion set fails (only rapid-acting insulin is used, so there is no long-acting depot).

Oral hypoglycaemic agents and injectable therapies

Drug class	Mechanism	Key agents	Notes
Biguanides	Activates AMPK; reduces hepatic gluconeogenesis; improves insulin sensitivity	Metformin	First-line for type 2; does not cause hypoglycaemia; GI side effects; rare lactic acidosis
Sulfonylureas	Close K-ATP channels on beta cells; stimulate insulin secretion	Glipizide, glibenclamide, glimepiride	Risk of hypoglycaemia; weight gain
SGLT2 inhibitors	Block sodium-glucose co-transporter 2 in proximal tubule; increase renal glucose excretion	Empagliflozin, dapagliflozin	Cardiovascular benefit (heart failure); risk of genital infections and euglycaemic DKA
GLP-1 receptor agonists	Activate GLP-1 receptor; glucose-dependent insulin secretion; suppress glucagon; slow gastric emptying; promote satiety	Semaglutide, liraglutide, dulaglutide	Weight loss; cardiovascular benefit; injectable (oral semaglutide also available)
DPP-4 inhibitors	Inhibit DPP-4; prolong endogenous GLP-1 and GIP action	Sitagliptin, linagliptin	Mild glucose-lowering; well tolerated
Thiazolidinediones (TZDs)	Activate PPAR-gamma; improve insulin sensitivity in adipose tissue	Pioglitazone	Fluid retention; weight gain; fracture risk
Alpha-glucosidase inhibitors	Delay carbohydrate absorption in the intestine	Acarbose	Modest effect; GI side effects

Continuous glucose monitoring

Continuous glucose monitors (CGMs) measure interstitial glucose (which lags plasma glucose by approximately 5--15 minutes) via a subcutaneous enzymatic electrode. Modern CGM systems provide:

• Real-time glucose readings updated every 1--5 minutes.
• Trend arrows indicating the direction and rate of glucose change.
• Alerts for hypoglycaemia and hyperglycaemia.
• Time in range (TIR): the percentage of time glucose is within 70--180 mg/dL, now considered a primary glycaemic outcome metric alongside HbA1c.

CGM data enables closed-loop insulin delivery ("artificial pancreas" systems): a CGM feeds glucose data to an algorithm that adjusts insulin pump delivery rate in real time. Hybrid closed-loop systems (e.g., Medtronic 670G/780G, Tandem Control-IQ) automatically adjust basal insulin while the user still initiates meal boluses. Fully automated dual-hormone systems (insulin and glucagon) are in development.

Connections Master

• Endocrine hormones and regulation 18.07.01 is the prerequisite unit covering general hormone classes and feedback principles. Insulin and glucagon are the specific implementation of a push-pull feedback system introduced there.

• Hypothalamic-pituitary axis 18.07.02 pending provides the context for counterregulatory hormones: cortisol (HPA axis), growth hormone (GH axis), and the stress response that modulates glucose homeostasis. The insulin tolerance test used to assess HPA axis integrity works precisely because hypoglycaemia is a potent activator of the stress axis.

• Cell signalling 17.07.01 provides the molecular machinery: the insulin receptor tyrosine kinase, PI3K/Akt signalling, GPCR-cAMP signalling (glucagon receptor), and receptor desensitisation mechanisms are all instances of the general signalling frameworks covered there.

• Renal physiology 18.08.01 is connected through SGLT2 inhibitors (acting on renal glucose reabsorption), diabetic nephropathy (glomerular pathology), and the osmotic diuresis of hyperglycaemia. The kidney's role in gluconeogenesis (contributing approximately 20--25% of endogenous glucose production in the fasted state) is also relevant.

• Cardiovascular physiology 18.02.01 is relevant to diabetic macrovascular complications: accelerated atherosclerosis, endothelial dysfunction, and the cardiovascular outcome benefits of SGLT2 inhibitors and GLP-1 agonists. Diabetic autonomic neuropathy causes resting tachycardia and orthostatic hypotension through loss of autonomic cardiovascular regulation.

• Digestive physiology 18.06.01 connects through the incretin effect: GLP-1 and GIP are gut hormones linking nutrient ingestion to insulin secretion. Gastric emptying (delayed by GLP-1, accelerated in gastroparesis from diabetic autonomic neuropathy) modulates postprandial glucose excursions.

• Respiratory physiology 18.03.01 connects through Kussmaul breathing in DKA: the deep, rapid respiratory pattern is a compensatory response to metabolic acidosis, blowing off CO2 to raise pH.

Historical & philosophical context Master

The discovery of insulin in 1921--1922 by Frederick Banting and Charles Best, in the laboratory of John Macleod at the University of Toronto, is one of the most dramatic stories in medical history ^{[Banting & Best 1922]}. Before insulin, type 1 diabetes was a death sentence: children died within months of diagnosis, typically in diabetic ketoacidosis. The only treatment was starvation diets that prolonged life by weeks to months at the cost of progressive emaciation.

Banting, an orthopaedic surgeon with no prior research experience, conceived the idea of ligating the pancreatic ducts to cause acinar degeneration while preserving the islets, thereby obtaining islet extracts free of digestive enzymes. Working with Best (a medical student), he produced extracts from degenerated dog pancreas that lowered blood glucose in diabetic dogs. The biochemist James Collip joined the team and developed purification methods that made the extract safe for human use. The first patient, 14-year-old Leonard Thompson, received an injection in January 1922; his blood glucose fell and his clinical condition improved dramatically.

Banting and Macleod received the 1923 Nobel Prize. Banting, furious that Best was excluded, shared his prize money with Best; Macleod similarly shared with Collip. The episode remains a case study in the sociology of scientific credit.

The understanding of insulin resistance as distinct from insulin deficiency emerged much later. Harold Himsworth proposed in 1936 that diabetes could be divided into "insulin-sensitive" and "insulin-insensitive" types, based on the glycaemic response to exogenous insulin. The distinction was confirmed by the development of the insulin radioimmunoassay by Rosalyn Yalow and Solomon Berson in 1959 (Yalow received the 1977 Nobel Prize; Berson had died and was ineligible). The radioimmunoassay revealed that many type 2 diabetic patients had elevated, not reduced, insulin levels -- proving that their hyperglycaemia was due to resistance, not deficiency.

The Bergman minimal model (1979) represented a conceptual advance by demonstrating that insulin sensitivity could be estimated from a single IVGTT, without requiring pancreatic clamping. The model's key insight was that insulin acts on glucose disposal through a remote ("action") compartment with its own dynamics, introducing a lag between plasma insulin and its metabolic effect. This formalisation made insulin sensitivity a quantifiable parameter ($S_I$) rather than a qualitative concept, enabling systematic study of insulin resistance in population-based research.

The discovery of GLP-1 as an incretin hormone in the 1980s, followed by the observation that the incretin effect is impaired in type 2 diabetes (Nauck et al., 1986), opened a new therapeutic paradigm. The development of GLP-1 receptor agonists, beginning with exenatide (derived from the saliva of the Gila monster, Heloderma suspectum, whose exendin-4 peptide shares 53% sequence homology with human GLP-1 and is resistant to DPP-4 degradation), demonstrated that gut-hormone biology could be pharmacologically exploited. Semaglutide, a once-weekly GLP-1 agonist, has since demonstrated cardiovascular benefit and substantial weight loss, blurring the boundary between diabetes treatment and obesity treatment.

The story of glucose homeostasis illustrates a recurring theme in physiology: homeostatic systems are not static equilibria but dynamic feedback processes that can fail in characteristic ways. The distinction between type 1 (loss of the controlled variable's effector) and type 2 (resistance to the effector at the target tissue) maps directly onto the control-theory framework of actuator failure versus plant gain reduction, and the clinical presentation (DKA in type 1, hyperosmolar hyperglycaemic state in type 2) reflects the different dynamical consequences of each failure mode.

Bibliography Master

1. Banting, F. G. & Best, C. H., "The internal secretion of the pancreas", J. Lab. Clin. Med. 7 (1922), 251-266.

2. Bergman, R. N., Ider, Y. Z., Bowden, C. R. & Cobelli, C., "Quantitative estimation of insulin sensitivity", Am. J. Physiol. 236 (1979), E667-E677.

3. Himsworth, H. P., "Diabetes mellitus: a differentiation into insulin-sensitive and insulin-insensitive types", Lancet 227 (1936), 127-130.

4. Yalow, R. S. & Berson, S. A., "Assay of plasma insulin in human subjects by immunological methods", Nature 184 (1959), 1648-1649.

5. Nauck, M. A., Homberger, E., Siegel, E. G. et al., "Incretin effects of increasing glucose loads in man calculated from venous insulin and C-peptide responses", J. Clin. Endocrinol. Metab. 63 (1986), 492-498.

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7. UK Prospective Diabetes Study (UKPDS) Group, "Intensive blood-glucose control with sulphonylureas or insulin compared with conventional treatment and risk of complications in patients with type 2 diabetes (UKPDS 33)", Lancet 352 (1998), 837-853.

8. Sherwood, L., Human Physiology: From Cells to Systems, 9th ed. (Cengage, 2016), Ch. 19.

9. Silverthorn, D. U., Human Physiology: An Integrated Approach, 8th ed. (Pearson, 2019), Ch. 22.

10. Guyton, A. C. & Hall, J. E., Textbook of Medical Physiology, 14th ed. (Elsevier, 2021), Ch. 78-80.

11. American Diabetes Association, "Standards of medical care in diabetes -- 2024", Diabetes Care 47 (Suppl. 1) (2024).