15.4 Diabetes, Glucose Homeostasis & Metabolic Syndrome
Key Takeaways
- Insulin is cleaved from proinsulin (C-peptide marker of endogenous secretion); glucose enters β cells via GLUT2 → ATP closes KATP → depolarization → Ca2+ influx → insulin release; GLUT4 is insulin-responsive in muscle/adipose.
- T1DM is autoimmune β-cell destruction (HLA-DR3/DR4, autoantibodies) with absolute insulin deficiency; T2DM is insulin resistance plus relative β-cell failure with amylin/amyloid and progressive secretory defect.
- DKA (usually T1): insulinopenia → lipolysis → ketogenesis, anion-gap acidosis, dehydration; HHS (usually T2): profound hyperglycemia/hyperosmolarity without significant ketoacidosis.
- Chronic complications: microangiopathy (AGEs, sorbitol, PKC pathways) → retinopathy, nephropathy, neuropathy; macrovascular atherosclerosis drives MI/stroke/PAD risk.
- High-yield drug mechanisms: metformin (↓hepatic gluconeogenesis, AMPK); sulfonylureas (close β-cell KATP); GLP-1 agonists/DPP-4 inhibitors (incretin axis); SGLT2 inhibitors (urinary glucose loss); TZDs (PPAR-γ insulin sensitizers).
15.4 Diabetes, Glucose Homeostasis & Metabolic Syndrome
Quick Answer: Glucose → GLUT2 β cell → ↑ATP → close KATP → insulin (± C-peptide). GLUT4 stores fuel in muscle/fat. T1 = autoimmune absolute deficiency; T2 = resistance + relative deficiency. DKA = ketosis/acidosis; HHS = extreme hyperosmolarity. Know metformin, SU, GLP-1, DPP-4, SGLT2, TZD mechanisms and micro- vs macrovascular complication logic.
Metabolic endocrine content is heavily tested because it links cell biology, pathology, and pharmacology in one system. Master the β-cell stimulus–secretion coupling first; everything else is a perturbation of that loop or of insulin action in target tissues.
Insulin Synthesis, Secretion, and GLUT Transporters
β cells synthesize preproinsulin → proinsulin → insulin + C-peptide in equimolar amounts. C-peptide measures endogenous insulin secretion (absent in exogenous insulin injection; low in T1DM; variable in T2DM).
Stimulus–secretion coupling:
- Glucose enters β cells via GLUT2 (insulin-independent).
- Glucokinase phosphorylates glucose (glucose sensor; MODY2 when mutated).
- Metabolism raises ATP/ADP.
- ATP closes the ATP-sensitive K+ channel (KATP, SUR1/Kir6.2).
- Membrane depolarizes → voltage-gated Ca2+ channels open → Ca2+ influx → insulin granule exocytosis.
Sulfonylureas and meglitinides close KATP pharmacologically; diazoxide opens it (inhibits insulin release). Incretins (GLP-1, GIP) amplify glucose-stimulated insulin secretion via cAMP.
| Transporter | Location | Insulin-dependent? | Role |
|---|---|---|---|
| GLUT1 | RBCs, brain (basal), placenta | No | Basal uptake |
| GLUT2 | β cell, liver, kidney, intestine | No | Glucose sensing/bidirectional hepatic flux |
| GLUT3 | Neurons | No | High-affinity brain uptake |
| GLUT4 | Skeletal muscle, adipose | Yes | Postprandial disposal |
| SGLT1/2 | Intestine/kidney (secondary active) | No | Na+-coupled absorption/reabsorption |
Insulin receptor is a receptor tyrosine kinase. Downstream IRS → PI3K–Akt pathway mediates GLUT4 translocation and metabolic effects; MAPK pathway contributes to growth effects.
Counterregulatory Hormones
When glucose falls, counterregulation defends the brain:
- Glucagon (α cells): glycogenolysis, gluconeogenesis, ketogenesis permission
- Epinephrine: rapid glycogenolysis, lipolysis, symptoms of hypoglycemia
- Cortisol and GH: longer-term insulin resistance and gluconeogenic support
Hypoglycemia unawareness after recurrent lows reflects blunted counterregulatory responses—clinically critical though mechanism depth varies by exam item.
Fed state: high insulin/glucagon ratio → glycogen synthesis, lipogenesis, protein synthesis. Fasted state: low insulin/glucagon → glycogenolysis, gluconeogenesis, lipolysis, ketogenesis.
Type 1 vs Type 2 Diabetes Mellitus
Type 1 DM
Autoimmune destruction of β cells (type IVa hypersensitivity patterns with T-cell mediation). Associated with HLA-DR3 and DR4, autoantibodies (anti-GAD65, IA-2, insulin autoantibodies) useful early. Absolute insulin deficiency → hyperglycemia and unrestrained lipolysis/ketogenesis when insulin is absent. Peak childhood/adolescence but adult latent autoimmune diabetes exists. Patients are often lean at onset; DKA is the classic acute presentation.
Type 2 DM
Insulin resistance in muscle, liver, and adipose (obesity, free fatty acids, inflammatory adipokines) plus progressive β-cell dysfunction (relative insulin deficiency). Islet amyloid (amylin) deposits are histologic associations. Stronger lifestyle and polygenic risk; acanthosis nigricans signals resistance. HHS is the classic hyperglycemic crisis more than DKA, though ketosis-prone T2 exists.
| Feature | T1DM | T2DM |
|---|---|---|
| Primary lesion | Autoimmune β-cell loss | Resistance + β-cell failure |
| Insulin/C-peptide | Very low | Normal/high early, later low |
| Body habitus | Often lean | Often overweight/obese |
| Ketosis tendency | High | Lower (not zero) |
| Twin concordance | Moderate | High |
| Treatment core | Insulin essential | Lifestyle ± non-insulin agents ± insulin |
DKA vs HHS Pathophysiology
Diabetic ketoacidosis (DKA): usually T1. Absolute insulin deficiency + counterregulatory excess → hyperglycemia (osmotic diuresis, dehydration, electrolyte depletion) and hormone-sensitive lipase activation → free fatty acids → hepatic ketogenesis (β-hydroxybutyrate, acetoacetate) → anion-gap metabolic acidosis. Kussmaul breathing, fruity breath, abdominal pain, altered mentation. Total-body K+ depleted despite possible hyperkalemia from acidosis/insulin lack—exam favorite. Triggers: infection, insulin omission, new-onset disease.
Hyperosmolar hyperglycemic state (HHS): usually older T2 patients. Enough insulin to suppress ketogenesis but not to prevent severe hyperglycemia → extreme hyperosmolarity, profound dehydration, neurologic changes; minimal ketones/acidosis. Mortality historically higher than DKA; fluid resuscitation is central conceptually.
| DKA | HHS | |
|---|---|---|
| Typical patient | T1, younger | T2, older |
| Glucose | High (often 300–600 mg/dL range conceptually) | Often higher (e.g., >600) |
| Ketones/acidosis | Prominent anion-gap acidosis | Absent/minimal |
| Osmolality | Elevated | Markedly elevated |
| Key driver | Insulinopenia → ketogenesis | Relative insulin + severe dehydration |
Hypoglycemia
Neuroglycopenic symptoms (confusion, seizure, coma) and autonomic symptoms (sweating, tremor, palpitations). Causes: excess insulin/secretagogue, missed meals, exercise, critical illness, insulinoma (high insulin + high C-peptide + low glucose), exogenous insulin (high insulin, low C-peptide). Factitious sulfonylurea use mimics insulinoma biochemically until drug screen—classic distinction.
Chronic Complications: Micro and Macrovascular
Chronic hyperglycemia drives:
- Advanced glycation end products (AGEs) cross-linking matrix proteins and activating RAGE inflammatory signaling
- Polyol (sorbitol) pathway via aldose reductase → osmotic/oxidative stress (lens, nerves, Schwann cells)
- Protein kinase C activation from de novo DAG synthesis → vascular dysfunction
- Hexosamine pathway excess
Microvascular (more glucose-specific, improved by glycemic control trials historically):
- Retinopathy: pericyte loss, microaneurysms, hemorrhages, neovascularization (VEGF) in proliferative disease
- Nephropathy: mesangial expansion, GBM thickening, nodular glomerulosclerosis (Kimmelstiel–Wilson), albuminuria; ACEI/ARB renoprotective concepts
- Neuropathy: distal symmetric polyneuropathy, autonomic neuropathy (gastroparesis, orthostasis, silent ischemia)
Macrovascular: accelerated atherosclerosis → coronary disease, stroke, peripheral arterial disease. Endothelial dysfunction, dyslipidemia pattern (high TG, low HDL, small dense LDL in insulin resistance). Glycemic control alone is less dominant than for microvascular disease—blood pressure and lipids matter enormously.
Metabolic Syndrome
Cluster of insulin resistance features (definitions vary slightly by organization): central obesity, hypertension, hyperglycemia/impaired fasting glucose, hypertriglyceridemia, low HDL. Shared pathophysiology: visceral adipose inflammation, free fatty acids, hepatic steatosis risk. Marks elevated ASCVD and T2DM risk—treat the components.
High-Yield Antihyperglycemic Mechanisms (CBSE Level)
| Class | Prototype idea | Primary mechanism | Notable effects / pearls |
|---|---|---|---|
| Biguanide | Metformin | ↓ hepatic gluconeogenesis (AMPK-related); ↑ insulin sensitivity modestly | First-line T2; GI side effects; lactic acidosis risk in severe renal/hypoxic states |
| Sulfonylureas | Glipizide, glyburide | Close β-cell KATP → insulin release | Hypoglycemia, weight gain; need functioning β cells |
| Meglitinides | Repaglinide | Shorter-acting KATP closure | Mealtime secretagogue |
| GLP-1 receptor agonists | Exenatide, liraglutide, etc. | Incretin: ↑ glucose-dependent insulin, ↓ glucagon, slow gastric emptying, satiety | Weight loss; low intrinsic hypo risk alone; nausea; injection |
| DPP-4 inhibitors | Sitagliptin | Block degradation of endogenous GLP-1/GIP | Weight neutral; milder than GLP-1 RAs |
| SGLT2 inhibitors | Canagliflozin, empagliflozin, etc. | Block renal proximal glucose reabsorption → glucosuria | Weight loss, BP ↓; genital mycotic infections; osmotic diuresis; cardio-renal outcome fame |
| TZDs | Pioglitazone | PPAR-γ agonists → insulin sensitization in adipose | Weight gain, edema, heart failure caution, bone fracture risk |
| α-glucosidase inhibitors | Acarbose | Block brush-border carb breakdown | Flatulence; ↓ postprandial glucose |
| Insulin | Multiple analogs | Activate insulin RTK | Essential in T1; hypo and weight gain risks |
Amylin analog (pramlintide) slows gastric emptying and suppresses glucagon—adjunct concept. Acarbose is less US-central but mechanism-clean for exams.
Clinical Vignette Reflexes
Newly diagnosed thin teen with polyuria, polydipsia, weight loss, and Kussmaul respirations = T1 DKA until proven otherwise. Older obese patient with glucose 900 mg/dL, stupor, minimal acidosis = HHS. Acanthosis nigricans + central obesity = insulin resistance/metabolic syndrome. Postprandial hypoglycemia after SU overdose = KATP closure toxicity. Rising creatinine and albuminuria after years of diabetes = microvascular nephropathy pathways. Choosing metformin first in uncomplicated T2 reflects hepatic gluconeogenesis suppression without hypoglycemia as monotherapy.
If you can draw β-cell depolarization, name the four major GLUT roles, split DKA from HHS in one sentence each, and recite the six core drug mechanisms in the table, you have covered the vast majority of CBSE diabetes and intermediate metabolism items in the reproductive–endocrine content band.
Which sequence best describes glucose-stimulated insulin secretion in a pancreatic β cell?
Which comparison between DKA and HHS is most accurate?
A drug lowers hepatic gluconeogenesis via AMPK-related pathways and is first-line for many patients with type 2 diabetes. Which agent matches this mechanism?