Endocrine Module 01Introduction to Hormones

1 · What defines "endocrine" (the one-line rule)

• Hormone =
  • A molecule secreted by a ductless gland into the bloodstream to act on a distant target cell.FA 2024 Endo
• Endocrine vs. exocrine:
  • Rules out exocrine glands (pancreatic acini → duodenum via duct, sweat glands, salivary glands).

• Dual role:
  • Endocrine: islets of Langerhans → blood.
  • Exocrine: acini → duct of Wirsung → duodenum.
  • Same organ, two modes.Costanzo Physiology 6e, ch 9

• Why endocrinopathies are systemic:
  • "Ductless" delivery means the hormone goes everywhere → multi-organ symptoms.

2 · Autocrine vs. paracrine — the tricky pair

• Core feature:
  • Receptor and ligand on the SAME cell.
• Tumor biology examples:
  • Glioblastomas often have PDGF autocrine loops.
  • Some breast cancers have IGF-1 autocrine loops.
• Immunology examples:
  • Activated T cells produce IL-2 AND express IL-2R → autocrine proliferation.FA 2024 Immuno

• Mechanism:
  • Signal diffuses to NEIGHBORING cells through interstitial fluid.
• Classic examples:
  • Somatostatin from pancreatic δ-cells inhibits adjacent α and β cells (paracrine) — same molecule from the hypothalamus is endocrine on the anterior pituitary.
  • Histamine from ECL cells → neighboring parietal cells → HCl secretion. Blocked by H2-blockers (famotidine).Guyton & Hall 14e ch 78

• Mode is defined by delivery, not the molecule:
  • A single molecule can be autocrine, paracrine, OR endocrine depending on which cell makes it and which cell listens.

3 · Neuroendocrine — the hypothalamus is the master regulator

• Neuroendocrine neuron:
  • A neuron whose axon terminal releases hormone into the bloodstream instead of across a synapse.

• Magnocellular neurons:
  • Located in supraoptic and paraventricular nuclei.
  • Produce ADH & oxytocin.
  • Stored/released from the posterior pituitary → systemic circulation.
• Parvocellular neurons:
  • Produce TRH, CRH, GnRH, GHRH, dopamine (PIH), somatostatin.
  • Released into the hypophyseal portal system → anterior pituitary (local, concentrated delivery).Costanzo 6e ch 9; B&B Endo

• Hypophyseal portal network:
  • A capillary → vein → capillary system that delivers hypothalamic hormones to the anterior pituitary.
  • Achieves ~1000× the concentration hormones would reach via systemic circulation.
  • Why lesions of the pituitary stalk cause anterior pituitary dysfunction.Guyton 14e ch 75
• Adrenal medulla:
  • A modified sympathetic ganglion.
  • Chromaffin cells are post-ganglionic sympathetic neurons that release Epi/NE into the blood.
  • Functions as a neuroendocrine organ.

4 · Specificity — receptor expression + local factors

• Receptor expression alone:
  • Necessary but not sufficient for hormonal response.

• Receptor isoform affinity:
  • α1 vs β1 both bind NE, but produce opposite vascular effects.
• Intracellular signaling machinery:
  • A cell with GPCR but no adenylyl cyclase won't respond to Gs-coupled signaling.
• Local concentration:
  • Paracrine signals achieve higher [hormone] than endocrine.
  • Explains why paracrine somatostatin is potent locally despite low systemic levels.
• Timing:
  • Some receptors are expressed only during certain cell-cycle or developmental windows.

1 · Amino acid derivatives (amines)

• Catecholamines:
  • Epinephrine, norepinephrine, dopamine.
  • Hydrophilic; short half-life (~1–2 min); stored in vesicles.
• Thyroid hormones T3/T4:
  • Made by coupling two iodinated tyrosines.
  • Iodination + ring structure = lipophilic.
  • Long half-life (T4 ~7 days).Costanzo 6e ch 9

• Serotonin (5-HT):
  • Mostly neurotransmitter.
  • Acts as paracrine/endocrine from enterochromaffin cells in GI tract (carcinoid syndrome).Pathoma 2024 ch 15
• Melatonin:
  • From pineal gland; peaks at night.
  • Acts via MT1/MT2 GPCRs for circadian rhythm.

• Histamine:
  • Mostly paracrine (gastric ECL cells, mast cells).

2 · Peptides & proteins — the synthesis pipeline

• Step 1 — Translation:
  • Transcribed → translated on ribosomes of the rough ER as a larger precursor (pre-pro-hormone).
• Step 2 — Signal cleavage:
  • Pre-sequence (signal peptide) targets the protein to the ER lumen → cleaved → pro-hormone.
• Step 3 — Packaging & processing:
  • Pro-hormone travels Golgi → packaged in secretory vesicles → proteolytically processed inside the vesicle to the final active form.

• Processing pathway:
  • Pre-pro-insulin (110 AA) → pro-insulin (86 AA with A and B chains connected by C-peptide) → insulin (A + B chains, disulfide-linked) + C-peptide (equimolar).FA 2024 Endo
• Why C-peptide matters clinically:
  • C-peptide is inert but clinically valuable.
  • It rises with endogenous insulin and is ABSENT with exogenous insulin.
  • Distinguishes insulinoma / endogenous hyperinsulinism from factitious (injected) insulin use.B&B Endo — Insulinoma

• Size range:
  • 3 AAs (TRH) to 200+ AAs (glycoprotein hormones like FSH).

• Members:
  • TSH, LH, FSH, hCG.
• Subunit structure:
  • Share a common α-subunit (encoded by a single gene — CGA).
  • Unique β-subunit confers receptor specificity.
• Why hCG is ideal for pregnancy testing:
  • Glycosylation patterns extend half-life and modulate affinity.
  • hCG has extra sialic acid → longer half-life than LH → detectable at low concentrations in urine.Guyton 14e ch 76

3 · Steroids — the cholesterol family

• Universal precursor:
  • All 5 steroid hormone classes (progestins, gluco/mineralocorticoids, androgens, estrogens) + vitamin D are built from cholesterol (27 carbons).

• Primary — LDL-receptor-mediated uptake:
  • Highest contribution in the adrenal gland.
• Secondary — de novo synthesis:
  • From acetyl-CoA via HMG-CoA reductase.
• Tertiary — stored cholesterol esters:
  • Held in lipid droplets, mobilized acutely during stimulation.Costanzo 6e ch 9

• Why steroids cannot be stored:
  • The lipid backbone diffuses through ANY membrane, including vesicle membranes.
  • Cells make steroids on demand and release them immediately.Guyton 14e ch 74
• Why steroids have long half-lives:
  • Carrier protein binding (CBG, SHBG, albumin) protects from hepatic metabolism.
  • Bound hormone cannot be filtered by the kidney, extending circulating time.
• Elimination:
  • Hepatic conjugation (glucuronide or sulfate) → excreted in bile or urine.
  • Liver disease can markedly prolong steroid half-life.Lippincott Pharm 7e

4 · Eicosanoids (lipid derivatives) — brief

• Precursor:
  • Arachidonic acid (20-C polyunsaturated fatty acid).
  • Released from membrane phospholipids by phospholipase A2 (PLA2).

• COX-1 / COX-2 pathway:
  • Produces prostaglandins (PGE2, PGI2) and thromboxane (TxA2).
  • Blocked by NSAIDs.
• 5-lipoxygenase (LOX) pathway:
  • Produces leukotrienes (LTB4, LTC4).
  • Enzyme blocked by zileuton.
  • Receptor blocked by montelukast (used in asthma).FA 2024 Pharm — NSAIDs

• Half-life:
  • Seconds.
  • Act as paracrine/autocrine — NOT classic endocrine.

5 · The thyroid rebel — why T3/T4 are lipophilic

• How T3 and T4 are built:
  • T4 = two DIT (di-iodotyrosine) residues coupled.
  • T3 = MIT + DIT.
  • Coupling + iodination creates a bulky, non-polar aromatic structure.

• Functional profile:
  • Fat-soluble; crosses membranes freely.
  • Carrier-bound in plasma (TBG).
  • Nuclear receptor (transcription factor).
  • Long half-life (T4 ~7 days).

• Circulating form:
  • T4 is the primary circulating form (~80% of thyroid output).
• Active form:
  • T3 is ~5× more potent at the TRβ receptor.
• Peripheral conversion:
  • 5'-deiodinase converts T4 → T3 in target tissues (liver, kidney, muscle).
  • Why a patient on levothyroxine still ends up with active T3 systemically.Guyton 14e ch 77; Pathoma ch 15

• Reverse T3 (rT3):
  • In severe illness / starvation, T4 is preferentially converted to reverse T3 — an inactive metabolite.
  • Patient appears hypothyroid biochemically but is NOT truly hypothyroid — no treatment needed. Classic USMLE trap.

1 · The order of the pathway (why each step matters)

• Context:
  • Not strictly part of catecholamine synthesis, but clinically important.
• Clinical relevance:
  • Deficiency = PKU (phenylketonuria) → intellectual disability, musty body odor, seizures.
  • Screened at birth in all US states.FA 2024 Biochem
  • Tyrosine becomes an "essential" AA in PKU patients — must be supplied in diet.

• Location & cofactors:
  • Cytoplasmic.
  • Requires BH4 (tetrahydrobiopterin) and O₂.
  • Same cofactor as phenylalanine hydroxylase — BH4 deficiency breaks multiple pathways.Costanzo 6e ch 9
• Regulation:
  • Feedback-inhibited by end products (DA, NE, Epi) binding an allosteric site.
  • Phosphorylation by PKA or CaMKII during sympathetic activation INCREASES activity (relieves feedback).

• Location & cofactors:
  • Cytoplasmic.
  • Requires vitamin B6 (PLP).
• Activity level:
  • High baseline activity — rarely rate-limiting.
  • This is why L-DOPA is effective in Parkinson's: the enzyme is already waiting to convert it.
• Pharmacologic target:
  • Blocked peripherally by carbidopa in Parkinson's treatment.FA 2024 Pharm — Parkinson's

• Unique location:
  • Occurs INSIDE the chromaffin granule / synaptic vesicle.
  • Dopamine is first pumped into the vesicle by VMAT (vesicular monoamine transporter), then hydroxylated.
• Cofactors:
  • Vitamin C (ascorbate).
  • Copper.
• Pharmacologic target:
  • Reserpine inhibits VMAT → blocks vesicular uptake → depletes catecholamines (old antihypertensive).Lippincott Pharm 7e

• Location & cofactor:
  • Cytoplasmic.
  • Requires SAM (S-adenosyl methionine) as methyl donor.
• Special requirement:
  • Only active where cortisol levels are high — see section 3 below.

2 · The rate-limiting step: tyrosine hydroxylase in depth

• Enzyme kinetics:
  • Slowest Vmax in the pathway and highly regulated.
  • Any change in catecholamine output requires a change in this enzyme's activity.

• Feedback control:
  • Allosteric end-product feedback by NE, Epi, and DA.
  • Phosphorylation by PKA (cAMP-dependent) and CaMKII during stress → relieves end-product inhibition → more NE/Epi produced.B&B Endo — Catecholamines

• Transcriptional upregulation:
  • Induced by chronic stress, glucocorticoids, and nerve growth factor (NGF).
  • This is why chronic stress or steroid excess can lead to elevated sympathetic tone.

• Metyrosine (α-methyl-p-tyrosine):
  • Competitive tyrosine hydroxylase inhibitor.
  • Used preoperatively in pheochromocytoma when α-blockade alone isn't enough.Lippincott Pharm 7e; UWorld Endo Qbank
  • Reduces catecholamine stores by 50–80% → helps prevent intraoperative hypertensive crisis.

• Reasoning:
  • DOPA decarboxylase has HIGH baseline activity and rapidly consumes any DOPA produced.
  • This is why L-DOPA therapy works — you give the substrate and the enzyme is ready and waiting.

3 · Why only the adrenal medulla makes epinephrine

• Gross structure:
  • Adrenal gland = cortex (outer) + medulla (inner).
• Cortical zones (outer → inner):
  • Zona Glomerulosa → aldosterone.
  • Zona Fasciculata → cortisol.
  • Zona Reticularis → adrenal androgens (DHEA).
• Medulla:
  • Modified post-ganglionic sympathetic neurons = chromaffin cells.
  • Innervated directly by pre-ganglionic sympathetic fibers (splanchnic nerves).Pathoma 2024 ch 15; Guyton 14e ch 78

• Path of blood flow:
  • Blood enters the adrenal cortex first via superficial capsular arteries.
  • Cortical capillaries load up with cortisol from the zona fasciculata.
  • Those cortisol-rich capillaries drain CENTRIPETALLY through the medulla before leaving via the central adrenal vein.
• Consequence:
  • The adrenal medulla bathes in cortisol at 10–100× systemic plasma concentration.
  • A concentration seen NOWHERE else in the body.Guyton 14e ch 78; Costanzo 6e ch 9

• Mechanism:
  • Cortisol binds glucocorticoid receptors in chromaffin cells → induces PNMT gene transcription.
  • Without high local cortisol, PNMT is not expressed → NE cannot be methylated to Epi.
• Why nerves elsewhere can't make Epi:
  • Sympathetic nerve terminals elsewhere (heart, vessels, bronchi) never see this cortisol concentration.
  • No PNMT → they release NE only.FA 2024 Endo

• Addison's disease (primary adrenal insufficiency):
  • ↓ cortisol → ↓ PNMT expression → impaired epinephrine synthesis.
  • Contributes to fatigue, hypoglycemia, and poor stress response.
• Pheochromocytoma (chromaffin cell tumor):
  • Releases a mix of Epi + NE.
• Paraganglioma (extra-adrenal sympathetic tumor):
  • Releases mostly NE — no local cortisol to induce PNMT.
  • HY distinction between pheo and paraganglioma.Pathoma 2024 ch 15
• Chronic exogenous glucocorticoid use:
  • May experience adrenal medullary atrophy AND HPA-axis suppression.
  • But the adrenal cortisol–medulla anatomy means patients often still make some Epi.

• Adrenal medulla:
  • ~80% Epi / ~20% NE.
• Sympathetic nerve terminals:
  • ~0% Epi / 100% NE.FA 2024 Endo

4 · Catecholamine degradation and lab markers

• MAO (monoamine oxidase):
  • Mitochondrial, intracellular.
  • MAO-A preferentially acts on NE and serotonin.
  • MAO-B preferentially acts on dopamine.
• COMT (catechol-O-methyltransferase):
  • Cytoplasmic, extracellular.
  • Uses SAM as methyl donor.Lippincott Pharm 7e

• Norepinephrine pathway:
  • NE → normetanephrine (COMT) → VMA (MAO).
• Epinephrine pathway:
  • Epi → metanephrine (COMT) → VMA (MAO).
• Dopamine pathway:
  • Dopamine → HVA (homovanillic acid).

• Best initial test:
  • Plasma free metanephrines — most sensitive (~99%).
• Confirmatory:
  • 24-hour urine metanephrines and VMA — specific.UWorld Endo

• MAOI + tyramine foods:
  • Hypertensive crisis.
  • MAO normally degrades dietary tyramine; without MAO, tyramine reaches nerve terminals and displaces stored NE.

1 · The chemistry: cholesterol → pregnenolone

• Carbon count change:
  • Removes 6 carbons from cholesterol's side chain (C27 → C21).
  • Hence the name "side-chain cleavage" (P450scc = CYP11A1 = desmolase — four names, same enzyme).Costanzo 6e ch 9
• Mechanism:
  • Two sequential hydroxylations at C20 and C22.
  • Followed by C-C bond cleavage.

• Required cofactors:
  • NADPH.
  • O₂.
  • Fe (as P450 heme cofactor).
• Location:
  • Inner mitochondrial membrane — so cholesterol must be imported first (see StAR, below).

2 · The real bottleneck: StAR and mitochondrial cholesterol import

• What StAR does:
  • Shuttles cholesterol from the OUTER to the INNER mitochondrial membrane.
  • This transport is the actual rate-limiting step of steroidogenesis.
• Why desmolase isn't the bottleneck:
  • Desmolase is constitutively active — it simply waits for substrate.
  • Block StAR and the downstream enzyme is idle.

• Adrenal cortex:
  • ACTH via cAMP-PKA.
• Gonads:
  • LH via cAMP-PKA.
• Zona glomerulosa:
  • Angiotensin II + K⁺ via IP3 / Ca²⁺.Boards & Beyond — Adrenal

• Preferred (most to least):
  • LDL-receptor uptake.
  • Stored cholesterol esters in lipid droplets.
  • De novo synthesis via HMG-CoA reductase.

• Cause:
  • Mutations in StAR.
• Pathology:
  • Adrenals become lipid-engorged (cholesterol cannot enter mitochondria).
• Clinical:
  • Profound deficiency of ALL steroid hormones → salt-wasting, hypotension.
  • 46,XY fetuses are phenotypic females (no androgens to drive male development).Pathoma ch 15

3 · Downstream branching — one precursor, five hormone families

• Tissue-specific enzymes determine the product:
  • All steroid hormones derive from pregnenolone.
  • Different organs and zones express different downstream enzymes, producing different end-products.

• Zona glomerulosa:
  • Has aldosterone synthase; lacks 17α-hydroxylase.
  • Makes aldosterone only.
• Zona fasciculata:
  • Has 17α-hydroxylase + 11β-hydroxylase.
  • Makes cortisol.
• Zona reticularis:
  • Has 17,20-lyase.
  • Makes DHEA (adrenal androgen).

• Testes (Leydig cells):
  • Full androgen pathway → testosterone.
• Ovary (granulosa + theca):
  • Theca makes androstenedione (LH-driven).
  • Granulosa aromatizes it to estradiol (FSH-driven).
  • Two-cell two-gonadotropin model.FA 2024 Repro; Costanzo 6e ch 10

• Pathways:
  • Δ5 pathway — the pregnenolone side.
  • Δ4 pathway — the progesterone side.
• Tissue preference:
  • Adrenal prefers Δ5.
  • Gonads prefer Δ4.

4 · Enzyme deficiencies you MUST know — CAH

• Block an enzyme:
  • Substrate backs up upstream.
  • Accumulated intermediates shunt into alternative pathways.

• HYPER vs. HYPO rule:
  • HYPER-tension = 11 or 17.
  • HYPO-tension = 21.
• Virilization rule:
  • 21 and 11 → virilize (shunt INTO sex steroids → females virilize).
  • 17 → does NOT virilize (blocks sex steroids → genitalia underdeveloped).Pathoma ch 15; FA 2024

5 · Drug targets on the pathway

• Ketoconazole:
  • Inhibits desmolase + 17α-hydroxylase + several other CYPs.
  • Net effect: ↓ cortisol, ↓ androgens.
  • Used in Cushing's syndrome and advanced prostate cancer.
  • Notable side effect: gynecomastia (from ↓ testosterone).Lippincott Pharm 7e
• Metyrapone:
  • 11β-hydroxylase inhibitor.
  • Used diagnostically (tests ACTH reserve) and therapeutically (Cushing's).
• Aminoglutethimide:
  • Desmolase inhibitor.
  • Older agent, mostly replaced by ketoconazole.
• Mitotane:
  • Adrenolytic — destroys the adrenal cortex.
  • Used in adrenocortical carcinoma.
• Finasteride:
  • 5α-reductase inhibitor (blocks T → DHT conversion).
  • Used in BPH and male-pattern baldness.
• Anastrozole / letrozole / exemestane:
  • Aromatase inhibitors (block androgen → estrogen conversion).
  • Used in ER+ breast cancer (postmenopausal).
  • Letrozole is also used for ovulation induction in fertility treatment.
• Abiraterone:
  • 17α-hydroxylase / 17,20-lyase inhibitor.
  • Used in metastatic prostate cancer.Lippincott Pharm 7e — chemotherapy chapter

1 · Binding kinetics — the Kd concept

• Dissociation constant:
  • The binding of hormone to carrier is reversible and governed by Kd.
  • Formula: Kd = [Hormone]_free × [Carrier]_free / [Complex]
• Interpretation:
  • LOW Kd = tight binding.
  • HIGH Kd = loose binding.
  • TBG has low Kd for T4 — very tight.

• Thyroid hormones:
  • T4: ~0.03% free.
  • T3: ~0.3% free.
• Steroids:
  • Cortisol: ~5% free.
  • Testosterone: ~2% truly free; ~40% is loosely albumin-bound and bioavailable.Costanzo 6e ch 9

• Homeostatic adjustment:
  • When carrier level changes, total hormone shifts but free hormone returns to baseline via negative feedback.
  • This is why a pregnant euthyroid patient has ↑ total T4 but normal free T4 and normal TSH.

2 · What regulates each carrier's production

• Increased by:
  • Estrogen (pregnancy, OCP, HRT, tamoxifen).
  • Acute hepatitis.
  • Heroin, methadone.
• Decreased by:
  • Androgens.
  • Glucocorticoids.
  • Nephrotic syndrome (urinary loss).
  • Cirrhosis.
  • Severe illness (euthyroid sick syndrome).FA 2024 Endo; Costanzo 6e

• Increased by:
  • Estrogen.
  • Pregnancy.
• Decreased by:
  • Cirrhosis.
  • Nephrotic syndrome.
  • Severe sepsis.

• Increased by:
  • Estrogen.
  • Hyperthyroidism.
  • Cirrhosis.
  • Anorexia.
  • Anticonvulsants.
• Decreased by:
  • Androgens.
  • Obesity.
  • Hypothyroidism.
  • Insulin excess (PCOS, T2DM).
• PCOS link:
  • Hyperinsulinemia ↓ SHBG → ↑ free testosterone.
  • Clinical picture: hirsutism, acne, oligomenorrhea.
  • Metformin / weight loss can reverse this.FA 2024 Repro; B&B — PCOS

• Decreased in:
  • Liver failure.
  • Nephrotic syndrome.
  • Malnutrition.
  • Burns.
• Clinical effect:
  • Low albumin shifts free fraction of MANY drugs and hormones.
  • Alters pharmacokinetics (e.g., phenytoin, warfarin dosing).Lippincott Pharm 7e

3 · Half-life values worth knowing

• Definition:
  • Time for plasma concentration to drop by 50% after secretion stops.
• Determinants:
  • Carrier binding (protects).
  • Hepatic metabolism.
  • Renal filtration.

• Catecholamines (seconds):
  • Epi, NE: 1–2 minutes.
• Peptide hormones (minutes):
  • PTH: ~4 minutes (very short — why bone/kidney effects are continuous).
  • Insulin: 5–8 minutes.
  • ACTH: ~10 minutes.
  • GH: ~20 minutes.
• Steroids (hours):
  • Aldosterone: ~20 minutes.
  • Cortisol: 60–90 minutes.
  • Testosterone: 1–2 hours.
• Thyroid hormones (days — titanic):
  • T3: 18–24 hours.
  • T4: ~7 days — why once-daily levothyroxine works.Guyton 14e ch 77

4 · Why "total hormone" can deceive you — worked examples

• Pregnancy:
  • ↑ estrogen → ↑ TBG → ↑ total T4 but normal free T4.
  • Patient is euthyroid — don't treat based on total T4.
• Nephrotic syndrome:
  • Protein loss in urine includes TBG → ↓ total T4.
  • Free T4 remains normal → still euthyroid.
• Cirrhosis:
  • ↓ albumin from failed hepatic synthesis.
  • ↑ SHBG from ↑ estrogen (liver can't clear it).
  • Total hormone levels and free fractions both altered → careful lab interpretation needed.B&B Endo — Thyroid labs
• Severe illness ("euthyroid sick syndrome"):
  • ↓ TBG, ↓ CBG, ↓ free T3 (peripheral conversion shunts T4 to inactive reverse T3).
  • Patient is NOT truly hypothyroid — no thyroid hormone treatment needed.

5 · Nuance: exceptions to "only free = active"

• Mechanism:
  • A few tissues internalize bound hormone-carrier complexes WHOLE.
  • Classic: kidney proximal tubule taking up vitamin D-binding protein complexes.
  • Some steroid-SHBG uptake also uses this pathway.Guyton 14e

• Local adjustments:
  • Certain tissues have tissue-specific deiodinase or carrier dissociation.
  • Can change the LOCAL free-hormone concentration from plasma.

• Default rule:
  • The "only free is active" rule is >95% correct.
  • It is the right default answer for USMLE questions.

1 · GPCR biology in depth

• Structure:
  • 7 transmembrane α-helices, single polypeptide chain.
  • Extracellular N-terminus, intracellular C-terminus.
  • Ligand binding pocket sits among the TM helices.
• Resting state:
  • Gα (bound to GDP) + Gβγ form a trimer.
  • Trimer attached to the inner membrane, loosely associated with the receptor.

• Steps:
  • Hormone binds → conformational change in receptor → receptor acts as a GEF (guanine exchange factor).
  • Gα exchanges GDP for GTP → dissociates from Gβγ.
  • Gα-GTP (and sometimes Gβγ) activates or inhibits downstream effectors.
  • Gα's intrinsic GTPase activity hydrolyzes GTP → GDP, terminating the signal and reassembling the trimer.Costanzo 6e ch 1

• Pathway:
  • Gs-α activates adenylyl cyclase → ATP → cAMP → activates PKA.
  • PKA phosphorylates substrates (enzymes, channels, CREB).
• Termination & modulation:
  • Terminated by phosphodiesterases (PDEs) that hydrolyze cAMP → AMP.
  • Caffeine inhibits PDEs non-selectively → ↑ cAMP duration → stimulation.
• Toxin disruption:
  • Cholera toxin ADP-ribosylates Gs-α → permanently "on" → ↑↑ cAMP → Cl⁻/water secretion → rice-water diarrhea.FA 2024 Micro — Vibrio cholerae

• Pathway:
  • Gq-α activates phospholipase C-β (PLC) → cleaves PIP2 → IP3 + DAG.
  • IP3 → opens ER calcium channel (IP3R) → ↑ cytoplasmic Ca²⁺.
  • DAG + Ca²⁺ → activate PKC → phosphorylate targets.
• Classic clinical example:
  • α1 agonist on vascular smooth muscle → Gq → Ca²⁺ → contraction → vasoconstriction.

• Pathway:
  • Gi-α inhibits adenylyl cyclase → ↓ cAMP.
  • Gβγ directly opens cardiac GIRK K⁺ channels — why M2 agonists slow the heart.
• Toxin disruption:
  • Pertussis toxin ADP-ribosylates Gi → permanently "off" → cAMP un-inhibited → ↑ cAMP → lymphocytosis, whooping cough.FA 2024 Micro — Bordetella

• Mechanism:
  • GRKs (G-protein-coupled receptor kinases) phosphorylate the activated receptor.
  • β-arrestin binds → uncouples receptor from G-protein.
  • Receptor internalized via clathrin-coated pits.

2 · Receptor tyrosine kinases (RTKs)

• Structure:
  • Single transmembrane (TM) domain.
  • Intracellular tyrosine kinase domain.

• Mechanism:
  • Ligand binding induces receptor dimerization.
  • Each receptor phosphorylates the OTHER on tyrosine residues (trans-autophosphorylation).
  • Phosphotyrosines become docking sites for SH2 or PTB domains on adaptor proteins.

• RAS-RAF-MEK-ERK (MAPK):
  • Drives growth, proliferation, differentiation.
• PI3K-AKT-mTOR:
  • Drives survival and metabolism.
  • GLUT4 translocation in response to insulin.Costanzo 6e ch 1; FA 2024 Biochem

• Structure:
  • α₂β₂ tetramer — two α chains outside, two β chains with kinase domains inside.
  • Pre-dimerized (doesn't need a second molecule to find a partner).
• Activation:
  • Insulin binding causes conformational change that activates the intracellular kinases.

• EGFR family:
  • HER1, HER2, HER3, HER4.
• Other growth-factor RTKs:
  • PDGFR, FGFR, VEGFR.
  • All are major cancer drug targets.

• Trastuzumab:
  • Targets HER2 in breast cancer.
• Imatinib:
  • Targets BCR-ABL fusion in CML.
• Cetuximab:
  • Targets EGFR in colorectal cancer.
• Bevacizumab:
  • Targets VEGF in many solid tumors.Lippincott Pharm 7e — oncology

3 · JAK-STAT (cytokine receptors)

• Structure:
  • Receptor itself has no intrinsic kinase activity.
  • Recruits JAK (Janus kinase) enzymes bound to the intracellular tail.

• Step-by-step:
  • Ligand → receptor dimerization → JAKs trans-phosphorylate each other.
  • JAKs phosphorylate the receptor tails creating docking sites for STATs (Signal Transducers and Activators of Transcription).
  • STATs get phosphorylated → dimerize → translocate to the nucleus.
  • STAT dimers bind DNA (GAS elements) → activate transcription.FA 2024 Immuno

• Classic endocrine / hematopoietic group (GPE/TPO):
  • GH, Prolactin, EPO, TPO.
• Immune cytokines:
  • All interleukins.
  • Interferons.
  • G-CSF.
• Metabolic:
  • Leptin.

• Tofacitinib:
  • Used in rheumatoid arthritis.
• Ruxolitinib:
  • Used in myelofibrosis and alopecia areata.

4 · Guanylyl cyclase receptors

• Membrane-bound GC:
  • Hormone (ANP, BNP, guanylin) binds extracellular domain.
  • Intracellular catalytic domain converts GTP → cGMP.
• Soluble GC:
  • NO (nitric oxide) freely diffuses across membranes.
  • Binds the heme of soluble guanylyl cyclase inside the cell → cGMP.Costanzo 6e ch 1

• cGMP → PKG:
  • Phosphorylates smooth-muscle proteins.
  • Leads to vasodilation and natriuresis.

• BNP:
  • Elevated in heart failure — diagnostic blood test.
• Nitroglycerin:
  • Releases NO → vasodilation in angina.
• Sildenafil:
  • Blocks PDE5 → preserves cGMP.
  • Acts in corpus cavernosum (erectile function) and pulmonary vasculature (pulmonary HTN).Lippincott Pharm 7e

5 · Nuclear receptor superfamily — intracellular signaling

• Three domains:
  • DNA-binding domain (DBD) — contains zinc fingers.
  • Ligand-binding domain (LBD).
  • Transactivation domains (AF-1 and AF-2).
• Where they bind DNA:
  • Specific hormone response elements (HREs) in the promoters of target genes.

• Type I — "steroid" receptors:
  • Members: GR (cortisol), MR (aldo), AR, PR, ER.
  • Resting location: cytoplasm, bound to HSP90 and HSP70.
  • On hormone binding: HSPs dissociate → receptor dimerizes → nuclear translocation → binds palindromic half-site HREs.
• Type II — "thyroid / vit D / retinoid" receptors:
  • Members: TR, VDR, RAR, PPAR.
  • Resting location: nucleus, pre-bound to DNA as heterodimers with RXR, holding a corepressor.
  • On hormone binding: swaps corepressor for a coactivator → transcription.Guyton 14e ch 74

• Classical "genomic" effects:
  • Minutes for transcription initiation.
  • Hours for full protein-level effects.
• Non-genomic effects:
  • Some steroid effects are rapid (seconds–minutes) via membrane-associated receptors.
  • Less well-characterized, but HY for vascular effects of aldosterone and estrogen.

6 · Quick decision tree — pick the receptor family

• Is the hormone lipid-soluble (steroid, thyroid, vit D, retinoid)?
  • → Intracellular receptor.
  • Cytoplasm for steroids; nucleus for thyroid / vit D / retinoid.
• Is it insulin, a growth factor, or a cancer-driver ligand?
  • → Receptor tyrosine kinase (RTK).
• Is it GH, prolactin, EPO, TPO, or a cytokine?
  • → JAK-STAT.
• Is it ANP, BNP, or NO?
  • → Guanylyl cyclase / cGMP pathway.
• Otherwise (most peptide hormones, most neurotransmitters):
  • → GPCR — then figure out Gs / Gi / Gq.

1 · Dose-response curves — the math you need

• Mathematical form:
  • Response = (Emax × [H]ⁿ) / (EC50ⁿ + [H]ⁿ)

• EC50:
  • The [hormone] producing 50% of maximal response.
  • Lower EC50 = more sensitive (curve shifted left).
• Emax:
  • The maximum achievable response (efficacy).
  • Drops when receptor machinery is globally impaired (few receptors OR broken downstream).
• Hill coefficient (n):
  • Steepness of the curve.
  • n>1 = positive cooperativity (classic example: hemoglobin–O₂).
  • Hormone-receptor binding is usually n≈1 (single-site binding).Rang & Dale Pharmacology 9e; Costanzo 6e ch 1

• Core idea:
  • Many tissues have more receptors than needed for Emax.
  • This creates high sensitivity even at low receptor occupancy.
• Practical consequence:
  • A tissue with 90% spare receptors can lose 80% of its receptors and still reach Emax — just with a right-shifted curve (↑EC50).
  • Explains why receptor down-regulation causes decreased sensitivity (EC50 shift) BEFORE decreased efficacy (Emax drop).Lippincott Pharm 7e

2 · Receptor regulation — multiple mechanisms, different timescales

• Mechanism:
  • GRKs phosphorylate activated receptors on C-terminal serine/threonine residues.
  • β-arrestin binds phosphorylated receptor → uncouples it from G-proteins.
• Classic example:
  • β2-receptor tachyphylaxis to albuterol in severe asthma overuse.FA 2024 Pharm — β-agonists

• Mechanism:
  • β-arrestin recruits clathrin adaptor proteins → receptor taken into endosomes.
  • Fate: recycled to plasma membrane (re-sensitized) OR trafficked to lysosome (degraded = down-regulated).
• Classic example:
  • Insulin receptor is classically internalized together with bound insulin → both degraded in lysosome.

• Direction of change:
  • Chronic hormone excess → ↓ receptor gene transcription → fewer new receptors made.
  • Chronic hormone deficit → ↑ receptor gene transcription → more receptors made.
• Classic example:
  • β-blocker withdrawal tachycardia: chronic β-blockade up-regulates β1 → sudden discontinuation → catecholamines meet a supersensitive receptor population.

3 · Down-regulation vs. resistance — clarify

• Down-regulation:
  • Fewer receptors on the cell surface.
• Resistance:
  • Decreased biologic response overall.
  • Can be from fewer receptors, impaired signal transduction, impaired downstream effector machinery, or all three.B&B Endo — Diabetes

• Initiating trigger:
  • Obesity / inflammation → ↑ TNF-α and free fatty acids.
• Molecular block:
  • Serine phosphorylation of IRS-1 (insulin receptor substrate-1).
  • Serine-phosphorylated IRS-1 cannot be tyrosine-phosphorylated properly → impaired PI3K/AKT signaling.
• Downstream consequence:
  • ↓ GLUT4 translocation in muscle.
  • ↓ Suppression of hepatic gluconeogenesis.
• Late compounding:
  • Chronic hyperinsulinemia eventually ALSO down-regulates receptors, compounding the problem.FA 2024 Endo; Pathoma ch 15

• Metformin:
  • Activates AMPK, partially bypasses insulin resistance.
  • Reduces hepatic gluconeogenesis.
• Thiazolidinediones (pioglitazone):
  • PPAR-γ agonists.
  • Improve insulin sensitivity at adipose tissue.
• GLP-1 agonists (semaglutide):
  • Enhance glucose-dependent insulin secretion.
  • Promote weight loss.Lippincott Pharm 7e

4 · Permissive, synergistic, antagonistic — used precisely

• Definition:
  • Hormone A is required for hormone B's effect, but A alone at physiologic levels has little direct effect.
• Cortisol permits catecholamine vasoconstriction:
  • Addisonian crisis patients don't respond to norepinephrine until hydrocortisone is given.
  • Cortisol up-regulates α1 receptor expression and preserves vascular responsiveness.Guyton 14e ch 78
• Thyroid hormone permits GH and catecholamines:
  • Permits GH effects on growth.
  • Permits catecholamine cardiac effects (↑ β-adrenergic receptor density in myocardium).

• Definition:
  • Two hormones together produce MORE than the sum of individual effects.
• Examples:
  • Glucagon + Epi + cortisol → hyperglycemia during stress (counter-regulatory response, more than additive).
  • FSH + estrogen in follicular development.

• Definition:
  • Opposing actions.
• Examples:
  • Insulin vs. glucagon on hepatic glucose output.
  • PTH vs. calcitonin on bone (PTH resorbs; calcitonin inhibits osteoclasts — clinically minor in humans).

5 · Hormone resistance syndromes — the "receptor is broken" diseases

• Complete androgen insensitivity (AIS):
  • Cause: loss-of-function mutation in androgen receptor (AR).
  • Presentation: 46,XY phenotypic female with testes, no axillary/pubic hair, blind-ending vagina, primary amenorrhea.FA 2024 Repro; Pathoma ch 15
• Laron syndrome:
  • Cause: GH receptor mutation.
  • Labs: ↑ GH (no negative feedback), ↓ IGF-1.
  • Clinical: short stature; paradoxically protective against cancer and diabetes.

• Pseudohypoparathyroidism type 1A (Albright hereditary osteodystrophy):
  • Cause: Gsα mutation → PTH doesn't signal despite high PTH levels.
  • Features: hypocalcemia + short 4th/5th metacarpals + round facies + mental disability.
• Nephrogenic diabetes insipidus:
  • Cause: V2 receptor mutation (X-linked) OR aquaporin-2 mutation.
  • Clinical: kidney cannot respond to ADH → polyuria, dilute urine.

• Thyroid hormone resistance:
  • Cause: TRβ mutation.
  • Labs: ↑ T3/T4 with inappropriately normal or ↑ TSH.
  • Clinical: often presents with goiter and ADHD-like features.

• Leptin receptor deficiency:
  • Cause: loss-of-function mutation in leptin receptor.
  • Clinical: severe early-onset obesity despite high leptin levels (no satiety signaling).

6 · Tachyphylaxis (rapid tolerance) — how and why

• β2-agonist overuse (albuterol) in asthma:
  • Receptor desensitization by GRK / β-arrestin.
  • Receptor internalization.
  • Net effect: diminished bronchodilation despite more puffs.
• Opioid tolerance:
  • μ-opioid receptor down-regulation plus post-receptor adaptations.
  • Requires higher doses over time for equivalent analgesia.
• Nitrate tolerance:
  • Depletion of intracellular sulfhydryl groups needed to convert nitroglycerin to NO.
  • Requires a nitrate-free interval each day to restore efficacy.Lippincott Pharm 7e — cardiovascular

• Big picture:
  • These mechanisms are variations of the same theme: receptors (and the systems behind them) adapt to keep a cell's response within a useful physiologic range.