Chapter 3 · Module 3 of 4 · Pharmacodynamics

Signal Transduction and Receptor Regulation

G protein second messenger pathways, receptor desensitization and downregulation, and the clinical consequences of abrupt drug discontinuation

The four major G protein pathways differ by effector: Gs activates adenylyl cyclase → cAMP → PKA; Gi inhibits adenylyl cyclase, opposing Gs; Gq activates PLC → IP₃ (calcium release) + DAG → PKC; the nitric oxide/cGMP pathway relaxes smooth muscle — organic nitrates and PDE5 inhibitors act here, and combining them causes severe hypotension
Receptor desensitization (seconds to minutes) involves GRK phosphorylation and beta-arrestin uncoupling from G protein; downregulation (hours to days) involves receptor internalization and degradation — recovery requires new receptor synthesis; tachyphylaxis occurs within minutes to hours via desensitization or substrate depletion (nitrates, indirect sympathomimetics)
Chronic antagonist exposure causes receptor upregulation — abrupt discontinuation exposes the surplus receptors to endogenous ligand and produces rebound responses; beta-blocker withdrawal causes rebound tachycardia, hypertension, and angina with risk of myocardial infarction in coronary artery disease
Clonidine (alpha-2 agonist) suppresses central sympathetic outflow; abrupt withdrawal unmasks a sympathetic surge causing severe rebound hypertension 18–24 hours later; both beta-blockers and clonidine must be tapered, never stopped abruptly
Opioid tolerance involves cellular cAMP superactivation — chronic Gi inhibition is compensated by adenylyl cyclase upregulation; withdrawal removes Gi input, leaving excess adenylyl cyclase unopposed → cAMP surge causing the autonomic and somatic withdrawal syndrome; clonidine partially suppresses withdrawal by reducing sympathetic outflow through Gi

Abbreviations: cAMP = cyclic adenosine monophosphate · cGMP = cyclic guanosine monophosphate · PKA = protein kinase A · PKC = protein kinase C · PLC = phospholipase C · IP₃ = inositol trisphosphate · DAG = diacylglycerol · GRK = G protein-coupled receptor kinase · NO = nitric oxide · PDE5 = phosphodiesterase 5 · HPA = hypothalamic-pituitary-adrenal · ACTH = adrenocorticotropic hormone

Section 1 — G Protein Second Messenger Pathways

Section 1

The Four Major G Protein Signaling Pathways

Gs Pathway

Gs → cAMP

Gs activates adenylyl cyclase
Adenylyl cyclase synthesizes cAMP
cAMP activates PKA
PDE terminates signal (inhibited by milrinone, theophylline)
Examples: beta-adrenergic agonists, glucagon, vasopressin V2

Gi Pathway

Gi → Inhibits cAMP

Gi inhibits adenylyl cyclase
Reduces cAMP and PKA activity
Opposes Gs-mediated effects
Examples: opioids (mu, delta, kappa), M2 muscarinic (↓ heart rate), alpha-2 agonists (clonidine), dopamine D2

Gq Pathway

Gq → Ca²⁺ + PKC

Gq activates PLC
PLC cleaves to IP₃ + DAG
IP₃ releases Ca²⁺ from endoplasmic reticulum
DAG activates PKC
Examples: alpha-1 adrenergic (vasoconstriction), M1/M3 muscarinic, angiotensin II AT1 receptor

NO/cGMP Pathway

NO → cGMP

NO activates soluble guanylyl cyclase
cGMP activates PKG → smooth muscle relaxation
Organic nitrates: prodrug NO donors
PDE5 inhibitors (sildenafil, tadalafil): prolong cGMP
Nitrates + PDE5 inhibitors: risk of severe hypotension — contraindicated

Section 2 — Receptor Desensitization and Downregulation

Section 2

Time Course and Mechanisms of Reduced Receptor Responsiveness

Process	Time Scale	Mechanism	Clinical Consequence
Desensitization	Seconds to minutes	GRK phosphorylates the receptor; beta-arrestin binds and uncouples G protein; receptor remains on cell surface but is non-functional	Waning response during continuous agonist exposure; diminished bronchodilation with chronic beta-agonist use in asthma
Downregulation	Hours to days	Receptor internalization into endosomes; degradation in lysosomes; reduced surface receptor number	Sustained loss of receptor-mediated effect; recovery requires new receptor synthesis over days
Tachyphylaxis	Minutes to hours	Rapid tolerance from desensitization or substrate depletion (nitrates deplete endogenous thiols; ephedrine exhausts norepinephrine stores)	Nitrates: 24-hour tolerance; require 8–12 hour nitrate-free interval to restore responsiveness. Ephedrine: progressive loss of pressor effect with repeated doses

Section 3 — Tolerance, Supersensitivity, and Withdrawal

Section 3

Clinically Important Withdrawal Syndromes

Antagonist Withdrawal

Beta-Blocker Withdrawal Syndrome

Chronic beta-blockade → beta-1 receptor upregulation
Abrupt stop: upregulated receptors + circulating catecholamines = rebound
Rebound tachycardia, hypertension, angina, arrhythmia
Risk: acute myocardial infarction in coronary artery disease
Management: taper over 1–2 weeks; never stop abruptly

Agonist Withdrawal

Clonidine Rebound Hypertension

Clonidine: central alpha-2 agonist → reduces sympathetic outflow via Gi
Chronic use → compensatory sympathetic upregulation
Abrupt stop: sympathetic surge unmasked
Onset 18–24 hours: severe hypertension, tachycardia, diaphoresis
Management: reinstate and taper; never stop abruptly

Opioid Tolerance and Withdrawal

Adenylyl Cyclase Superactivation

Chronic opioids → Gi inhibits adenylyl cyclase → cell compensates by upregulating adenylyl cyclase
Withdrawal: adenylyl cyclase overexpressed and unopposed → cAMP surge
Symptoms: lacrimation, rhinorrhea, piloerection, diarrhea, tachycardia, dysphoria
Clonidine (Gi → ↓ cAMP) partially suppresses withdrawal symptoms
Methadone/buprenorphine: sustained low-level mu activation prevents withdrawal

Glucocorticoid Withdrawal

Adrenal Insufficiency

Chronic supraphysiological glucocorticoids → HPA axis suppression
Adrenal cortex atrophies without ACTH stimulation
Abrupt stop after >3 weeks: adrenal insufficiency, hypotension, circulatory collapse
Management: slow taper; stress-dose steroid coverage during illness or surgery

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