PHA 334 - ANS (Lecture #3) AFTER QUIZ #1

G proteins (SCAG)

Gq Increases intracellular Ca2+ via IP3/DAG Smooth muscle contracts Cardiac muscle contracts Glands increase secretion Gs Increases cAMP Smooth muscle relaxes Cardiac muscle increases HR and contractility Adipose increases lipolysis Glands increase secretion Gi Decreases cAMP Smooth muscle contracts Cardiac muscle decreases HR and contractility Adipose decreases lipolysis Glands decrease secretion

Gs Gi Gq effects depend on cell type

Gs activation relaxes smooth muscle (β2 β3) contracts cardiac muscle (β1) Gi activation contracts smooth muscle (α2) relaxes cardiac muscle (M2) Gq activation contracts smooth muscle (α1 M3) contracts cardiac muscle (α1)

Ligand-gated ion channels

Neurotransmitter binds opens channel Rapid response Nicotinic receptors Found at ganglia

GPCR receptors

Seven transmembrane receptors Activate G proteins Slower longer-lasting responses Found on SCAG

G protein activation

1. NT binds to a GPCR (which is a guanine nucleotide exchange factor, GEF) which binds to the G protein complex—GDP releases from α subunit

2. α subunit binds GTP and recruits downstream effectors

3. GTP is hydrolyzed by a GTPase activating protein (GAP)

4. the inactive, GDP-bound state

Second messengers

cAMP cGMP DAG IP3 Ca2+ regulate enzymes ion channels gene expression

Kinases

Add phosphate groups to proteins using ATP

Phosphatases

Remove phosphate groups from proteins

Phosphodiesterases (PDEs)

Break down cAMP and cGMP 11 isoforms

Effect of PDE inhibition

Increases cAMP and cGMP Smooth muscle relaxation

cAMP pathway

Activates PKA Decreases Ca2+ Inhibits MLCK Leads to relaxation

cGMP pathway

Activates PKG Promotes MLCP Leads to relaxation

Smooth muscle contraction mechanism

Ca2+ + calmodulin → MLCK activation → myosin phosphorylation → contraction

Smooth muscle relaxation mechanism

1⃣ Decrease MLCK activity (cAMP pathway – Gs)

Seen with:

• β2

• β3

• Some Gs receptors

Mechanism:

1. ↑ cAMP

2. Activates PKA

3. PKA ↓ intracellular Ca²⁺

4. PKA inhibits MLCK

5. Less myosin phosphorylation
   → Relaxation

Think:
👉 Gs → cAMP → PKA → inhibits MLCK → relaxation

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2⃣ Increase MLCP activity (cGMP pathway – NO)

Seen with:

• Nitric oxide (NO)

• Endothelium-dependent M3 vasodilation

Mechanism:

1. NO → ↑ cGMP

2. Activates PKG

3. PKG activates MLCP (myosin light chain phosphatase)

4. MLCP dephosphorylates myosin
   → Relaxation

Think:
👉 cGMP → PKG → MLCP → dephosphorylate → relax

Acetylcholine release

PNS and SNS preganglionic neurons All PNS postganglionic neurons

Norepinephrine release

SNS postganglionic neurons except sweat glands

ACh termination

Acetylcholinesterase

NE termination

Norepinephrine transporter (NET)

PNS characteristics

Local short-lived effects ACh rapidly degraded Low divergence

SNS characteristics

Diffuse long-lasting effects NE slower degradation High divergence Epinephrine hormone release

Nicotinic receptors

Ion channel Excitatory Always stimulatory

NG receptor

Ganglia

NN receptor

Neuronal

NM receptor

Neuromuscular junction

Muscarinic receptors overview

PNS GPCR SSLUDDD-C Located on SCAG M1 M3 M5 = Gq M2 M4 = Gi

M1 M3 M5 receptors

M1 Gq Gland secretion

M3 Gq Smooth muscle contraction and secretion Sweat glands SNS exception

M5 Gq CNS role

M2 M4 receptors

M2 Gi Decreases heart rate SA AV nodes

M4 Gi CNS inhibitory

SSLUDDD-C

◦Slow heart rate (effect generated by cardiac muscles “C”)

◦Salivation (effect generated by glands, “G”)

◦Lacrimation/crying (effect generated by “G”)

◦Urination (effect generated by smooth muscles, “S”)

◦Digestion (effect generated by S and G)

◦Defecation (effect generated by S)

◦Dilation of sexual organs (effect generated by S, indirectly)

◦Constriction of bronchioles & pupils (effect generated by S)

Adrenergic receptors overview

SNS GPCR Alpha 1 = Gq Alpha 2 = Gi Beta 1 2 3 = Gs

Alpha 1 receptor

Gq Smooth muscle contraction Vasoconstriction

α1A receptor

Gq Bladder neck prostate contraction

α1B receptor

Gq Vasoconstriction arteries veins Cardiac growth

α1D receptor

Gq Vasoconstriction aorta coronary arteries

Alpha 2 receptor

Gi Inhibits NE release Decreases sympathetic outflow

α2A receptor

Gi Presynaptic inhibits NE Hypotension sedation analgesia

α2B receptor

Gi Postsynaptic smooth muscle contraction

α2C receptor

Gi Presynaptic inhibits catecholamine release adrenal medulla

Beta receptors

Gs Increase cAMP

β1 receptor

Gs Heart increased HR contractility Kidney renin release

β2 receptor

Gs Smooth muscle relaxation Bronchodilation Vasodilation

β3 receptor

Gs Adipose lipolysis Bladder relaxation

Adrenal medulla exception

Preganglionic ACh directly to adrenal medulla Releases epinephrine norepinephrine

Dual innervation

Organs receive both SNS and PNS input Opposing effects

Presynaptic regulation

M2 M4 alpha 2 inhibit neurotransmitter release

SKAVAS

Skin Kidneys Adipose Vasculature Adrenal glands Sweat glands

Net physiological response depends on

Receptor density Receptor affinity Cell type Signal pathway Drug concentration

Dilator pupillae

SNS α1 Gq Contraction pupil dilation

Sphincter pupillae

PNS M3 Gq Contraction pupil constriction

Ciliary muscle

PNS M3 Contracts near vision SNS β2 Relaxes distance vision

M3 in blood vessels

Located on endothelium NOT smooth muscle

M3 vasodilation mechanism

M3 → Gq → Ca2+ → NO synthase → NO → cGMP → relaxation

Nitric oxide (NO)

Paracrine molecule Causes smooth muscle relaxation

Endothelium requirement

ACh causes vasodilation only if endothelium intact

SNS eyes

α1 Pupils dilate Lenses flatten

SNS skin

α1 Piloerection Emotional sweating M3 heat sweating

SNS salivary glands

α1 Inhibit secretion β thick saliva

SNS blood vessels

α1 vasoconstriction β2 vasodilation

SNS lungs

β2 relax bronchi decrease mucus

SNS heart

β1 increase HR contractility

SNS GI sphincters

α1 contract sphincters

SNS GI motility

β2 decrease motility

SNS pancreas

α2 decrease insulin β2 increase glucagon

SNS liver

α1 β2 increase glycogenolysis gluconeogenesis

SNS gallbladder

β2 decrease bile secretion

SNS adrenals

NN release epinephrine norepinephrine

SNS kidneys

β1 increase renin

SNS bladder

β2 β3 relax detrusor α1 contract sphincter

SNS genitals

α1 ejaculation

PNS eyes

M3 pupils constrict lenses bulge

PNS glands

M1 M3 salivation lacrimation

PNS heart

M2 decrease HR

PNS lungs

M3 bronchoconstriction mucus

PNS GI

M3 increase motility relax sphincters

PNS pancreas

M3 pancreatic secretion insulin glucagon somatostatin

PNS gallbladder

M3 bile secretion

PNS bladder

M3 contract detrusor relax sphincter

PNS genitals

M3 vasodilation erection

Pilocarpine (Vuity Salagen)

Muscarinic receptor agonist Causes miosis Contracts ciliary muscle Used for glaucoma xerostomia

Methacholine (Provocholine)

Muscarinic receptor agonist Causes bronchoconstriction Used for asthma test

Atropine

Muscarinic receptor antagonist Blocks PNS Mydriasis increased HR decreased secretions Antidote organophosphate poisoning

Scopolamine

Muscarinic receptor antagonist Motion sickness CNS effects

Pyridostigmine (Mestinon)

Acetylcholinesterase inhibitor Increases ACh Treats myasthenia gravis

Varenicline (Chantix)

Nicotinic receptor partial agonist Smoking cessation

Nicotine (Nicorette)

Nicotinic receptor agonist

Prazosin (Minipress)

Alpha 1 antagonist Vasodilation decreases BP

Clonidine (Catapres)

Alpha 2 agonist Decreases sympathetic outflow

Guanfacine (Tenex)

Alpha 2 agonist ADHD decreases SNS

Lofexidine (Lucemyra)

Alpha 2 agonist Opioid withdrawal decreases SNS

Propranolol

Nonselective beta antagonist Decreases HR BP

Metoprolol

Beta 1 selective antagonist Decreases HR

Albuterol (Ventolin)

Beta 2 agonist Bronchodilation SABA

Formoterol (Perforomist)

Beta 2 agonist Long acting bronchodilation LABA

Mirabegron (Myrbetriq)

Beta 3 agonist Relaxes bladder

Sympathomimetics

Increase SNS activity Methamphetamine Propylhexedrine Pseudoephedrine increase NE

Fluticasone (Flovent)

Inhaled corticosteroid Anti-inflammatory asthma COPD

Fluticasone + Salmeterol (Advair Diskus)

◦Anti-inflammatory drug for chronic asthma or COPD

◦Inhaled corticosteroid (ICS)

◦Often used in combination therapy, e.g. fluticasone+salmeterol (beta2 agonist) (Advair Diskus)

Budesonide + Formoterol (Symbicort)

ICS + beta2 agonist asthma COPD

Trelegy

Combination steroid + muscarinic antagonist + beta2 agonist

Roflumilast (Daliresp)

PDE4 inhibitor Increases cAMP Smooth muscle relaxation COPD

Ensifentrine (Ohtuvayre)

PDE3 PDE4 inhibitor Increases cAMP cGMP Smooth muscle relaxation