Anesthesia Pharmacology: Autonomic Pharmacology: Cholinergic Drugs

 

Antimuscarinic Effects on Organ Systems

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Muscarinic Type M2

Agonist

Antagonist

Tissue (Heart)

Responses

Molecular Aspects

Methacholine

Atropine

SA node

decreased phase 4 depolarization; hyperpolarization

  • K+ channel activation (hyperpolarizing) through -γ Gi subunits*.

  • Gi -mediated inhibition of adenylyl cyclase* (negative inotropism)

    (Gi can inhibit directly Ca2+ channel opening)

     

Atrium

decreased contractility; decreased AP duration

AV node

decreased conduction velocity

Ventricle

decreased contractility

* K+ channel activation has a hyperpolarizing effect. The increase in membrane-potential (more negative) moves the membrane-potential away from threshold. The consequence of this effect is to take longer for diastolic depolarization (phase 4) to reach threshold in SA nodal pacemaker cells. Accordingly, muscarinic receptor activation decreases rate and muscarinic receptor blockade increases rate.

Muscarinic Type M3

Agonists

Antagonists

Tissue

Responses

Molecular Aspects

Acetylcholine,

Methacholine

Atropine

Smooth muscle

Contraction

  • Phospholipase C (PLC) stimulation through Gq/11

  • Formation: (IP3 ) + diacylglycerol (DAG): increased cytosolic Ca2+

Secretory glands

Increased Secretion

Adapted from Table 6-2: Lefkowitz, R.J, Hoffman, B.B and Taylor, P. Neurotransmission: The Autonomic and Somatic Motor Nervous Systems, In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics, (Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc.,1996, p119.

Brown, J.H and Taylor, P. Muscarinic Receptor Agonists and Antagonists In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) The McGraw-Hill Companies, Inc.,1996, pp.149-159.

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