Dr. Hesham Aly Salem Dept. of Pharmacology & Toxicology F O P C U 2016

Transcription

1 Dr. Hesham Aly Salem Dept. of Pharmacology & Toxicology F O P C U 2016

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3 Release Acetylcholine (Ach) 1. All motor nerves leaving CNS: 1. All preganglionic fibers (sympathetic & parasympathetic) 2. Somatic nerves 2. All postganglionic parasympathetic fibers 3. Postganglionic sympathetic fiber supplying sweat glands Co-transmission Release of other mediators together with the Ach to modulate its effect e.g. ATP, NO, Proteoglycan

4 Acetate Choline Ca ++ ChT Choline AcCoA ChAT CoA Ach Ach Ca ++ Ach ChEs

5 Choline (precursor) uptake Ach synthesis Ach granular uptake & storage Ach (+co-transmitters) release following NAP Ach cholinoceptors interaction Ach degradation

6 Cholinesterase True (Acetylcholinesterase) Synapses RBCs CNS Pseudo (Butyrocholinesterase) Plasma liver

7 Binds Ach Nicotinic (N) Muscarinic (M) N m MEP N n 1. autonomic ganglia 2. adrenal medulla M 1 M 2 M 3 M 4 M 5

8 Some M receptors present in all autonomic ganglia

9 Ionotropic receptors Nicotinic receptor ION Ach ION Ach Ach Channels open Increase Na influx & K efflux Depolarization of postsynaptic membrane in MEP or. Contraction Autonomic ganglia or Activation of postganglionic fiber Adrenal medulla...release of A, NA ION Excitatory effects of Ach

10 1. In all effector organs supplied by cholinergic fibers (expt. Sk.m.) 2. In some organs not supplied by cholinergic fibers (heart ventricle & vascular endothelium & vascular sm.m.) 3. In autonomic ganglia M 1 M 2 M 3 In gastric parietal cells In autonomic ganglia In Heart In smooth m. In exocrine glands In smooth m. GPCR

11 GPCR M 1 M 3 M 2 Gq Gi

12 Ach Cell membrane M1,3 Gq PL-C PIP 2 DAG IP 3 Stimulation of Ca channel Ca influx Inhibition of K channel K efflux Ca ++ Stores (ER) Ca ++ Release PKC Excitatory effects of Ach Contraction Secretion.. Phosphorylation of various proteins

13 Ach Cell membrane M2 Gi AC Inhibition of Ca channel Ca influx camp Stimulation of K channel K efflux Inhibitory effects of Ach

14 Nicotinic Muscarinic N m N n M 1 M 2 M 3 Main Location MEP ganglia+ adrenal medulla Exocrine glands + ganglia Heart + Sm. m. Exocrine glands + Sm. m. Receptor Type Ionotropic Metabotropic (GPCR) Gq: PL-C Gi: AC Gq: PL-C Receptor Activation Na,K permeability IP3, DAG camp IP3, DAG Main Effect Excitatory Excitatory Inhibitory Excitatory (except in sphincters) Antagonist NMB Ganglionic blockers Muscarinic blockers (Atropine)

15 Receptor Effect Atria Heart Ventricles M 2 Inhibition SAN Bronchi Eye (ciliary m., circular m.) M 3 M 3 Contraction Contraction GIT Smooth m. Sphincters M 3 M 3 Contraction Relaxation Urinary Bladder Detrusor m. Sphincters M 3 M 3 Contraction Relaxation Lacrimal Glands Salivary Bronchial M 3 Secretion Sweat GIT M 1 M 3 Secretion

16 Structures received PARASYMPATHETIC supply Smooth muscles Cardiac muscles Exocrine glands Except Except Except 1. Radial m. of iris 2. Pilomotor m. of skin 3. Vascular sm.m. 4. Pregnant uterus 1. Ventricle myocytes 1. Sweat gland Structures NOT received PARASYMPATHETIC supply

17 Heart Bronchi Eye GIT Exocrine Glands Urinary Bladder Sex organ Atria SAN Smooth m. Glands Circular muscle Ciliary muscle Smooth m. Sphincters Glands Salivary Lacrimal Detrusor m. Sphincters Inhibition Inhibition Contraction Secretion Miosis Contraction Motility Relaxation Secretion Secretion Secretion Contraction Relaxation Erection

18 Drugs that are agonists of M receptors peripherally Parasympathetic-like actions Direct Indirect Choline Esters Natural Alkaloids Synthetic Anticholinesterase 1. Acetylcholine 2. Methacholine 3. Carbachol 4. Bethanechol 1. Muscarine 2. Pilocarpine 1. Cevimeline

19 Quaternary ammonium (permanently charged) Relatively insoluble in lipids Poorly absorbed & Poorly distributed into CNS. Hydrolyzed in GIT less active p.o. β-methyl group (methacholine, bethanechol) reduces N effects + resistant to hydrolysis by pseudoches longer durations of action. Carbamoyl group (carbachol, bethanechol) resistant to hydrolysis by true & pseudoches longer durations of action.

20 Ach & methacholine are not effective systemically.why? Carbachol is not used systemically..why?

21 Muscarine is quaternary amine Less completely absorbed from GIT NB: Muscarine (in certain mushrooms) is toxic when ingested & even enters the brain. Pilocarpine is tertiary amine Well absorbed from most sites of administration

22 Hydrolysis by ChEs N effect M effect Acetylcholine True & Pseudo + M 1 =M 2 =M 3 Methacholine True - M 2 >M 1,3 Carbachol - + M 1,3 >M 2 Bethanechol - - M 1,3 >M 2 Pilocarpine - - M 1,3 >M 2 Cevimeline - - M 3

23 Cardiovascular: Pharmacological Actions Heart: (M 2 ) HR (bradycardia: -ve chronotropic effect) force of contraction (-ve inotropic effect) AV conductivity (-ve dromotropic effect) Blood v.: (M 3 ) VD induced by NO released by intact v. endothelium

24 The vascular effect of M 3 -agonists Ach M3 Endothelial Cell Vascular Smooth m.

25 Cardiovascular: Pharmacological Actions Heart: (M 2 ) HR (bradycardia: -ve chronotropic effect) force of contraction (-ve inotropic effect) AV conductivity (-ve dromotropic effect) Blood v.: (M 3 ) VD induced by NO released by intact v. endothelium If endothelium is damaged (as in atherosclerosis): Ach vasoconstriction due to a direct effect on vascular sm.m. in which activation of M 3 receptors stimulates IP 3 production & releases intracellular Ca

26 Cardiovascular: Blood Pressure: Pharmacological Actions Direct M-effect HR, VD BP However, the net effect on BP depends on: 1. Reflex sympathetic discharge 2. Stimulation of presynaptic M receptors on Sympathetic n. inhibition of NE release Parasympathetic n. inhibition of Ach release 3. Stimulation of ganglionic M 1 receptors on sympathetic discharge Stimulates NE release

27 Pharmacological Actions Eye: (M 3 ) Contraction of circular m. or iris Miosis Contraction of ciliary m. Accommodation for near vision Constricted iris widens the filtration angle Contracted ciliary m. opens trabecular meshwork Both effects facilitate aqueous humor outflow into the canal of Schlemm, which drains the anterior chamber IOP

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29 Pharmacological Actions Exocrine glands: (M 1,3 ) All secretions (lachrymal, salivary, GIT, bronchial, sweat) Smooth muscles: (M 3 ) Contraction of all smooth muscles Bronchial bronchoconstriction GIT gut motility Bladder voiding Relaxation of: Sphincters (due to activating hyperpolarizing K channels) Blood vessels (due to NO release VD)

30 Pharmacological Actions N-Actions: NOT REQUIRED (Cholinergic but not parasympathomimetic actions) 1. Skeletal muscles (NMJ): (N m ) Twitches if given i.v. 2. Autonomic ganglia (& adrenal medulla): (N n ) Complex effects with predominance of sympathetic actions on CVS and parasympathetic actions on others.

31 Simple alcohol Carbamates Organophosphates Quaternary ammonium Quaternary ammonium Tertiary amine Quaternary ammonium Edrophonium Neostigmine Physostigmine Echothiophate Pyridostigmine Distigmine Ambenonium Quaternary ammonium: Less lipid solubility Poor absorption - Less CNS distribution Tertiary amines: More lipid solubility Well absorption CNS distribution

32 Mechanism of Action Compete with Ach for active sites on ChEs Steps of Ach hydrolysis by ChEs: 1. Ach binds to the enzyme s active site 2. Ach is hydrolyzed free choline released 3. The ChEs is covalently bound to acetyl group (acetylated enzyme) 4. The covalent acetyl-enzyme bond is split, with the addition of water (hydration) free enzyme. The entire process occurs in approximately 150 ms.

33 Hydration CH 3 COOH H 2 O Acetylated ChEs AntiChEs Compete with Ach for active sites on ChEs

34 Mechanism of Action Simple alcohol antiches: Not form covalent bond with the enzyme (but form reversible H-bonds) Short duration inhibition (2 10 min). Carbamate antiches: Form covalent bond with the enzyme (carbamylated active site) more resistant to the hydration step Intermediate duration inhibition (30 min - 6 h) Organophos. antiches: Form covalent bond with the enzyme (phosphorylated active site) extremely resistant to the hydration step Long duration inhibition (hundreds of hrs).

35 Acetyl Acetylated ChEs: Immediate Hydration (150 ms) S. alco. Carbamyl Phosphoryl Simple alcohol Carbamylated ChEs Phosphorylated ChEs Short duration (2-10 min) Slow Hydration ( 30 min 6h) Classification according to duration of action Very slow Hydration (~100 h)

36 Aging Phosphoryl Phosphoryl non-enzymatic hydrolysis Phosphorylated ChEs Aged Phosphorylated ChEs Classification according to reversibility of action

37 Aging Phosphorylated ChEs is very slowly hydrolyzed by water spontaneous reactivation When one of the two R groups of OP on OP-ChEs complex is lost due to non-enzymatic hydrolysis The phosphorylated AchEs is Before ageing, a strong nucleophiles like pralidoxime are able to break the phosphorus-enzyme bond and can be used as cholinesterase reactivator. However, once ageing occurs inhibition becomes permanent till synthesis of new enzyme.

38 Pharmacological Actions Inhibit true & pseudo ChEs enzymes Inhibit Ach degradation Accumulation of Ach M actions N actions CNS actions (For centrally distributed antiches)

39 Simple alcohol Carbamates Organophosphates Quaternary ammonium Quaternary ammonium Tertiary amine Quaternary ammonium Edrophonium Neostigmine Physostigmine Echothiophate Pyridostigmine Distigmine Ambenonium N m action > M action GIT & UT > Eye M action > N m action Eye > GIT & UT

40 Central effects: Pharmacological Actions In low concentrations activation & alerting response In higher concentrations generalized convulsions, may be followed by coma & respiratory arrest. Eye, RT, GIT, UT: Similar to the effects of the direct-acting parasympathomimetics

41 NMJ: Pharmacological Actions Therapeutic doses strength of contraction, especially in weakened muscles due to competitive NMBs or myasthenia gravis. Neostigmine have an additional direct Nm agonist effect. Toxic doses Ach accumulation twitches & fasciculation followed by depolarizing neuromuscular blockade (cholinergic crisis) paralysis

42 Cardiovascular: Heart: Pharmacological Actions Parasympathetic effect -ve chrono, dromo, ionotropic effects. Blood v. & Blood pressure: At normal dose minimal effects. At moderate doses Hypertention (Due to stimulation of sympathetic ganglia peripherally and/or sympathetic centers centrally) At high (toxic) doses Hypotension (Due to marked bradycardia and fall in CO).

43 To be therapeutically effective systemically the drug must be: Lack of N action as it is not preferred due to complex effects & un-wanted CV effects. Less selective toward M 2 receptors to reduce un-wanted CV effects. Not hydrolyzed by ChEs to be active, otherwise it will be rapidly hydrolyzed Very short duration of action

44 Topical use 1. Glaucoma 2. Iris/lens adhesion 3. Counteract mydriatic effect of atropine 4. Accommodative esotropia

45 Systemic use Urinary retention (postoperative, postpartum, neurogenic bladder). Systemic use: 1. Postoperative ileus (atony or paralysis of stomach or bowel following surgical manipulation) 2. Congenital megacolon 3. Dry mouth: Associated with Sjögren s syndrome (AID vs exocrine salivary & lacrimal glands) Caused by radiation damage of the salivary glands Ganglionic blocker-induced.

46 Systemic use 1. Treatment & diagnosis of myasthenia gravis 2. Treatment (antidote & reversal) for competitive NMB overdose Autoimmune antibodies against N m receptors Muscular weakness & rapid fatigability & inability to produce sustained contractions & drooping eyelids

47 Diagnosis: Edrophonium (i.v.): (short duration AntiChEs) Used to differentiate between myasthenia gravis & cholinergic crisis HOW? Treatment: 1. Neostigmine, Pyridostigmine, Ambenonium:(intermediate duration AntChEs) Protect Ach from hydrolysis by ChEs thus allowing Ach to diffuse further within synaptic cleft of MEP chance of Ach to find unoccupied (hidden; spare) receptors. 2. Immunosuppressant drugs 3. Surgical removal of thymus gland 4. Plasma exchange to remove antibodies (transient effectiveness)

48 Systemic use Topical use (Hair lotion) Systemic use Physostigmine is used because it enters CNS and reverses central as well as peripheral signs of muscarinic blockade Topical use Systemic use

49 Diseases Direct Indirect Use Glaucoma - Iris adhesion Accommodative esotropia Stimulate hair growth Diagnosis of asthma Pilocarpine Carbachol Ach Pilocarpine Methacholine Physostigmine Echothiophate Topical Urinary retention Bethanechol Neostigmine Systemic Paralytic ileus Bethanechol Neostigmine Edrophonium Dry mouth Myesthenia gravis - diagnosis Myesthenia gravis - treatment NMB reversal Atropine antidote Paroxysmal SVT Pilocarpine Cevimeline Edrophonium Neostigmine, Pyridostigmine, Ambenonium Neostigmine Physostigmine* Edrophonium*