Cholinergic Antagonist Course Coordinator Jamaluddin Shaikh, Ph.D. School of Pharmacy, University of Nizwa Lecture-16 October 31, 2011 Cholinergic Antagonist Cholinergic antagonists bind to cholinoceptors, but do not trigger receptor-mediated intracellular effects Types of antagonists:
1. Selectively block muscarinic synapses of the parasympathetic nerves 2. Ganglionic blockers, show a preference for the nicotinic receptors of the sympathetic and parasympathetic ganglia 3. Neuromuscular blocking agents, interfere with transmission of efferent impulses to skeletal muscles Peripheral Nervous System Cholinergic Antagonists Antimuscarinic agents Atropine
Mecamylamine Neuromuscular Blocker Pancuronium Tubocurarine Atracurium Mivacurium Succinylcholine Antimuscarinic agents: Atropine A tertiary amine, has high affinity for muscarinic receptors, preventing ACh from binding
Atropine acts both centrally and peripherally General actions last about 4 hours Atropine: Actions Eye: Blocks all cholinergic activity, resulting in mydriasis, unresponsiveness to light. In narrow-angle glaucoma, intraocular pressure may rise dangerously Gastrointestinal (GI): Reduce activity of the GI tract Although gastric motility is reduced, hydrochloric acid production is not significantly affected.
Urinary system: Reduce hypermotility states of the urinary bladder Atropine: Actions, continued.. Cardiovascular: At low doses: decrease cardiac rate (bradycardia) At higher doses: cardiac rate increases modestly Secretions: Blocks the salivary glands, producing a drying effect on the oral mucous membranes Sweat and lacrimal glands are also affected
Atropine: Therapeutic Use Ophthalmic: Topical atropine exerts mydriatic effect May induce an acute attack of eye pain due to sudden increases in eye pressure in individuals with narrow-angle glaucoma Antidote for cholinergic agonists: Used for the treatment of overdoses of cholinesterase inhibitor insecticides and some types of mushroom poisoning Antisecretory: Used as an antisecretory agent to block secretions in the upper
and lower respiratory tracts prior to surgery Atropine: Pharmacokinetics & Adverse Effects Pharmacokinetics: Readily absorbed Partially metabolized by the liver Eliminated primarily in the urine Half-life is about 4 hours
Adverse effects: Dry mouth, blurred vision, tachycardia, and constipation Effects on CNS include restlessness, confusion, hallucinations, Children are sensitive to the rapid increases in body temperature Scopolamine Tertiary amine alkaloid, produces effects like atropine Therapeutic uses: Therapeutic use is limited to prevention of motion and to blocking short-term memory Pharmacokinetics and adverse effects:
Similar to those of atropine Ipratropium A quaternary derivative of atropine Useful in treating asthma in patients who are unable to take adrenergic agonists Beneficial in the management of chronic obstructive pulmonary disease. It is inhaled for these conditions Does not enter the systemic circulation or the CNS Cholinergic Antagonists Antimuscarinic agents
Atracurium Mivacurium Succinylcholine Ganglionic Blockers: Nicotine A component of cigarette smoke No therapeutic benefit and is deleterious to health Depending on the dose, nicotine depolarizes autonomic ganglia, resulting first in stimulation and then in paralysis of all ganglia The stimulatory effects include increased blood pressure and cardiac rate At higher doses, the blood pressure falls because of
Neuromuscular Blocker Pancuronium Tubocurarine Atracurium Mivacurium Succinylcholine Neuromuscular Blocking Drugs Block cholinergic transmission between motor nerve endings and the nicotinic receptors on the neuromuscular end plate of skeletal muscle
Structural analogs of acetylcholine Clinically useful during surgery for producing complete muscle relaxation Nondepolarizing Blockers Mechanism of Action At low doses: Interact with the nicotinic receptors to prevent the binding of acetylcholine Prevent depolarization of the muscle cell membrane and inhibit muscular contraction At high doses:
Block the ion channels of the end plate Nondepolarizing Blockers, continued. Therapeutic uses: Used in anesthesia during surgery Pharmacokinetics: Injected intravenously Quaternary amines structure, making them orally ineffective, and do not cross the blood-brain barrier Many of the drugs are not metabolized Excreted unchanged in the bile
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