Although choline acetyltransferase (ChAT) catalyzes the final transfer of an acetyl group from acetyl-CoA to choline, the actual physiological rate-limiting step in acetylcholine synthesis is the carrier-mediated transport of free choline into the presynaptic nerve terminal via the sodium-dependent, hemicholinium-sensitive high-affinity choline transporter (CHT1).
Bethanechol is a synthetic choline ester structurally resistant to both acetylcholinesterase and pseudocholinesterase. It exerts selective agonist activity at muscarinic M3 receptors located on the urinary bladder detrusor muscle, stimulating intracellular Gq-PLC-IP3 signaling to produce powerful detrusor contractions and trigone/sphincter relaxation, facilitating micturition.
A) เคชเคพเคเคฐเคฟเคกเฅเคธเฅเคเคฟเคเฅเคฎเคพเคเคจ
B) เคซเคพเคเคธเฅเคธเฅเคเคฟเคเฅเคฎเคพเคเคจ
C) เคจเคฟเคฏเฅเคธเฅเคเคฟเคเฅเคฎเคพเคเคจ
D) เคเคกเฅเคฐเฅเคซเฅเคจเคฟเคฏเคฎ
Answer: (B)
Physostigmine is a naturally occurring tertiary amine alkaloid obtained from Physostigma venenosum. Because its tertiary nitrogen lacks a permanent quaternary charge at physiological pH, it readily crosses lipophilic membranes, including the blood-brain barrier, making it the drug of choice for reversing central and peripheral delirium in acute atropine poisoning. Neostigmine and pyridostigmine are quaternary amines that cannot penetrate the CNS.
Edrophonium reversibly inhibits acetylcholinesterase for a short period, so its effect begins rapidly and lasts only minutes. Historically, the Tensilon test used transient improvement in myasthenic weakness as a diagnostic clue. It is not a preferred routine modern diagnostic test because of potential serious adverse effects and the availability of other diagnostic approaches.
Organophosphate compounds irreversibly phosphorylate the esteratic site serine hydroxyl group of acetylcholinesterase. Pralidoxime (2-PAM) possesses an oxime group (-NOH) with high nucleophilic affinity for the electrophilic phosphorus atom; it binds the anionic site, attacks the organophosphate moiety, and hydrolyzes the phosphorus-enzyme covalent bond to liberate active functional enzyme, provided aging has not occurred.
When neostigmine is administered postoperatively to reverse non-depolarizing neuromuscular junction blockade, it raises acetylcholine levels across all nicotinic and muscarinic synapses, risking profound parasympathetic bradycardia, bronchospasm, and hypersecretion. Glycopyrrolate (or atropine) is co-administered as an anticholinergic countermeasure; glycopyrrolate is often preferred due to its quaternary structure preventing CNS adverse effects.
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