How is the action of succinylcholine terminated?
Succinylcholine is rapidly metabolized by plasma cholinesterase (pseudocholinesterase) to succinylmono-choline and choline. When 1 mg/kg of succinylcholine is injected intravenously, about 90% is metabolized within 1 minute, so very little of the drug actually reaches the neuromuscular junction.
Plasma cholinesterase has no effect on succinylcholine once it has reached the neuromuscular junction. The action of succinylcholine at the neuromuscular junction is terminated when succinylcholine moves away from the neuromuscular junction, which occurs when its concentra-tion gradient has reversed. When succinylcholine is first injected, its concentration is greatest in the plasma and lowest at the neuromuscular junction. As succinylcholine is metabolized in the serum and extracellular space, its concentration declines and succinylcholine diffuses away from the junction along this reversed gradient, and the clinical effect (block) of succinylcholine decreases. The onset of blockade from 1 mg/kg is typically 1 minute, while the normal duration of block is 5–8 minutes. Plasma cholinesterase is produced in the liver under genetic con-trol. Its production is related to a pair of allelic genes EU and EU. The enzyme has a half-life of 5–12 days.