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Actions of the selective inhibitor of cholinesterase tetramono iso propyl pyrophosphortetramide on the rat phrenic nerve‐diaphragm preparation
Author(s) -
HEFFRON P. F.
Publication year - 1972
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1972.tb06896.x
Subject(s) - chemistry , cholinesterase , acetylcholine , neuromuscular transmission , curare , neuromuscular junction , phrenic nerve , acetylcholinesterase , diaphragm (acoustics) , stimulation , tetanic stimulation , edrophonium , biophysics , neostigmine , pharmacology , endocrinology , medicine , biochemistry , neurotransmission , respiratory system , enzyme , biology , neuroscience , acoustics , receptor , physics , loudspeaker
Summary1 . Tetramono iso propyl pyrophosphortetramide ( iso ‐OMPA) added for 15 min to the rat isolated phrenic nerve‐diaphragm in a concentration of 30 μ m , produced a complete selective and stable inhibition of cholinesterase. A concentration of 3 μ m produced near complete inhibition of cholinesterase, and a concentration of 300 μ m also inhibited acetylcholinesterase marginally. 2 . Inhibition of cholinesterase was associated with a sustained increase in the neuromuscular blocking action of exogenous butyrylcholine but not of exogenous acetylcholine. Iso ‐OMPA, 300 μ m , in addition caused transient increases in the sensitivity of the rat diaphragm to exogenous acetylcholine and butyrylcholine. In the same concentration, it had a curare‐like action on the frog rectus abdominis muscle preparation. 3 . Iso ‐OMPA, 30 μ m , caused reversible increases in the amplitude of the twitch response and tetanic responses, which were of a similar magnitude in the indirectly stimulated preparation and the directly stimulated curarized preparation. Caffeine had a similar effect on the twitch response and its effectiveness was increased by iso ‐OMPA, and vice‐versa . Amongst anticholinesterases, octamethyl pyrophosphortetramide and tetraethylpyrophosphate also enhanced the amplitude of the tetanic response, but paraoxon, dyflos, and mipafox did not. 4 . It is concluded that iso ‐OMPA, in concentrations (3 and 30 μ m ) which in 15 min give near maximal or maximal selective inhibition of cholinesterase, has no effect on the transmission of nerve impulses at the neuromuscular junction, but enhances reversibly the amplitude of the contractile response to stimulation by a direct action upon the muscle fibre, which involves a mechanism related to but not identical with that by which caffeine potentiates twitch tension. In higher concentrations, iso ‐OMPA has a curare‐like action at the neuromuscular junction.