The Interaction Between Edrophonium (Tensilon) and Acetylcholine at the Motor End‐plate

Abstract

The effect of edrophonium (3-hydroxy-phenyl-dimethylethylammonium chloride) on the motor end-plate and its interaction with acetylcholine and carbachol has been investigated. Use was made of intracellular recording of membrane potential and of ionophoretic micro-application of drugs from single and twin-pipettes.Small doses of edrophonium potentiate the depolarizing effect of acetylcholine, but not that of carbachol. This action can be observed with doses of edrophonium which have no depolarizing effect by themselves. Large doses of edrophonium have some depolarizing action and, at the same time, inhibit depolarizations produced by carbachol. After treatment with neostigmine, edrophonium fails to potentiate the acetylcholine response. The observations are in agreement with the view that the principal action of edrophonium on the neuromuscular junction is that of a potent and rapidly acting anticholinesterase.The effects of anticholinesterases on muscle are usually tested under conditions in which the inhibitor/enzyme reaction has approached or reached equilibrium. In recent experiments (Castillo and Katz, 1957c), a different method was used, brief localized doses of the drug being applied to an end-plate with the help of an ionophoretic micro-technique. Under these conditions, the observed potency of a drug depends on the kinetics, rather than the equilibrium constant, of the reaction. In such experiments it was found that substances like neostigmine, which are strong but slowly acting enzyme inhibitors, produced no potentiation of the acetylcholine response, while less powerful but more rapidly acting esterase inhibitors (choline, decamethonium) caused a marked increase in the acetylcholine effect. Similar experiments will be described in which edrophonium (3-hydroxy-phenyl-dimethylethylammonium chloride) was allowed to interact with acetylcholine (ACh), by applying the substances from micropipettes placed at close range to an end-plate of the frog's sartorius muscle. The membrane potential of the muscle fibre was recorded with an intracellular electrode inserted within a few hundred microns of the point of drug action.The effect of edrophonium on neuromuscular transmission in the frog has previously been studied by Nastuk and Alexander (1954), who concluded that the anticurare action of this substance and the modifications which it produced in the shape of the electric end-plate response could be attributed to its anti-esterase activity (see also Smith, Cohen, Pelikan, and Unna, 1952). The present experiments confirm this view and provide additional evidence for the high speed at which the reaction between edrophonium and ACh-esterase proceeds. METHOD The technique has been described in detail in previous papers (Castillo and Katz, 1955, 1957a, c; see also Katz and Thesleff, 1957). The experiments were made on isolated sartorius muscles of R. temporaria at about 200 C. The preparations were mounted in a bath of Ringer solution which contained the electrodes for the recording of membrane potentials and for the...

The effect of diffusional barriers upon the pharmacology of cells within the central nervous system

“…Similar results were obtained with edrophonium (Tensilon: 3-hydroxyphenyldimethylethyl-ammonium chloride, Roche Products Ltd.), which is structurally related to prostigmine and has a similar anticholinesterase activity (Randall, 1950;Nastuk & Alexander, 1954;Katz & Thesleff, 1957). When administered intravenously this drug has no effect on Renshaw cells.…”

“…However, the present series of experiments does not exclude the possibility that these drugs also have a depolarizing action (cf. Katz & Thesleff, 1957).…”

An electrophysiological study of chelonian skeletal muscle

“…190 msec. These times are quite similar to those reported for the time course of action of edrophonium at the junctional membrane of frog muscle fibres by Katz & Thesleff (1957c (Nastuk & Alexander, 1954; Katz & Thesleff, 1957 c) could be demonstratedl at these sites with relatively large doses (Fig. 10 C).…”

A study of supersensitivity in denervated mammalian skeletal muscle

“…its effect is to inhibit the response to ACh and to produce a slight membrane depolarization (upper record). The general view is that the main action of edrophonium is inhibition of cholinesterase (Smith, Cohen, Pelikan & Unna, 1952;Nastuk & Alexander, 1954;Katz & Thesleff, 1957b); hence the absence of ACh potentiation indicates that the membrane is, in a practical sense, devoid of cholinesterase. Electrical excitability It has been suggested that, unlike the rest of the muscle membrane, the ACh receptors are not excitable by electrical stimulation (del Castillo & Katz, 1954).…”