A NOTE ON AN UNUSUAL EFFECT OF GALLAMINE AND TUBOCURARINE

Increase in twitch force of the adductor pollicis muscle with stabilized preload at constant thumb abduction before and after administration of muscle relaxant

OBJECTIVE

To determine whether the twitch force of the adductor pollicis remains stable when 0.1 Hz single twitch stimulation is started after stabilization of the thumb preload at a constant degree of thumb abduction; also to study any possible increase in twitch force before the onset of and after the recovery from neuromuscular block.

METHODS

Measurements were performed in thirty patients under general anaesthesia. Twitch forces were first allowed to stabilize after allowing the preload to drift to its resting tension at a constant degree of thumb abduction. Three groups of ten patients then each received either vecuronium (2, 4, 8, 16 and 32 microg/kg(-1), successively at intervals of 2 min), d-tubocurarine (5, 10, 20, 40 and 80 microg/kg(-1), successively at intervals of 2 min), or suxamethonium (0.025, 0.05, 0.1, 0.2 and 0.4 mg/kg(-1), successively at intervals of 2 min). Measurements were continued until twitch forces had recovered from neuromuscular block and were stable.

RESULTS

Twitch forces stabilized at 114% (sd = 8.9) of the initial value after 10.9 (6.1) min of stimulation. Increase in twitch force before the onset of neuromuscular block was seen in two patients receiving vecuronium and in two patients receiving d-tubocurarine. Increase in twitch force after recovery from neuromuscular block was seen in all patients receiving suxamethonium.

CONCLUSIONS

Twitch forces may increase when stimulation is started after stabilization of thumb preload at a constant degree of thumb abduction. In some patients twitch forces may increase before the onset of neuromuscular block with vecuronium or d-tubocurarine; twitch forces increase after recovery from suxamethonium.

Gallamine triethiodide (flaxedil): tetraethylammonium- and pancuronium-like effects in myelinated nerve fibers

Gallamine triethiodide (Flaxedil) is commonly used as a neuromuscular blocking agent. Voltage-clamp studies show that gallamine also directly affects amphibian and mammalian myelinated nerve fibers. Externally, gallamine is about five times more potent than tetraethylammonium in blocking potassium conductance, where this is present, but has no effect on the sodium channel. Internal application slows sodium inactivation, which in addition is often incomplete. At positive potentials, gallamine can occlude sodium channels, thereby almost eliminating outward sodium currents.

The modes of action of gallamine

The action of gallamine, a classical competitive neuromuscular blocking agent, has been examined on voltage-clamped endplates of frog skeletal muscle fibres. Gallamine produces a parallel shift of the equilibrium log (concentration)--response curves in concentrations of up to about 40 microM. At a membrane potential of -70 mV the Schild plot of the dose ratios so measured has a gradient of slightly less than the theoretical value, for a competitive antagonist, of unity. The apparent equilibrium constant for 'competitive' block is about 2 microM, and is approximately independent of the membrane potential. Fluctuation analysis of the endplate current shows two components in the presence of gallamine. The results can be fitted, over the range tested, by a mechanism that involves block of open ion channels by gallamine in a manner similar to that by procaine or quaternary local anaesthetic analogues. The rate constants for this action are strongly dependent on the membrane potential. At -100 mV the association rate constant is about 4 x 10(7) M-1S-1, the dissociation rate constant is about 600 s-1, and the equilibrium constant about 15 microM. Other kinetic measurements (voltage-jump relaxation, and nerve-evoked endplate currents) give results consistent with this conclusion, but apparently these results are valid over a range of conditions narrower than that for fluctuation analysis.

The effect of suxamethonium alone and its interaction with gallamine on the indirectly elicited tetanic and single twitch contractions of skeletal muscle in man during anaesthesia

Simultaneous measurements of tetanic and single twitch contractions of the adductor politics muscle in man were made during neuromuscular blockade induced by suxamethonium (0.2 mg/kg) under nitrous oxide, oxygen and pentazocine anaesthesia. 2. After a suitable control period, suxamethonium (0.2 mg/kg) was given intravenously and the same dose was repeated 15 min later. After a further 15 min gallamine (0.2 mg/kg) was administered intravenously. When recovery from gallamine reached a plateau on the tetanic contraction recording a third injection of suxamethonium was given. 3. In most patients, suxamethonium (0.2 mg/kg) caused a partial blockade of the single twitch and complete blockade of the tetanic contraction. In every instance, the tetanic contraction was more depressed and recovered more slowly than the single twitch which not only recovered promptly but also showed overshoot. 4. After gallamine marked antagonism to suxamethonium block developed and this was greater with the single twitch than with the tetanic contractions. 5. Tetanic-tension fade was observed after every injection of suxamethonium and the degree of fade was dependent on the frequency of the tetanic stimulation.

The mechanism of the facilitatory action of edrophonium in cat skeletal muscle

1. The effects of edrophonium have been observed in the transversely cut tenuissimus muscle of the cat.2. Concentrations of edrophonium 10(-7)M to 10(-5)M increased the amplitude of the end-plate potential (e.p.p.) but produced no greater increase in time course than previously observed in curarized muscle.3. When the e.p.p. and gross nerve action potential were recorded simultaneously, antidromic discharges were observed in the motor nerve concomitantly with repetitive e.p.ps in the presence of edrophonium.4. Edrophonium produced no effect on the input resistance or equilibrium potential of the end-plate.5. The fractional release of transmitter was significantly increased by edrophonium but there was no increase in the quantal release of transmitter in the transversely cut muscle preparation.6. In curarized muscle edrophonium also increased the quantal release, the size of the available store of transmitter and the rate of refilling of the available store.7. It is concluded that edrophonium facilitates transmission to skeletal muscle by inducing a repetitive antidromic discharge in the nerve, following orthodromic stimulation. The antidromic discharge propagates by axon reflex to other nerve terminals of the same motor unit producing repetitive e.p.ps. It is also suggested that edrophonium antagonizes tubocurarine by acting as a partial agonist at the motor nerve terminal.

The effect of some foreign anions on suxamethonium blockade of the isolated rat diaphragm preparation

1. The pharmacology of suxamethonium blockade of the rat phrenic nerve-diaphragm preparation has been studied in the presence of some foreign anions.2. Blockade proceeded in two distinct phases in both normal and modified Krebs solution.3. In normal Krebs solution the characteristics bore no close resemblance to either depolarizing or competitive type blockade.4. In the presence of foreign anions the characteristics of the blockade more closely resembled those of depolarization.5. There was an increase in the sensitivity of the motor end-plate region of the muscle to the depolarizing action of acetylcholine, carbachol and suxamethonium in the presence of the anions.6. Although the anions enhanced the depolarizing activity of suxamethonium, the blocking potency of the drug was unaltered.7. It is suggested that end-plate depolarization plays little part in suxamethonium blockade of the isolated rat diaphragm and that desensitization is the primary cause of the blockade.

Actions of gallamine and tetraethylammonium at the frog neuromuscular junction

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