Actions of triethylcholine on neuromuscular transmission. 1961

Abstract

The effects of the triethyl analogue of choline (triethyl 2-hydroxyethyl ammonium) on muscular activity have been studied in conscious rabbits, chicks, dogs and a cat. The contractions of the tibialis anticus and soleus muscles of cats under chloralose anaesthesia, and of the tibialis anticus muscle of rabbits under urethane anaesthesia and the isolated diaphragm preparation of the rat were also used. In conscious animals, triethylcholine caused a slowly developing muscular weakness which was more severe after exercise and which resembled the symptoms of myasthenia gravis. In nerve-muscle preparations triethylcholine had a selective action in reducing the contractions of muscles elicited by a high rate of nerve stimulation while leaving unaffected the contractions caused by slower rates of stimulation. During the paralysis of the tibialis muscle of the cat produced by triethylcholine, action potentials recorded from the motor nerve were unaffected and the muscle responded normally to injected acetylcholine and to direct electrical stimulation. The failure of neuromuscular transmission produced by triethylcholine was reversed by injection of choline, but anticholinesterases were ineffective. Choline reduced the toxicity of triethylcholine in mice. It is concluded that triethylcholine produces transmission failure at the neuromuscular junction by interfering with the ability of the nerve endings to synthesize acetylcholine. The possibility that triethylcholine is itself acetylated by the nerve endings and released as an inactive neurohormone is discussed. It was shown that triethylcholine was devoid of depolarizing action and curare-like blocking action. It possesses a transient ganglion blocking action of the tetraethylammonium-type as shown in experiments in which it caused a fall in blood pressure and blocked the response of the nictitating membrane to pre- but not to post-ganglionic stimulation of the cervical sympathetic nerve.

Fig. 1

The effect of choline in reducing the toxicity of triethylcholine. The circles indicate the LD50 of triethylcholine alone, of triethylcholine in combination with 4 different doses of choline, and of choline alone. The 95% confidence limits of the LD50 of triethylcholine alone and in combination with choline are indicated by the vertical lines, and of choline alone by the horizontal line. The straight line joining the LD50 points for triethylcholine alone and choline chloride alone is that to be expected if the toxicities were additive (Gaddum, 1959). All the data for this figure were obtained from one batch of mice.

Fig. 2

Cat, 4.0 kg. Upper record, blood pressure, middle and lower records, maximal twitches of right and left tibialis anticus muscles elicited indirectly 1/sec and 1/10 sec respectively. At A, 10 μg acetylcholine was injected close-arterially to the right tibialis anticus muscle, electrical stimulation being temporarily stopped during the injection. At TEC in a, 10 mg/kg, and at TEC in b, 30 mg/kg of triethylcholine chloride were injected intravenously. At CH, 5 mg/kg choline chloride was injected intravenously.

Fig. 3

Cat, 5.5 kg. Maximal twitches of the tibialis anticus (upper record) and soleus muscles (lower record) elicited indirectly once every second. At TEC, 35 mg/kg of triethylcholine iodide, and at CH, 5 mg/kg of choline chloride were injected intravenously. At T, tetani were elicited at a frequency of 50/sec for approximately 6 sec. During the tetani the kymograph speed was increased. During the period marked , the muscles were stimulated directly. The animal had previously recovered from an initial injection of 35 mg/kg of TEC.

Fig. 4

Cat, 3.2 kg. Upper and middle records: maximal twitches of the right tibialis anticus and soleus muscles respectively, elicited indirectly once every sec. Lower record: maximal twitches of the left tibialis anticus muscle elicited indirectly once every 10 sec. At TEC, 30 mg/kg of triethylcholine iodide, and at NEO, 0.1 mg/kg neostigmine methylsulphate injected intravenously.

Fig. 5

Effect of triethylcholine on respiratory movements (upper) and contractions of tibialis anticus muscle (lower) in cat. Inspirations are downwards and tibialis contractions upwards. The sciatic nerve was stimulated at 100/sec for 0.2 sec every 5 sec. The doses of triethylcholine (TEC) refer to the iodide.

Fig. 6

Cat, 2.1 kg. Maximum action potentials recorded from the peripheral end of the left common peroneal nerve and from the right tibialis anticus muscle (concentric needle electrodes) in response to stimulation of the nerve once every second. S, Stimulus artifact; N, nerve action potential; M, muscle action potential; a and b were recorded before injection with an interval of 10 min between them; c, d, e and f were recorded 12, 14, 16 and 20 min respectively after the intravenous injection of 75 mg/kg of triethylcholine iodide; g and h were recorded 1 and 3 min after the intravenous injection of 5 mg/kg choline chloride.

Fig. 7

Effect of triethylcholine on respiratory movements (upper trace) and contractions of tibialis anticus muscle (lower trace) of rabbit. Inspirations are downwards and muscle contractions upwards. The sciatic nerve was stimulated at 100/sec for 0.2 sec every 5 sec. The tetanic contractions of the tibialis muscle were depressed, but respiration was not impaired after the intravenous injection of 40 mg/kg triethylcholine iodide.

Fig. 8

Maximal twitches of 2 hemidiaphragms from one rat mounted in the same 100 ml. organ bath. Contractions are downwards. In the upper tracing the phrenic nerve was stimulated at 1/sec and in the lower at 1/10 sec. At TEC 40 mg of triethylcholine iodide and 1 hr later at Chol. 10 mg of choline chloride were added to the bath.

Fig. 9

Maximal twitches of isolated rat diaphragm (contractions downwards) elicited by stimulation of the phrenic nerve at a frequency of 1/sec. At TEC 200 μg/ml. of triethylcholine chloride and at Chol. 50 μg/ml. of choline chloride were added to the bath. In the upper tracing choline reversed the action of triethylcholine. In the lower tracing, which was obtained from the same preparation, choline added first reduced the action of triethylcholine.

Fig. 10

Cat, 3.4 kg. Upper record: blood pressure; middle and lower records: maximal twitches of the tibialis anticus and soleus muscles respectively elicited indirectly once every second and, towards the end of the record, once every 10 sec. At A, 8 μg acetylcholine injected close-arterially to the tibialis anticus muscle. Electrical stimulation was temporarily stopped while acetylcholine injections were made. At HC-3,2 mg/kg of hemicholinium dibromide, and at CH, 5 mg/kg choline chloride were injected intravenously.

Fig. 11

Cat, 2.8 kg. Effect of triethylcholine iodide on blood pressure (upper tracing), and on the contractions of nictitating membranes in response to sympathetic nerve stimulation with 0.5 msec pulses at 20/sec for 12 sec every 2 min. The contractions of the left nictitating membrane (middle trace) are in response to preganglionic nerve stimulation and the contraction of the right nictitating membrane (lower trace) are to postganglionic nerve stimulation.