A physiological study of chick myotubes grown in tissue culture

1. A study has been made of some passive and active membrane properties of myotubes of different ages obtained in culture from explants of chick embryo thigh muscle.2. After 3 days in vitro the mean values for the myotube resting membrane potential and input resistance were - 63.8 mV and 1.30 MOmega respectively. By 13 days these values had fallen to - 51.0 mV and 0.80 MOmega.3. Current/voltage relations were measured in the presence of tetrodotoxin. The relations were linear for membrane potentials between - 120 and - 35 mV. Further depolarization usually resulted in a delayed increase in conductance which inactivated with time.4. All myotubes tested using anodal break excitation were capable of generating action potentials. Action potentials were blocked by tetrodotoxin, saxitoxin and procaine.5. All myotubes were sensitive to iontophoretically applied ACh. The potential change produced by ACh reversed polarity at a membrane potential between 0 and + 10 mV. The depolarization produced by ACh was unaffected by anticholinesterases.6. The ACh response was blocked by cobra neurotoxin, D-tubocurarine and atropine.7. The electrical properties of the myotubes appear to resemble those of normal adult twitch-type skeletal muscle fibres.8. The pharmacological properties of the myotube cholinergic receptor have been compared with those of the neuromuscular junction and the denervated muscle fibre membrane.

Effects of an isolated toxin from Australian tiger snake (Notechis scutatus scutatus) venom at the mammalian neuromuscular junction

1. The acute effects of a purified toxin from Australian Tiger snake (Notechis scutatus scutatus) venom have been investigated at the mammalian neuromuscular junction.2. The toxin was injected into the tail vein of mice. Death was due to respiratory paralysis.3. The resting membrane potential, and action potential of muscle fibres in muscles from in vivo intoxicated animals were normal.4. The frequency of miniature end plate potentials (m.e.p.p.s) from intoxicated nerve-muscle preparations was reduced, although m.e.p.p. amplitude was unaltered.5. Nerve stimulation resulted in end plate potentials (e.p.p.s) of quantal amplitude; only rarely was the e.p.p. large enough to give rise to an action potential.6. High (20 mM) K(+) did not increase m.e.p.p. frequency in intoxicated preparations.7. The toxin was largely ineffective in vitro.8. The similarities and differences between this toxin, beta-bungarotoxin and botulinum toxin are discussed.

Inhibition of denervation changes in skeletal muscle by blockers of protein synthesis

1. A study was made of the effects of inhibitors of protein synthesis (actinomycin D, chloramphenicol and cycloheximide) on the in vivo development of extrajunctional cholinergic receptors, tetrodotoxin-resistant action potentials, slowing of the time course of the action potential, and on the fall in the resting membrane potential in denervated mouse skeletal muscle.2. Actinomycin D (0.5 mg/kg, I.P.) reduced the incorporation of uridine into skeletal muscle by 50-60% for 4-5 days. There was no simultaneous reduction in L-leucine incorporation.3. Actinomycin D (0.5 mg/kg I.P., 1 day after denervation) inhibited the development of extrajunctional cholinergic receptors, tetrodotoxin-resistant action potentials and the fall in resting membrane potential for approximately 4 days.4. The post-denervation fall in the maximum rate of rise and in the amplitude of the overshoot of the muscle fibre action potential was unaffected by actinomycin D treatment.5. Actinomycin D failed to inhibit the appearance of the membrane changes if given later than 2 days after denervation.6. Chloramphenicol (6 g/kg in divided doses) and cycloheximide (40 g/kg every 12 hr) were also able to inhibit the appearance of the membrane changes.7. It is concluded that some denervation induced changes in the muscle fibre membrane depend on the synthesis of new proteins. The results imply that the motor nerve cell normally exerts a regulatory influence on the genome of the muscle cell.

Mechanical properties of dystrophic mouse muscle

A physiological method of estimating the number of motor units in normal and dystrophic muscles has revealed a large reduction in the number of motor units in dystrophic muscles. Anatomical confirmation of the results has been obtained. Dystrophic muscles also exhibit a `myasthenic-like' response to high frequency stimulation. It is suggested that the nerve fibre is primarily affected in murine dystrophy.

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.

Adenosine-3',5'-monophosphate: intracellular mediator for methyl xanthine stimulation of gastric secretion

We postulated that methyl xanthines stimulate hydrochloric acid production in the isolated frog gastric mucosa by inhibiting the phosphodiesterase that destroys adenosine-3',5'-monophosphate (cyclic AMP). In support of this theory, several criteria were satisfied. Exogenously supplied cyclic AMP stimulated hydrogen and chloride transport in the spontaneously secreting mucosa and in the non-acid secreting mucosa in a similar fashion as did the methyl xanthines. Methyl xanthines increased the mucosal content of cyclic AMP, and the increase preceded the secretory response; thereafter, the magnitude of these two quantities paralleled each other. A correspondence was found between the concentration of methyl xanthines that affected acid secretion and the concentration of methyl xanthines that affected the tissue content of cyclic AMP. Theophylline was more effective than caffeine in increasing cyclic AMP content, which is in accord with the previously reported differences in their effect on acid secretion and phosphodiesterase activity.

The effect of temperature on end-plate depolarization of the rat diaphragm produced by suxamethonium and acetylcholine

End-plate depolarization in the rat isolated diaphragm by suxamethonium and acetylcholine was measured at 37° and 20° using a modification of the method of Fatt (1950). At 20° the response of the motor end-plates to the depolarizing activity of acetylcholine and suxamethonium was enhanced. Since the presence of neostigmine in the bath fluid did not modify the response of the motor end-plates to the depolarizing activity of suxamethonium at 37°, the results suggest the enhanced response of the motor end-plates at 20° is due to a change in the properties of the motor end-plate, and not to a change in the activity of cholinesterases present in the tissues.