Mfm. Braga et al., INTERACTIONS BETWEEN SUXAMETHONIUM AND NONDEPOLARIZING NEUROMUSCULAR BLOCKING-DRUGS, British Journal of Anaesthesia, 72(2), 1994, pp. 198-204
In anaesthetized cats, we have confirmed that previously injected suxa
methonium potentiates non-depolarizing neuromuscular blocking drugs wh
ereas, when injected during the block, suxamethonium antagonizes the p
aralysis. We have attempted to explain these interactions by studying
the effects of suxamethonium on extracellularly recorded nerve ending
waveforms that correspond to the ionic currents in the mouse triangula
ris sterni isolated nerve-muscle preparation. The preparations were pa
ralysed with mu-conotoxin (obtained from the cone snail), which is bel
ieved to act by selectively blocking sodium channels in muscle, and wh
ich therefore should not interfere with currents at the nerve endings.
Suxamethonium, in concentrations of 0.5-300 mu mol litre(-1), produce
d a concentration-dependent increase in the amplitude of the waveform
corresponding to the inward calcium current evoked by a nerve impulse.
This effect did not occur in the presence of tubocurarine, suggesting
that suxamethonium, which is a nicotinic agonist, may have been actin
g on a nicotinic receptor on the nerve endings that is coupled to the
voltage-operated calcium channels. The inward calcium current is belie
ved to be responsible for neurotransmitter (acetylcholine) release. It
is concluded, therefore, that its enhancement by suxamethonium contri
butes to the ability of this drug to reverse non-depolarizing block. S
uxamethonium also exerted complex effects on the waveform correspondin
g to the outward flowing calcium-activating potassium current at the n
erve endings, but no effect was observed in this isolated nerve-muscle
preparation that could obviously explain the ability of suxamethonium
to potentiate subsequently injected non-depolarizing blocking drugs.