Neuromuscular interactions between suxamethonium and magnesium sulphate in the cat

In order to study the neuromuscular interactions between suxamethoniumand magnesium sulphate (MgSO₄), we have determined the dose-responserelationship of suxamethonium and the neuromuscular actionsof 1.25xED₅₀ dose of suxamethonium, both before and after pretreatmentwith MgSO₄. We have also compared the effect of 1.25xED₅₀ doseof suxamethonium in the absence and in the presence of 50%neuromuscularblock, established previously by infusion of MgSO₄. Twenty-onecats were anaesthetized with urethane. Train-of-four stimulationwas applied every 12s to the sciatic nerve and the force ofcontraction of the tibialis anterior muscle was measured. Thepotency of suxamethonium decreased in each instance with pretreatmentwith MgSO₄. The ED₅₀ of suxamethonium increased significantlyfrom mean 21.0 (SEM 1.9) µg kg^–1 before MgSO₄ to25.6 (2.3) µg kg^–1 after MgSO₄ 60 mg kg^–1and to 26.6 (2.2) µg kg^–1 after MgSO₄ 90 mg kg^–1(P<0.05). Twitch depression produced by 1.25xED₅₀ dose ofsuxamethonium decreased significantly with MgSO₄ pretreatment,from 76.7 (2.6)% before MgSO₄ to 61.7 (6.4)% after MgSO₄ 60mg kg^–1 and 48.7 (7.5)% after MgSO₄ 90 mg kg^–1 (P<0.05).With stable 50% neuromuscular block, established previouslyby infusion of MgSO₄ the 1.25xED₅₀ dose of suxamethonium producedmore twitch augmentation (133 (6.3)%vs 108.3 (1.3)%; P<0.05)and less twitch depression (31.6 (9.6)% vs 74.1 (0.6)%, P<0.05)than in the absence of MgSO₄. The results of all three methodsdemonstrated that the pharmacological interaction between suxamethoniumand magnesium was antagonistic.