Neuromuscular interactions between suxamethonium and magnesium sulphate in the cat

In order to study the neuromuscular interactions between suxamethonium and magnesium sulphate (MgSO4), we have determined the dose-response relationship of suxamethonium and the neuromuscular actions of 1.25 x ED50 dose of suxamethonium, both before and after pretreatment with MgSO4. We have also compared the effect of 1.25 x ED50 dose of suxamethonium in the absence and in the presence of 50% neuromuscular block, established previously by infusion of MgSO4. Twenty-one cats were anaesthetized with urethane. Train-of-four stimulation was applied every 12 s to the sciatic nerve and the force of contraction of the tibialis anterior muscle was measured. The potency of suxamethonium decreased in each instance with pretreatment with MgSO4. The ED50 of suxamethonium increased significantly from mean 21.0 (SEM 1.9) micrograms kg-1 before MgSO4 to 25.6 (2.3) micrograms kg-1 after MgSO4 60 mg kg-1 and to 26.6 (2.2) micrograms kg-1 after MgSO4 90 mg kg-1 (P < 0.05). Twitch depression produced by 1.25 x ED50 dose of suxamethonium decreased significantly with MgSO4 pretreatment, from 76.7 (2.6)% before MgSO4 to 61.7 (6.4)% after MgSO4 60 mg kg-1 and 48.7 (7.5)% after MgSO4 90 mg kg-1 (P < 0.05). With stable 50% neuromuscular block, established previously by infusion of MgSO4, the 1.25 x ED50 dose of suxamethonium produced more 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 methods demonstrated that the pharmacological interaction between suxamethonium and magnesium was antagonistic.