Magnesium sulfate (MgSO₄) has been used in the treatment of pre-eclampsia, hypertension and arrhythmia. This study was been conducted to evaluate the potency and time course of rocuronium-induced neuromuscular block after prolonged pretreatment with MgSO₄ in rabbits.

Forty rabbits were randomly assigned to two groups; the control group: normal saline 0.1 ml/kg sq for 6 weeks; and the study group: 50% MgSO₄ 60 mg/kg sq for 6 weeks. The dose-response relationships of rocuronium were studied during thiopental anesthesia in twenty rabbits. Each group received rocuronium 40, 60 and 80 µg/kg rocuronium, respectively, and the time course of rocuronium, at 0.6 mg/kg, evaluated in the twenty rabbits in each group.

The mean serum magnesium concentration were 2.3-2.6 and 3.3-3.9 mEq/L in the control and study groups, respectively (P < 0.001). The calculated ED₅₀s for rocuronium were 52.7 and 61.2 µg/kg in the control and study groups, respectively (P < 0.05). The times after rocuronium administration until the 95% twitch recovery were 49.3 and 34.8 min in the control and study groups, respectively (P < 0.05).

Rocuronium, following prolonged MgSO₄ pretreatment had a decreased effect and short duration of action compared with the control, for which increased doses of nondepolarizing muscle relaxants would be required to obtain a similar effect.