The effect of magnesium deficiency on volatile anaesthetic requirement in the rat: the role of central noradrenergic neuronal activity.

Volatile anaesthetic minimum alveolar concentration (MAC, a measure of anaesthetic requirement) increased in a time-dependent manner in rats fed a Mg2+-deficient diet. MAC values in hypomagnesemic rats were 22-30 per cent greater than those in age-matched controls at 12 and 17 days after starting the diet (p < 0.01). Noradrenergic neuronal activity, as assessed from the ratio of the concentration of 3,4-dihydroxyphenylethylene-glycol (DHPG) to that of norepinephrine (NE), decreased in the brain stem and cerebrum-cerebellum in hypomagnesemic rats owing to an increase in NE concentration in both regions of the brain (p < 0.025). We conclude that prolonged hypomagnesemia (> or = 12 days) increases volatile anaesthetic MAC in the rat. The concomitant decrease in the ratio of DHPG/NE suggests that this increase in MAC cannot be attributed to an increase in noradrenergic neuronal activity in brain.