The antiadrenergic effects of hypermagnesemia: an experimental study

The circulatory effects of norepinephrine (4 micrograms/kg), isoproterenol (10 micrograms/kg) and phenylephrine (20 micrograms/kg) were determined in anesthetized dogs with normal plasma magnesium and with induced hypermagnesemia. Norepinephrine caused a 24% increase in heart rate and a 103% increase in the systemic vascular resistance index in normomagnesemic dogs, while with hypermagnesemia the variations were of 13% and 1%, respectively. Isoproterenol increased heart rate by 48% and 18% in dogs with normo- and hypermagnesemia, respectively. Phenylephrine increased the systemic vascular resistance index (74%) only in the normomagnesemic state. The effects of all the drugs were significantly different (P less than 0.01), without and with the simultaneous administration of magnesium sulfate (plasma magnesium, 1.3 +/- 0.2 mEq/l and 6.8 +/- 1.1 mEq/l, respectively). We conclude that acute induced hypermagnesemia antagonizes the circulatory effects of adrenergic stimulation, a fact that may explain its antiarrhythmic and hemodynamic effects during acute myocardial ischemia.