Background: infusion of Mg for therapeutic purposes is still a matter for debate. Dosages vary considerably, yet subclinical effects on normal physiology are largely ignored. In human and animal models, interactions between Mg and insulin exist, thus we have investigated the effect of infusing Mg on serum insulin, ionised Mg (Mg2+) and Ca (Ca2+) and plasma glucose in human volunteers.\ud Methods: six male volunteers were infused with magnesium sulphate (MgSO4) dissolved in normal saline, using a high-dose “loading” bolus, followed by a lower-level “maintenance” period. \ud Findings: serum Mg2+ rose rapidly throughout the bolus infusion, declined during the maintenance phase, but remained higher than pre-infusion levels throughout the experimental period; serum Ca2+ rose when serum Mg2+ was highest. Infusion of MgSO4 had no effect on heart rate or blood pressure, but caused a rapid, pronounced drop in circulating fasting insulin (p < 0.0005), which slowly recovered to basal values during the course of the maintenance infusion. A slight, transient rise in plasma glucose (p < 0.05) concomitant with the decline in serum insulin was also observed.\ud Interpretation: it is possible that the effect of Mg2+ on insulin may have been due to antagonism of Ca2+ entry in pancreatic beta-cells, the insulin decline causing a subsequent rise in circulating glucose levels. We suggest that these effects of MgSO4 infusions should be considered where the aim is to achieve high doses of blood Mg2+ levels by clinical intervention
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