EFFECTS OF MAGNESIUM-SULFATE ON THE NORADRENALINE-INDUCED CEREBRAL VASOCONSTRICTOR AND PRESSOR-RESPONSES IN THE GOAT

Objective To examine the ability of magnesium sulphate to counteract t he noradrenaline-induced cerebral vasoconstrictor and presser response s in goats by using both in vivo and in vitro techniques. Design Cereb ral blood flow was measured in vivo by means of an electromagnetic flo w probe around the internal maxillary artery. Isometric tension was re corded in vitro from rings of goat middle cerebral artery maintained i n an organ bath. Results 1. In vivo. Continuous infusion of noradrenal ine (10 mu g/min) directly into the cerebral arterial supply elicited sustained decrease in cerebral blood flow (61% [SEM 3] of control valu es) and increase in cerebral vascular resistance (178% [SEM 9] of cont rol values). Magnesium sulphate, injected directly into the cerebral a rterial supply (10-300 mg) or infused intravenously (0.3 g and 3 g dur ing 15 min) at the noradrenaline-induced steady state, increased cereb ral blood flow by decreasing cerebral vascular resistance in a dose-de pendent manner. A similar result was obtained when intravenous magnesi um sulphate (3 g/15 min) was tested against the cerebral vasoconstrict or and presser responses induced by intravenous infusion of noradrenal ine (30 mu g/min). 2. In vitro. When compared with the response obtain ed in a control medium (1 mmol/L Mg2+), 10 mmol/L Mg2+ significantly i nhibited the maximum contraction elicited by noradrenaline (10(-8) to 3 x 10(-3) mol/L) from 45% [SEM 4] to 26% [SEM 4]. Conclusions Magnesi um sulphate reverses the noradrenaline-induced cerebral vasoconstricto r and presser responses by a direct inhibitory action of Mg2+ on the a ctions of noradrenaline in the cerebral and peripheral vascular beds, which leads to a decrease in vascular resistance. These results could explain, at least in part, the beneficial effects of magnesium sulphat e in the management of preeclampsia and eclampsia.