Effects of magnesium on the vasoconstrictor responses to norepinephrine and potassium chloride in the rat mesenteric artery.

In the perfused rat mesenteric vascular bed, the effects of magnesium and calcium on the vasoconstrictor responses to norepinephrine or potassium chloride were studied. Higher concentrations of magnesium in the perfusate attenuated the vascular responses to norepinephrine and potassium chloride in a dose-related manner. Higher concentrations of calcium in the perfusate enhanced greatly the vascular responses to potassium chloride, while the response to norepinephrine was only slightly potentiated by increasing calcium concentration. The attenuating effect of magnesium on the responses to norepinephrine and to potassium chloride were reversed by elevation of calcium concentration in the perfusate. The potentiating effects of calcium on the responses to potassium chloride were also reversed by increasing the concentration of magnesium. However, an inhibitory effect of magnesium on the responses to norepinephrine was not observed in the presence of higher concentrations of calcium. These results indicate that magnesium acts antagonistically on calcium movement in vascular contractions due to potassium chloride, but that the relationship between magnesium and calcium is more complex in vascular contractions induced by norepinephrine.