Effect of pregnancy on relaxation of rat aorta to magnesium

There is evidence for involvement of the endothelium in magnesium induced vasodilatation. In view of the use of magnesium in the treatment of women with pre-eclampsia, and because the mechanism of the vascular effects of magnesium in pregnancy is not fully understood, this study examined the dilator response to magnesium of aortic rings from pregnant and non-pregnant rats. The role of the endothelium was also evaluated.

Rings from descending thoracic aorta of pregnant and non-pregnant rats, contracted with either 10(-7) M phenylephrine or 40 mM potassium chloride, were relaxed with increasing concentrations of MgSO4 in the presence or absence of 10(-6) M indomethacin or endothelium. The rings were also contracted to 10(-5) M phenylephrine, in calcium-free medium containing 0, 1.2, or 4.8 mM MgSO4.

The relaxation of aortic rings from pregnant rats to MgSO4 was greater when stimulated with potassium chloride but that of rings from non-pregnant rats was greater when stimulated with phenylephrine. Neither the presence of MgSO4 nor pregnancy had any effect on intracellular calcium dependent contraction. The relaxations of rings from either pregnant or non-pregnant rats to MgSO4 were not significantly altered by de-endothelialisation or pretreatment with 10(-6) M indomethacin.

The effect of pregnancy on magnesium induced relaxation of rat aortic smooth muscle was dependent on the agent used to induce contraction in the tissue, probably because pregnancy exerted different actions on receptor and voltage operated calcium channels. This effect of pregnancy was independent of either endothelial function or prostaglandin synthesis. Neither pregnancy nor the presence of magnesium affected the release of intracellular stored calcium.