EFFECT OF PREGNANCY ON RELAXATION OF RAT AORTA TO MAGNESIUM

Objective: There is evidence for involvement of the endothielium in ma gnesium induced vasodilatation. In view of the use of magnesium in the treatment of women with pre-eclampsia, and because die mechanism of t he 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. Methods: Rings from descending thoracic aorta of pregn ant and non-pregnant rats, contracted with either 10(-7) M phenylephri ne or 40 mM potassium chloride, were relaxed with increasing concentra tions of MgSO4 in the presence or absence of 10(-6)M indomethacin or e ndothelium. The rings were also contracted to 10(-5) M phenylephrine, in calcium-free medium containing 0, 1.2, or 4.8 mM MgSO4. Results: Th e relaxation of aortic rings from pregnant rats to MgSO4 was greater w hen stimulated with potassium chloride but that of rings from non-preg nant rats was greater when stimulated with phenylephrine. Neither the presence Of MgSO4 nor pregnancy had any effect on intracellular calciu m dependent contraction. The relaxations of rings from either pregnant or non-pregnant rats to MgSO4 were not significantly altered by de-en dothelialisation or pretreatment with 10(-6) M indomethacin. Conclusio ns: The effect of pregnancy on magnesium induced relaxation of rat aor tic smooth muscle was dependent on the agent used to induce contractio n in the tissue, probably because pregnancy exerted different actions on receptor and voltage operated calcium channels. This effect of preg nancy was independent of either endothelial function or prostaglandin synthesis. Neither pregnancy nor the presence of magnesium affected th e release of intracellular stored calcium.