Pregnancy-induced alterations of relaxation in response to magnesium in the rat aorta may be due to plasma-borne agents.

The influence of plasma from pregnant and nonpregnant humans was examined, using magnesium-induced relaxation of precontracted rat aortic rings. The results showing magnesium-induced relaxation of aortic rings from pregnant and nonpregnant rats were compared. In rat aortic rings incubated in plasma from pregnant patients, magnesium was more potent in relaxing the precontractions induced by potassium chloride than in relaxing those induced by phenylephrine. The magnesium-induced relaxation of rings incubated in plasma from normal pregnant subjects was similar to that in unincubated rings from normal pregnant rats. Neither the removal of endothelium nor pretreatment with indomethacin affected the pattern of responses in the rings. The results suggest that the effects of pregnancy on magnesium-induced relaxation of the rat aorta may be mediated by plasma-borne agents, and the mechanisms by which the agents alter the relaxation do not involve either the vascular endothelium or prostaglandin synthesis.