The mechanism of relaxation in response to magnesium by the aorta of pregnant rats with salt-induced hypertension.

The study examined the effect of salt-induced hypertension on vascular relaxation in response to magnesium sulphate during pregnancy. Pregnant Wistar rats were fed for 6 weeks on a diet containing 0.3% (control) and 8.0% (test) sodium chloride. Aortic rings were then removed and contracted with 10(-7) M phenylephrine or 30 mM potassium chloride. High salt intake increased the systolic blood pressure of the rats and increased the relaxation of phenylephrine-contracted intact rings in response to magnesium sulphate. Neither endothelium removal nor treatment with 10(-6) M indomethacin altered the relaxation of rings from the two groups of rats, when contracted with potassium chloride. Both processes significantly (P < 0.05) and similarly decreased the sensitivity and the maximal relaxation of rings from test rats contracted with phenylephrine; the relaxation of rings from the control rats was not altered. The results suggest that the relaxation of isolated rat aortic rings contracted with phenylephrine is enhanced in pregnant rats with salt-induced hypertension. The mechanism involved in this enhancement is dependent on the vascular endothelium and receptor activation, and is indomethacin sensitive.