Effect of magnesium on posterior pituitary hormone action on vascular smooth muscle

Abstract

The contractile effects of oxytocin and of vasopressin and their interaction with magnesium were studied. Oxytocin (0. 01-0. 2 U/ml) without preservative contricted canine arteries. Oxytocin with chlorobutanol as a preservative (Syntocinon, 0. 2 [mu]/ml) dilated canine artery strips, but the vasodilator effect could be reproduced by chlorobutanol (100 /[mu]g/ml) alone. The dilator effects of both Syntocinon and chlorobutanol were blocked by nethalide (10-50 [mu]g/ml). The contractile response of Mg-depleted vascular strips to vasopressin was decreased, although their Mg content (2. 6 mmoles/kg tissue water) was sufficient for full activity of the contractile elements, as indicated by the normal responses to maximally stimulating concentrations of 1-epinephrine. The contractions produced by posterior pituitary polypeptides in isolated human umbilical veins, canine arteries, and rabbit mammary strips were each potentiated by magnesium. Oxytocin- and vasopressin-induced contractions of depolarized vascular strips were also potentiated by magnesium. A semiquantitative analysis of the interaction of Mg with neurohypophyseal hormones suggests that these results are compatible with the hypothesis that Mg increases the affinity between oxytocin, vasopressin, and receptors in contractile tissue.