What is the primary action of magnesium sulfate as a tocolytic?

Magnesium sulfate primarily acts as a tocolytic by acting on vascular smooth muscle to induce vasodilation. This mechanism is important because the relaxation of smooth muscles helps in decreasing uterine contractions, which is essential in managing preterm labor. By promoting vasodilation, magnesium sulfate can increase blood flow and reduce the chances of uterine hyperactivity, thereby serving as an effective tocolytic agent.

The ability of magnesium sulfate to cause relaxation in the smooth muscle also contributes to its effects in preventing seizures in conditions such as eclampsia, but its primary use as a tocolytic lies in its ability to reduce uterine tone and help in managing preterm contractions. This is why the option regarding its action on vascular smooth muscle is correct in the context of its use as a tocolytic.