Magnesium sulfate tocolysis works by which mechanism?

• A. Interferes with calcium uptake, reducing contractions
• B. Directly relaxes smooth muscle by potassium blockade
• C. Increases uterine contractility
• D. Inhibits prostaglandin synthesis only

Answer: (A)

Magnesium sulfate tocolysis works by blocking calcium entry into uterine smooth muscle, acting as a calcium antagonist. By reducing intracellular calcium, it diminishes calcium-calmodulin activation of myosin light-chain kinase, lowering cross-bridge cycling and causing uterine relaxation to suppress contractions. It can also dampen neuromuscular transmission, further reducing excitability. It’s not achieved through potassium blockade, it does not increase contractility, and it is not solely about inhibiting prostaglandin synthesis.