Effect of magnesium sulfate on contractile force and intracellular calcium concentration in pregnant human myometrium

Objective This study was undertaken to evaluate the effects of magnesium sulfate (MgSO4) on contractile force and increases in free intracellular calcium concentration ([Ca2+]i) in human myometrial strips from pregnant women. Study design Simultaneous measurements of isometric tension and [Ca2+]i were measured in myometrial strips obtained at the time of cesarean delivery from pregnant nonlaboring women at term with the use of a fluorescence spectrometer equipped with a displacement force transducer. Changes in [Ca2+]i were measured with fura-2, a Ca2+-sensitive fluorescent probe. Myometrial strips were exposed to MgSO4 (5 or 10 mmol/L) for 5, 10, 20, and 30 minutes and observed for spontaneous contractions or stimulated with either oxytocin (OT; 0.1 μmol/L) or potassium chloride (KCl; 90 mmol/L). Results MgSO4 reduced spontaneous, OT, and KCl-evoked contractions and increases in [Ca2+]i in a time and concentration-dependent manner. After 20 minutes exposure to 5 mmol/L MgSO4, the OT-elicited changes in contractile response and [Ca2+]i were significantly decreased. MgSO4 did not change [Ca2+]i/force relationship of the responses to OT or KCl, or during spontaneous activity. Conclusion At a pharmacologic concentration (5 mmol/L), MgSO4 inhibits contractile response and [Ca2+]i in pregnant human myometrial strips by a pattern that is consistent with both extra- and intracellular mechanisms. At a suprapharmacologic concentration (10 mmol/L), the more immediate effect of MgSO4 is consistent with an extracellular mechanism. MgSO4 does not appear to interfere at the level of the calcium-calmodulin interface, since the [Ca2+]i/force relationship was not changed.