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.


METHODS
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 Ca(2+)-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 micromol/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.


CONCLUSIONS
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.
Fomin
Victor P
VP
Department of Obstetrics and Gynecology, Indiana University School of Medicine, Indianapolis, IN, USA.
Gibbs
Shawn G
SG
Vanam
Ram
R
Morimiya
Akira
A
Hurd
William W
WW
D002118:Calcium / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D007425:Intracellular Membranes / Q000378:metabolism*; D008278:Magnesium Sulfate / Q000008:administration & dosage / Q000494:pharmacology*; D009215:Myometrium / Q000378:metabolism / Q000502:physiology*; D009994:Osmolar Concentration; D011247:Pregnancy; D013997:Time Factors; D015149:Tocolytic Agents / Q000008:administration & dosage / Q000494:pharmacology*; D014590:Uterine Contraction / Q000187:drug effects*
D015149:Tocolytic Agents; D008278:Magnesium Sulfate; D002118:Calcium