Role of the sodium-calcium exchange mechanism and the effect of magnesium on sodium-free and high-potassium contractures in pregnant human myometrium.
OBJECTIVE
Periodic uterine relaxation in labor relieves the fetus from hypoxia related to sustained contractions. A reduction in intracellular calcium is essential for relaxation. Therefore we aimed to clarify the extrusion mechanisms of intracellular calcium ions in pregnant human myometrium.


METHODS
Isometric contraction of small muscle bundles obtained from human myometrium at term was recorded. High-potassium and sodium-free solutions could induce stable contractures that revealed initial phasic contractions followed by tonic contractions. The effects of external calcium, external magnesium, nifedipine, and calcium adenosine triphosphatase inhibitors on both contractures were then examined.


RESULTS
Because maximum tonic contraction was induced by 48 mmol/L potassium, this concentration was used. Both contractures depended on external calcium concentrations. Nifedipine, 10(-7) mol/L inhibited both initial phasic contractions and the tonic phase of the high-potassium contracture; however, it could not suppress the tonic phase of the sodium-free contracture. Calcium adenosine triphosphatase inhibitors, cyclopiazonic acid and lauryl sulfate, potentiated the tensions of tonic phases in both contractures; the effect of lauryl sulfate was stronger than that of cyclopiazonic acid. When external magnesium was gradually increased (from 0 to 9.6 mmol/L), both phasic contractions and the tonic phase of the high-potassium contracture transiently increased, followed by a reduction of tension, whereas the tonic phase of the sodium-free contracture was markedly inhibited by magnesium in a dose-dependent manner.


CONCLUSIONS
The presence of the sodium-calcium exchange mechanism and the specific inhibitory effect of magnesium were indicated in human myometrium during pregnancy. These mechanisms might prevent the long tonic contractions, to protect the fetus from hypoxia during pregnancy and parturition.
Morishita
F
F
Department of Obstetrics and Gynecology, School of Medicine, Fukuoka University, Japan.
Kawarabayashi
T
T
Sakamoto
Y
Y
Shirakawa
K
K
D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002352:Carrier Proteins / Q000502:physiology*; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D007537:Isometric Contraction / Q000187:drug effects; D008274:Magnesium / Q000494:pharmacology*; D009215:Myometrium / Q000187:drug effects; D009543:Nifedipine / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology*; D011247:Pregnancy / Q000502:physiology*; D012964:Sodium / Q000494:pharmacology*; D012967:Sodium Dodecyl Sulfate / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger; D014590:Uterine Contraction / Q000187:drug effects*
D002352:Carrier Proteins; D007211:Indoles; D019831:Sodium-Calcium Exchanger; D012967:Sodium Dodecyl Sulfate; D012964:Sodium; C028913:dodecyl sulfate; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D009543:Nifedipine; D011188:Potassium; C000543:cyclopiazonic acid