Ca(2+)-dependent inhibition of Ca(2+)-independent contraction in uterine smooth muscle.
Uterine smooth muscle of the rat shows Ca(2+)-independent contraction in response to oxytocin in Ca(2+)-free medium. Micromolar Ca2+ inhibits this contraction. We now tested whether Ca2+ itself is the cause of this inhibition. The ratio of fura-2 fluorescence, the indicator of the intracellular level of Ca2+, was increased in parallel with the degree of inhibition by Ca2+. When inhibition was elicited by Ca2+, EGTA released the inhibition. Comparison of the dose-response curve for oxytocin in Ca(2+)-free solution and that in the medium with 1 microM Ca2+ showed that the inhibition by Ca2+ is non-competitive. EGTA chelation of the intracellular Ca2+ by loading of EGTA as its acetoxymethylester resulted in diminution of inhibition by Ca2+. EGTA suppressed the Ca(2+)-induced contraction but did not affect Ca(2+)-independent contraction. It is concluded that the inhibition is induced by intracellular Ca2+ itself.
Ishine
T
T
Department of Molecular Pharmacology, Meiji College of Pharmacy, Tokyo, Japan.
Miyauchi
Y
Y
Gokita
T
T
Matsuo
K
K
Uchida
M K
MK
D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D004533:Egtazic Acid / Q000494:pharmacology; D005260:Female; D016257:Fura-2; D009130:Muscle, Smooth / Q000187:drug effects; D010121:Oxytocin / Q000494:pharmacology*; D051381:Rats; D013050:Spectrometry, Fluorescence; D014590:Uterine Contraction / Q000187:drug effects / Q000502:physiology*
D010121:Oxytocin; D004533:Egtazic Acid; D002118:Calcium; D016257:Fura-2