Effect of ovarian steroids on membrane ATPase activities in microsomes (microsomal fractions) from rat myometrium. Inhibition of a component of the Mg2+-activated ATPase by Ca2+-calmodulin and by oxytocin.
The activities of Mg2+-ATPase (Mg2+-activated ATPase), (Ca2+ + Mg2+)-activated ATPase and (Na+ + K+)-activated ATPase have been determined in microsomes (microsomal fractions) obtained from rat myometrium under different hormonal conditions. Animals were either ovariectomized and treated for a prolonged period of time with 17 beta-oestradiol or progesterone, or myometria were obtained at day 21 of pregnancy. In each case the endometrium was carefully removed. The Mg2+-ATPase consists of two components: an inactivating labile component and a second constant component. The rate of ATP hydrolysis by the labile component of the Mg2+-ATPase declines exponentially as a function of time after adding the membranes to the assay medium; this inactivation is caused by the presence of ATP in the medium. This ATPase activity inhibited by ATP is catalysed by a labile enzyme and hence it gradually diminishes within a few hours, even when the microsomes are kept on ice. This labile component has the highest activity in microsomes from pregnant rats, a lower activity in progesterone-treated rats, and the lowest in 17 beta-oestradiol-treated rats. This component of the Mg2+-ATPase is not affected by 90 nM-oxytocin. The constant component of the Mg2+-ATPase must be ascribed to a different enzyme, which, in contrast with the labile component, is very stable and not affected by the hormonal status of the animal. This constant component of the Mg2+-ATPase is inhibited both by Ca2+-calmodulin, and by oxytocin in microsomes from pregnant and from progesterone-treated animals, whereas such inhibition does not occur in microsomes from 17 beta-oestradiol-treated animals. The activity of the (Na+ + K+)-activated ATPase is not dependent on the hormonal status of the animal. Myometrial microsomes present an ATP-dependent Ca2+ transport, irrespective of the hormonal condition, but only in microsomes obtained from rats treated with 17 beta-oestradiol, can a (Ca2+ + Mg2+)-activated ATPase activity be demonstrated. This activity can be stimulated by calmodulin.
Missiaen
L
L
Department of Obstetrics and Gynaecology, Katholieke Universiteit Leuven, Belgium.
Wuytack
F
F
Casteels
R
R
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D002118:Calcium / Q000493:pharmacokinetics; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002147:Calmodulin / Q000494:pharmacology*; D004958:Estradiol / Q000494:pharmacology*; D005260:Female; D008861:Microsomes / Q000187:drug effects / Q000201:enzymology; D009215:Myometrium / Q000187:drug effects / Q000201:enzymology*; D010121:Oxytocin / Q000494:pharmacology*; D011247:Pregnancy; D011374:Progesterone / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism
D002147:Calmodulin; D011374:Progesterone; D004958:Estradiol; D010121:Oxytocin; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium