Hormonal control of uterine contraction. Characterization od cyclic AMP-dependent membrane properties in the myometrium.

A mitochondria-free membrane fraction prepared from rat myometrium accumulated 45Ca2+ in the presence of oxalic acid and ATP. The rate of transport of Ca2+ into the membranous vesicles was increased by greater than 50% in the presence of 3',5'-cyclic AMP, but not by 2',3'-cyclic AMP or 5'AMP. Membrane ATPase activity was stimulated by Mg2+; slight additional stimulation was obtained in the presence of Na+ and K+ but not in the presence of Ca+2. Despite the cyclic AMP sensitivity of membrane ATPase activity, the absence of any effect of inhibitors of Ca2+-transport suggest it has little to do with Ca2+ accumulation by the membranes. Cyclic AMP-induced increase in Ca2+-transport and membrane ATPase activity was duplicated in vivo by incubating uteri in 10(-4)M isoproterenol prior to membrane isolation. Isoproterenol has been previously shown to increase myometrial cyclic AMP levels, and changes in Ca2+-transport by cell membranes in relation to intracellular cyclic AMP levels may be the mechanism through which hormones modulate uterine contractility.