The initiation of parturition is caused by the interaction of various hormonal stimuli. Their precise mode of action on the cellular and subcellular level is largely unknown. Prostaglandin F2 alpha appears to be the final activator of the uterus. Since calcium is required for contraction, in muscle systems, we hypothesize that hormones such as prostaglandins (PG) and oxytocin exert their effect through regulation of subcellular calcium transport, and we have shown such effects in vitro during the previous period of this research grant. We plan to purify and characterize the subcellular organelles, enzymes and proteins involved in myometrial calcium transport, isolate model systems and study their interaction. Sarcoplasmic reticulum and cell membrane fractions have been separated from one another. ATP-dependent calcium binding and specific binding of (3H)PGE2 and other hormones will be studied in the isolated fractions. Calcium binding proteins and Ca-activated ATPase will be isolated and characterized using chromatography and electrophroesis.