The subcellular distribution of endogenous progesterone in pregnant rat myometrium

Abstract

The concentration of endogenous progesterone in the nuclear, mitochondrial, microsomal and cytosolic fractions of rat myometrium was determined during the last trimester of pregnancy and up to 20 h post partum. Progesterone was determined by a radioimmunoassay technique developed for this study. The cytosolic concentration of progesterone in the myometrium was constant from the 14–16th days of pregnancy (9–11 pmol of progesterone/mg cytosolic protein) and began to decrease after the 16–17th day. The cytosolic concentration of progesterone became low on the 21st day of pregnancy (0.5–1.0 pmol/ mg protein) and was practically the same on the 22nd day and 20 h post partum. The decrease in the concentration of endogenous progesterone in the myometrial cytosol was inversely correlated to foetal weight from the 18th to the 21st day of pregnancy. The concentration of endogenous progesterone in the microsomal, mitochondrial and nuclear fractions of rat myometrium was lower than that in the cytosol on the 14th day of pregnancy. Thereafter the concentrations of endogenous progesterone in these particulate fractions approached that of the cytosol and the amounts in the microsomal fraction exceeded the cytosolic concentration after the 18th day. Mitochondrial progesterone reached cytosolic concentrations on the 18th day of pregnancy and nuclear on the 22nd day. The concentration of progesterone decreased steeply in all fractions during the last 4 days of pregnancy and was low in every fraction on the 21st day (one day before parturition). The present results show that the ratios of endogenous progesterone concentrations in the 4 subcellular fractions studied do not vary much on a given day of pregnancy but these ratios change gradually during the last trimester of pregnancy.