[Biochemical and morphological study of pregnant rat myometrium: role of steroid receptors and gap junction formation in onset of parturition].

In order to understand the mechanism involved in the onset of labor in rats, a series of biochemical and morphological studies were done on pregnant rat myometrium, during the latter half of pregnancy and in the postpartum period. It was observed that plasma progesterone (P) levels gradually declined between day 15 and 20 of pregnancy as did cytosol (PRC) and nuclear P receptors (PRN). In contrast, nuclear estrogen receptors (ERN), both occupied and unoccupied, increased in number during this period of time. Between 24 to 6 hrs prior to labor, plasma P levels and PRN reached a nadir (day 15 v.s. day 21; 776 +/- 88.5 and 187.4 +/- 40.4 fmol/mg DNA, mean +/- S.E., p less than 0.01), while ERN showed an abrupt increase (day 21 v.s. day 22; 1710.6 +/- 61.1 and 3254.8 +/- 203.8 fmol/mg DNA, p less than 0.01), and remained high during labor. After delivery, ERN gradually decreased, but PRN showed a surge between 6 to 12 hrs postpartum in concert with the rise in plasma P levels due to ovulation. Electromicroscopic quantitation of gap junctions (gis) also showed a sharp increase in the number of gjs during labor (day 21 v.s. during labor; 2.0 +/- 0.4 and 17.9 +/- 1.5 gjs per 1,000mcm of plasma membrane, p less than 0.01). A large number of gjs were observed during the first 12 hrs postpartum, but they abruptly disappeared thereafter. These observations suggest that in rats, prepartum accumulation of ERN may facilitate the formation of gjs, which serve as a means for either electrical or chemical excitations to synchronize myometrial contractions and initiate/sustain labor. After delivery, on the other hand, P seems to down-regulate the number of gjs and induce rapid myometrial quiescence during postpartum ovulation which allows the myometrium to be receptive to fertilized ovum.