In the present study, we characterized four myometrial contraction-associated proteins (mCAPs): oxytocin receptor (OTR), prostaglandin H synthase 2 (PGHS2), estrogen receptor alpha (ERalpha), and heat shock protein 90 (Hsp90) messenger RNA (mRNA) expression in the nongravid horn of pregnant sheep and compared them with their expression in the gravid horn that is exposed to a greater degree of stretch. We also examined the regulatory effects of estrogen and progesterone on OTR mRNA expression in ovariectomized nonpregnant sheep. In addition, we determined the ontogeny of mCAP expression in the gravid horn throughout late pregnancy and during spontaneous term labor. Gravid horn and nongravid horn myometria were removed under general anesthesia from control ewes not in labor at 130-140 days gestational age (dGA; n = 3) and during betamethasone-induced labor (n = 6) at the same gestational age. Gravid horn myometrium was also collected from ewes not in labor at 95 dGA (n = 3), 101-110 dGA (n = 3), 111-120 dGA (n = 3), 121-130 dGA (n = 3), 131-140 dGA (n = 3), and 141-145 dGA (n = 4) and from ewes in spontaneous term labor (n = 4). All ewes were carrying single fetuses. Myometrium was also collected from ovariectomized nonpregnant ewes treated with saline (n = 5), estradiol (50 microg/day; n = 5), progesterone (0.3 g, intravaginally; n = 5), and estradiol plus progesterone (n = 5). Myometrial RNA was extracted and analyzed by Northern blot for OTR, PGHS2, ERalpha, and Hsp90 mRNA, normalized for 18S ribosomal RNA or beta-actin. ERalpha, Hsp90, OTR, and PGHS2 mRNA were all significantly up-regulated during betamethasone-induced labor (P < 0.01) in gravid and nongravid horn myometrium. The level of gravid horn OTR mRNA during labor was 3 times the level of nongravid horn OTR mRNA (P < 0.0001). Gravid horn PGHS2 mRNA was also higher than nongravid horn PGHS2 (P < 0.02). In contrast, in spontaneous term labor nongravid horn, ERalpha and Hsp90 mRNA were similar to gravid horn. Myometrial ERalpha and Hsp90 mRNA remained unchanged throughout late pregnancy and increased at spontaneous term labor (P < 0.05). In contrast, myometrial OTR increased around 130 dGA (P < 0.01) and further increased at spontaneous term labor (P < 0.02). Progesterone significantly inhibited myometrial OTR mRNA expression in nonpregnant sheep and estradiol antagonized progesterone's inhibitory effect. Mechanical stretch differentially regulated mCAP mRNA expression in the ovine gravid horn and nongravid horn. Mechanical stretch appears largely responsible for increased OTR mRNA and to a lesser degree PGHS2 mRNA. In addition, endocrine factors may be required for full activation of OTR and PGHS2 mRNA associated with labor. ERalpha and Hsp90 mRNA are not under the control of uterine stretch in keeping with our previous results, indicating that systemic hormones such as estradiol, are prime regulators for these two mCAP mRNA expression during labor.
