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Parathyroid Hormone-Related Protein Treatment of Pregnant Rats Delays the Increase in Connexin 43 and Oxytocin Receptor Expression in the Myometrium¹

Samuel Lunenfeld Research Institute,³ Mount Sinai Hospital, Toronto, Ontario M5G 1X5, Canada Institute of Medical Science⁴ and Departments of Obstetrics & Gynecology and Physiology, ⁵ University of Toronto, Toronto, Ontario M5S 1A8 , Canada Department of Obstetrics and Gynecology,⁶ University of Alberta, Edmonton, Alberta T5H 3V9, Canada

Myometrial quiescence during pregnancy is maintained by progesterone, which suppresses the expression of labor-associated genes such as connexin 43 (Cx43) and the oxytocin receptor (OTR). Parathyroid hormone-related protein (PTHrP) is a smooth muscle relaxant that inhibits myometrial contractions and therefore may act in synergy with progesterone to maintain myometrial quiescence during late pregnancy. We investigated the possibility that PTHrP, like progesterone, could act to suppress the expression of labor-associated genes. Pregnant rats were treated starting on Day 19 with daily i.p. injections of 100 µg/kg PTHrP (human synthetic fragment 1-34). On Day 22 of gestation, there was a significant reduction in the expression of Cx43 (mRNA and protein) and OTR (mRNA) in the myometrium of PTHrP-treated animals, whereas on Day 23 (labor) the expression of both Cx43 and OTR was unchanged by PTHrP treatment. Treatment of pregnant rats with PTHrP did not affect the time of delivery, concentrations of progesterone in maternal plasma, or levels of c-fos, fra-2, or parathyroid hormone/PTHrP receptor mRNA on any gestational day. Because PTHrP treatment delayed the dramatic increase in the expression of Cx43 and OTR, it may be an important factor in the maintenance of the quiescent state of the myometrium at a time when the concentrations of progesterone in maternal circulation decrease. PTHrP treatment did not prevent the increase in Cx43 and OTR gene expression on Day 23 or the timing of labor, suggesting that the effects of PTHrP signaling are overridden with the onset of labor.

¹ This study was supported by a grant from the Canadian Institutes of Health Research (37775) and a Doctoral Research Award from the Canadian Institutes of Health Research to J.A.M.

² Correspondence: Stephen J. Lye, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Ave., Suite 982, Toronto, ON, Canada M5G 1X5. FAX: 416 586 8857; lye{at}mshri.on.ca

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