Characterization of Porcine Endometrial, Myometrial, and Mammary Oxytocin Binding Sites during Gestation and Labor1

Abstract

Changes in oxytocin (OT) receptor expression have been found to be an important determinant of the physiological effect of OT in several species. To date there are no published studies of OT binding sites during pregnancy in the pig. The purpose of the present study is to improve understanding of the role of OT in porcine parturition. The concentration and affinity of OT binding sites were determined for myometrium and endometrium from pregnant and postpartum gilts. Tissues were obtained after slaughter from 7 animals in each of four groups: 1) 90 days gestation, 2) 112 days gestation, 3) term after milk letdown (before delivery), and 4) within 1-3 h after farrowing. Mammary tissues were obtained for some animals in each group (n = 3-5/group). Before slaughter, blood was collected from each animal and assayed for estradiol-17 beta, progesterone, 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM), and OT. Binding of 3H-OT in the three tissues was concentration- and time-dependent. Sites of 3H-OT binding (fmol/mg protein +/- SEM) increased toward term for each tissue and remained elevated in the postpartum group. Endometrial and mammary tissues displayed the most acute increases in OT binding site concentrations while myometrial tissues displayed a more gradual increase in OT binding sites over the times studied. The binding sites displayed high affinity for 3H-OT and were characterized by linear Scatchard plots. Concentrations of estradiol-17 beta, PGFM, and OT (pg/ml +/- SEM) were positively correlated with 3H-OT binding site concentrations, whereas progestrone concentrations (ng/ml +/- SEM) were negatively correlated with binding site concentration, as determined by Pearson's Correlation Analysis. The data represent the first account of changes in the expression of OT binding sites on porcine tissues during gestation and labor.

Uterine and Placental Expression of Canine Oxytocin Receptor During Pregnancy and Normal and Induced Parturition

“…Uterine sensitivity to OT is greatly increased during parturition in all species investigated (Blanks and Thornton 2003). This change in uterine OTR content, which is strongly related to the rapid increase in endometrial and myometrial OTR expression around the onset of labour, has been demonstrated in several species, for example in humans (Kimura et al 1992), rats (Ou et al 1998), sheep (Meier et al 1995) and pigs (Lundin-Schiller et al 1996). Besides myometrial contractile activity, OT-mediated effects may include ripening of the cervix (Kelly and Tan 2001).…”

“…) and pigs (Lundin‐Schiller et al. ). Besides myometrial contractile activity, OT‐mediated effects may include ripening of the cervix (Kelly and Tan ).…”

Oxytocin Secretion and Management of Parturition in the Pig

“…The influence of steroid hormones and PGF2a on OT receptors has been demonstrated in pregnant rats (Gorodeski et al 1990, Elalj et al 1993, Liu et al 1996. In pigs, myometrial OT receptors are up-regulated in pregnancy (Lundin-Schiller et al 1996, Yu et al 1995 although the exact time at which these receptors appear relative to birth has been difficult to determine. It is therefore unknown whether the formation of OT receptors is normally rate limiting to parturition.…”

“…Non-genomic inhibition of OT binding to uterine receptors at parturition by progesterone has recently been described (Grazzini et al 1997). Endometrial OT receptors also rise towards parturition (Lundin-Schiller et al 1996), providing a means by which OT could stimulate uterine PGF2a secretion (Tysseling et al 1996). The gradual increase in excitability of the porcine uterine musculature from 7-10 hours preceding birth has been characterised (Watts et al 1988, Porter et al 1986.…”

The central interaction of relaxin with oxytocin

“…Oxvtocin is also responsible for milk letdown. Oxytocin receptors are upreguLued in the uterus on the day of parturition (Alexandrova and Soloff, 1980) and in the mammarv gland immediatelv following parturition (Lundin-Schiller et al, 1996). Oxytocin has also been implicated as an osmoregulator, due to its response to slight variation^ in the plasma osmolalitv ot rats (LeiVIoine and Young, 1992).…”