CHARACTERIZATION OF PORCINE ENDOMETRIAL, MYOMETRIAL, AND MAMMARY OXYTOCIN-BINDING SITES DURING GESTATION AND LABOR

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