Secretion of oxytocin in pregnant and parturient cows: Corpus luteum may contribute to plasma oxytocin at term

Plasma oxytocin (OT) concentrations were determined in 14 late-pregnant and parturient Angus-Hereford cows. jugular and utero-ovarian veins were cannu lated for simultaneous withdrawal of blood samples. Samples were collected at 10-min intervals for 6 h once weekly beginning 60-14 days before the dat e of expected delivery (group 1), or daily 3-7 days before the due date (gr oup 2). In a third group, samples were collected at 15-min intervals every other day for 12 h beginning I wk before calving. Basal levels of OT were l ow, the overall mean for both veins was 0.46 +/- 0.03 muU/ml until a week b efore parturition, and then increased to 0.77 +/- 0.1 muU/ml (P < 0.02). Sp urts of OT occurred intermittently on all days. Interpeak intervals average d 71.0 +/- 10.7 min until Day -14, and from Day -14 to Day -1 the intervals were 44.0 +/- 5.3 min (P < 0.05). From Day -60 to Day -25 the amplitudes o f OT peaks were low and similar in both veins (mean 1.37 +/- 0.1 muU/ml). F rom Day -14 to Day -1 the peak amplitudes were 3.6 +/- 0.4 muU/ml on averag e (P < 0.02). During the last 2 wk the utero-ovarian peak of OT was frequen tly higher than the peripheral peak. In addition, a number of spurts were o bserved in the utero-ovarian vein only (solo peaks). On the day of parturit ion during the first stage of labor, peak amplitudes had increased to 7.3 /- 2.0 muU/ml, and the interpeak intervals had become shorter than before l abor (mean 25.1 +/- 2.6 min). A large surge of OT initiated the expulsive s tage of labor. Basal levels rose to 43.1 +/- 16 muU/ml and 38.7 +/- 12.6 mu U/ml, and peak levels to 77.4 +/- 19.1 muU/ml and 91.6 +/- 21 muU/ml in the jugular and utero-ovarian veins, respectively. Interpeak intervals had dec reased to 17.2 +/- 3.3 min (P < 0.05). Oxytocin levels remained high after delivery of the calf until the placenta was expelled. The posterior pituita ry was the source of circulating OT during most of gestation and labor, but the solo peaks observed during late gestation in the utero-ovarian vein we re probably of luteal origin or possibly of caruncular origin, because near term, both tissues express OT mRNA. Fetal posterior pituitary is another p ossible source for these peaks. Our conclusions are that during bovine preg nancy, low amplitude spurts of OT are secreted intermittently; near term, b oth the frequency and peak amplitude of the spurts increase; and during lab or, a dramatic increase in plasma OT precedes the expulsion of the calf. Th e main source of OT is the posterior pituitary, but near term, a utero-ovar ian source secretes additional OT into the systemic circulation.