Oxytocin, Oxytocin-Associated Neurophysin, and Prostaglandin F_2αConcentrations in the Utero-Ovarian Vein of Pregnant and Nonpregnant Sheep*

Abstract

Oxytocin, oxytocin-associated neurophysin (neurophysin), prostaglandin F2 alpha (PGF2 alpha), and progesterone concentrations were measured in the utero-ovarian vein (UOV) of sheep during the estrous cycle and early pregnancy. On days 13-16 of the cycle, large pulses of PGF2 alpha, oxytocin, and neurophysin were measured in samples collected at hourly intervals from the UOV draining a corpus luteum (UOV/CL). Most of the PGF2 alpha pulses (96.5%) coincided with a pulse of both oxytocin and neurophysin, whereas only 55.6% of oxytocin pulses coincided with a pulse of PGF2 alpha. Therefore, during luteolysis in sheep, uterine PGF2 alpha release is closely associated with ovarian oxytocin release, and oxytocin release is unlikely to be dependent upon a uterine PGF2 alpha stimulus. During frequent sampling, coincident oxytocin pulses were measured in 1) both UOVs when a CL was present in both ovaries and 2) the jugular vein, carotid artery, and UOV/CL, with a significantly higher oxytocin pulse concentration occurring in jugular venous compared with carotid arterial plasma. Pituitary and luteal release of oxytocin may, therefore, occur simultaneously and be controlled by a circulating factor in sheep. Compared to days 13-16 of the cycle, significantly (P less than 0.001) fewer pulses of PGF2 alpha, which were significantly (P less than 0.001) smaller in amplitude, were measured in UOV samples collected frequently during early pregnancy. The frequency of oxytocin pulses observed in the UOV/CL of pregnant sheep was not significantly (P greater than 0.1) different from that observed in cyclic ewes, although most (86.4%) oxytocin pulses occurred in the absence of a PGF2 alpha pulse. In contrast, when a pulse of PGF2 alpha was observed in the UOV/CL of pregnant ewes, it usually coincided with a pulse of oxytocin. The suppression of uterine PGF2 alpha release during early pregnancy is not considered to result from a lack of stimulation by oxytocin.

Relationships among Endometrial Oxytocin Receptors, Oxytocin-Stimulated Phosphoinositide Hydrolysis and Prostaglandin F2α Secretion in Vitro, and Plasma Concentrations of Ovarian Steroids before and during Corpus Luteum Regression in Cyclic Heifers1

“…The increased ability of oxytocin to induce endometrial PI hydrolysis in sheep during late diestrus [18,19] is consistent with similar increases occurring late in the estrous cycle in oxytocin receptor concentrations [7,14,15], oxytocin-induced PGF 2 . secretion [7][8][9], and pulsatile PGF 2 a release [20,21]. A similar mechanism may also be present for oxytocin-induced pulsatile secretion of PGF2, in cattle.…”

Detection of Functional Oxytocin Receptors on Endometrium of Pigs1

“…A second possibility may be that endogenous OT released in vivo may have rendered the endometrium from some gilts somewhat refractory to OT treatment in vitro. The pattern of OT release during diestrus in pigs has not been thoroughly defined; however, release of uterine PGF 2 a near the time of luteolysis appeared to occur more frequently in swine [4] than in sheep [33,34], and PGF 2 a elicited OT secretion from the neurohypophysis of cyclic pigs [6]. Those results indicated a positive feedback loop between PGF 2 a and OT release similar to that present in sheep, although OT was primarily of luteal origin during luteolysis in ruminants [2].…”

Components of a platelet-activating factor-signaling loop are assembled in the ovine endometrium late in the estrous cycle

“…The pulsatile release of luteolytic PGF 2␣ in ewes requires the action of the ovarian sex steroids (29). The release of PGF 2␣ from the endometrium coincides with the pulsatile release of oxytocin from the corpus luteum and posterior pituitary (13). The oxytocin receptor is expressed in the luminal epithelium, caruncular stroma, and deep glands of the endometrium (27,40) and is regulated by the presence of estradiol (10).…”