Oxytocin, Oxytocin-Associated Neurophysin, and Prostaglandin F2αConcentrations in the Utero-Ovarian Vein of Pregnant and Nonpregnant Sheep*

Publisher Website
Google Scholar
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 weith 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 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 < 0.001) fewer pulses of PGF2.alpha., which were significantly (P < 0.001) smaller in amplitide, 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 during early pregnancy. the frquency of oxytocin pulses observed in the UOV/CL of pregnant sheep was not significantly (P > 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.