Oxytocin, oxytocin-associated neurophysin, and prostaglandin F2 alpha concentrations in the utero-ovarian vein of pregnant and nonpregnant sheep.

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.