Oxytocin plasma levels during spontaneous and cloprostenol-induced luteolysis in sows

Plasma concentrations of oxytocin (OT) in both the jugular vein (JV) and utero-ovarian vein (UOV) and progesterone and prostaglandin F_2α (PGF_2α) in the UOV were measured in four sows on days 13-17 of the oestrous cycle (Exp. 1). Blood samples were taken every hour. At the day of luteolysis mean daily concentrations of OT in the UOV (4.1 ± 0.2 pg/ml) and in the JV (3.6 ±0.2 pg/ml) were significantly higher (P < 0.05) than the OT plasma levels at the day before luteolysis (3.0 ± 0.1 and 3.1 ±0.2 pg/ml in the UOV and JV, respectively). During luteolysis the level of OT in both the UOV and JV was correlated with PGF_2α level (r = 0.31, P < 0.05). In Exp. 2 the OT concentration in seven sows was measured in blood samples taken frequently from the JV and UOV on day 13 (n = 4) and from the JV, UOV and carotid artery (CA) on day 14 (n = 3) of the oestrous cycle, before and after cloprostenol (PG) injection. A significant rise (P < 0.05) of OT concentration was measured after PG injection at 4 and 6 min in the JV and UOV, respectively, on day 13, whereas on day 14 the rise occurred after 15 min in the JV and CA and after 20 min in the UOV. Maximal OT concentration was significantly higher in the JV compared to the level in the UOV (P < 0.001) on day 13 and to that in the UOV and CA (P < 0.01) on day 14. In contrast, the levels of OT in the UOV and CA were not significantly different (P < 0.1). Our results indicate that: (1) PGF_2α during spontaneous or cloprostenol-induced luteolysis stimulates secretion of OT in sows; and (2) during luteolysis OT is secreted from the pituitary but not from the ovary.