Acute changes in circulating concentrations of progesterone and nitric oxide and partial pressure of oxygen during prostaglandin F_2α-induced luteolysis in cattle

To examine whether oxygen (O2) and nitric oxide (NO) are temporally associated with the acute changes in luteal function during luteolysis, we determined the real-time changes in the circulating concentrations of progesterone (P4) and nitrite/nitrate (the stable metabolites of NO) and the partial pressure of oxygen (pO₂) during prostaglandin F_2α (PGF_2α)-induced luteolysis in cattle. Catheters for frequent blood sample collection were inserted into the ovarian vein (OV), jugular vein (JV) and aorta abdominalis (AA) in 12 cows on Day 9 of the oestrous cycle (oestrus=Day 0). On Day 10, the cows were randomly divided into two groups and treated with a luteolytic dose of a PGF_2α analogue or saline solution (control). Blood samples were collected at -2, -1, 0, 0.25, 0.5, 0.75, 1 and 2 h and then at 2-h intervals until 12 h after treatment (0 h). Injection of a PGF_2α induced a significant decrease in the concentrations of P4 in OV plasma within 2 h. The decrease in P4 concentrations was preceded by an increase in the NO concentrations in the blood collected from OV, JV and AA. Basal pO² was significantly higher in OV blood than in JV blood (P<0.05). PGF_2α injection increased pO² in OV blood between 0.5 and 2 h. These results demonstrate that PGF_2α induced an acute increase in pO² and NO in the ovarian circulation and suggest that O² and NO are involved in the early events of CL regression, including inhibition of P4 secretion and output, in cattle.