Acute Changes in Circulating Concentrations of Progesterone and Nitric Oxide and Partial Pressure of Oxygen During Prostaglandin F2α-induced Luteolysis in Cattle

To examine whether oxygen (O-2) 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 (pO(2)) during prostaglandin F-2 alpha (PGF(2 alpha)-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(2 alpha) 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(2 alpha) 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 pO(2) was significantly higher in OV blood than in JV blood (P<0.05). PGF(2 alpha) injection increased pO(2) in OV blood between 0.5 and 2 h. These results demonstrate that PGF(2 alpha) induced an acute increase in pO(2) and NO in the ovarian circulation and suggest that O-2 and NO are involved in the early events of CL regression, including inhibition of P4 secretion and output, in cattle.