PARENT SESSION
SLIDE SESSION 10: MECHANISMS OF LUTEAL REGRESSION
Chairs: Bo Rueda, Jack Carlson, Carlos Stocco (Trainee)
Arts Hall 026
1:30 PM-3:30 PM

EFFECT OF ENDOTHELIN-1 ON FUNCTIONAL LUTEOLYSIS IN THE COW.

Miyamoto, Akio¹, Oki, Naoko¹, Ohtani, Masayuki¹, ¹

ABSTRACT- Prostaglandin F₂ (PGF₂) is the primary luteolysin in the cow. During PGF₂-induced luteolysis, the blood flow decreases in the ovarian artery, and the endothelin-1 (ET-1) secretion within the corpus luteum (CL) increases, suggesting the important roles of ET-1 in luteolysis. Thus, this study was aimed to examine the effect of intraluteal injection of ET-1 and the interaction between ET-1 and PGF₂ on the function of midcycle-CL (PGF₂-responsive: Exp. 1) and early-CL (PGF₂-resistant: Exp. 2). Before starting Exp. 1, we established a model in which 125 g cloprostenol (PG) injection (a quarter amount of normal luteolytic dose of 500 g) causes the temporary reduction of CL function, but the CL recovers to the same ability as normal CL. In this model, the cow was injected with PG i.m. first, and then ET-1 or vehicle was injected directly into the CL 30 min after PG. In Exp. 1 (Day 10), the treatment groups (n=5-7/group) are as follows: 1) no treatment (normal cycle), 2) PG (PG full-dose), 3) 0.25 mg ET-1 (ET-1), 4) 1/4 dose PG and vehicle (PG(1/4)), and 5) 1/4 dose PG and ET-1 (PG(1/4)+ET-1). In Exp. 2 (Day 5), the treatment groups are as follows: 1) PG and vehicle (PG), 2) PG and ET-1 (PG+ET). Blood samples were collected by caudal venipuncture once a day from 3 days before treatment until 5 days after treatment (8 days in total). In Exp. 1, PG(1/4)+ET-1 resulted in the rapid reduction of plasma progesterone (P₄) concentration from the next day as similar as the treatment of PG full-dose. The plasma P₄ concentration in PG(1/4)+ET-1 maintained very low (1-2 ng/ml) which was significantly higher than that of PG full-dose (0.5 ng/ml), but lower than those of other treatment groups (4-7 ng/ml). The ET-1 concentration was rapidly increased by the PG full-dose injection. In Exp. 2, both treatments (PG and PG+ET-1) have no clear effect on hormonal concentration in plasma. Overall results support the concept that the interaction of ET-1 with PGF₂ induces the functional luteolysis in the cow. On the contrary, the reason why the early CL is resistant to luteolytic injection of PGF₂ remains unclear. Supported by JSPS grants.

KEY WORDS: endothelin-1, prostaglandin F₂, luteolysis, cow