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Positive Association, in Local Release, of Luteal Oxytocin with Endothelin 1 and Prostaglandin F_2alpha During Spontaneous Luteolysis in the Cow: A Possible Intermediatory Role for Luteolytic Cascade Within the Corpus Luteum¹

Graduate School of Animal and Food Hygiene³ and Department of Clinical Veterinary Science,⁴ Obihiro University of Agriculture and Veterinary Medicine, Obihiro 080-8555, Japan

ABSTRACT

Luteolysis is caused by a pulsatile release of prostaglandin F_2alpha (PGF_2alpha) from the uterus in ruminants, and a positive feedback between endometrial PGF_2alpha and luteal oxytocin (OXT) has a physiologic role in the promotion of luteolysis. The bovine corpus luteum (CL) produces vasoactive substances, such as endothelin 1 (EDN1) and angiotensin II (Ang II), that mediate and progress luteolysis. We hypothesized that luteal OXT has an additive function to ensure the CL regression with EDN1 and Ang II, and that it has an active role in the luteolytic cascade in the cow. Thus, the aim of the present study was to observe real-time changes in the local secretion of luteal OXT and to determine its relationship with other local mediators of luteolysis. Microdialysis system (MDS) capillary membranes were implanted surgically into each CL of six cyclic Holstein cows (18 lines total among the six cows) on Day 15 (estrus == Day 0) of the estrous cycle. Simultaneously, catheters were implanted to collect ovarian venous plasma ipsilateral to the CL. Although the basal secretion of OXT by luteal tissue was maintained during the experimental period, the intraluteal PGF_2alpha secretion gradually increased up to 300% from 24 h after the onset of luteolysis (0 h; time in which progesterone started to decrease). In each MDS line (microenvironment) within the CL, the local releasing profiles of OXT were positively associated with PGF_2alpha and EDN1 within the CL in all 18 MDS lines implanted in the six CLs (OXT vs. PGF_2alpha, 50.0%; OXT vs. EDN1, 72.2%; P < 0.05). On the other hand, the intraluteal OXT was weakly related to Ang II (OXT vs. Ang II, 27.7%). In the ovarian vein, the peak concentration of PGF_2alpha increased significantly when the peak of PGF_2alpha coincided with the peak of OXT after the onset of spontaneous luteolysis (P < 0.05). In conclusion, intraluteal OXT may locally modulate secretion of vasoactive substances, particularly EDN1 and PGF_2alpha within the CL, and thus might be one of the luteal mediators of spontaneous luteolysis in the cow.

corpus luteum,, corpus luteum function, endothelin 1, luteolysis, ovary, oxytocin, progesterone, prostaglandinF₂

¹Supported by the Grant-in-Aid for Scientific Research of the Japan Society for the Promotion of Science (JSPS) and the 21st Century COE Program (A-1), Ministry of Education, Culture, Science, and Technology, Japan. K.S. and K.G.H. are supported by a JSPS Research Fellowship for Young Scientists.

Correspondence: ²FAX: 81 155 49 5459; e-mail: akiomiya{at}obihiro.ac.jp