Ovary

Real-Time Relationships in Intra-Luteal Release among Prostaglandin F₂, Endothelin-1, and Angiotensin II During Spontaneous Luteolysis in the Cow

Abstract
It is well known that prostaglandin F₂ (PGF₂) is a physiological luteolysin and its pulsatile release from the endometrium is a luteolytic signal in many species. There is now clear evidence that the vasoactive peptides, endothelin-1 (ET-1) and angiotensin II (Ang II) interact with PGF₂ in the luteolytic cascade during PGF2-induced luteolysis in the cow. Thus, we investigated the local secretion of PGF₂, ET-1 and Ang II in the corpus luteum (CL) and their real-time relationships during spontaneous luteolysis in the cow. For this purpose, an in vivo microdialysis system (MDS) implanted in the CL was utilized to observe changes in the local secretion in the microenvironment within the CL. Each CL of cyclic Holstein cows (n=6) was surgically implanted with MDS capillary membranes (18 lines/6 cows) on Day 15 (estrus=Day 0) of the estrous cycle. The concentrations of PGF₂, ET-1, Ang II and progesterone (P) in the MDS samples were determined by enzymeimmunoassays. The intra-luteal PGF2 secretion slightly increased from 12 h after the onset of luteolysis (0 h) and drastically increased (about 300 %) from 24 h. The intra-luteal ET-1 secretion increased from 12 h. The intra-luteal Ang II secretion was elevated from 0 h and maintained at high levels (about 180 %) toward the estrus. In each MDS lines (same microenvironment) within the regressing CL, the local releasing profiles of PGF₂, ET-1 and Ang II CL positively correlated with each other (P < 0.05) at the high proportion in 18 MDS lines (PGF₂ vs. ET-1, 44.4 %; PGF₂ vs. Ang II, 55.6 %; ET-1 vs. Ang II, 38.9 %). In contrast, there was no clear relationship among these substances released into different MDS lines implanted in the same CL (different microenvironment). In conclusion, we propose that the increase of PGF₂, ET-1 and Ang II within the CL is common phenomenon for both PGF₂-induced and spontaneous luteolysis. Moreover, this study illustrated the in vivo relationships in intra-luteal release among PGF₂, ET-1 and Ang II during spontaneous luteolysis in the cow. The data suggest that these vasoactive substances may interact with each other in a local positive feedback manner to activate their secretion in the regressing CL, thus accelerate and complete luteolysis.

Key words: Ovary • Corpus luteum • Corpus luteum function • Progesterone

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