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LOCAL CHANGES IN THE BLOOD FLOW WITHIN THE EARLY AND MIDCYCLE CORPUS LUTEUM AFTER PGF2
INJECTION IN THE COW.
Acosta, Tomas1, Yoshizawa, Nobuyuki1, Ohtani, Masayuki1, Miyamoto, Akio1, 1
ABSTRACT- One of the main luteolytic actions of prostaglandin (PG) F2 has been postulated to be a decrease in the ovarian blood flow. However, before Day 5 of the normal cycle, the corpus luteum (CL) is resistant to the luteolytic action of PGF2. Therefore, we aimed to determine in detail the real-time changes in the intraluteal blood flow after PGF2 injection at the early and mid stage of the estrous cycle in the cow. Normal cycling cows at Day 4 (early CL) or Day 10-12 (midcycle CL) of the estrous cycle (estrus = Day 0), were examined by transrectal color and pulsed Doppler ultrasonography to determine the blood flow area, the time averaged maximum velocity (TAMXV) and the volume of the CL after an intramuscular injection of PGF2. Blood samples were collected at the time of each ultrasonographic examination that has been carried out just before PG injection (0 h) and then at 0.5, 1, 2, 4, 8, 12, 24, and 48 h for progesterone (P) determination. The ratio of the colored area to a sectional plane at the maximum diameter of the CL was used as a quantitative index of the changes in the blood flow within the luteal tissue. The blood flow within the midcycle CL firstly increased (P < 0.05) at 0.5-2 h, decreased at 4 h to the same levels observed at 0 h, and then further decrease to a lower level (P < 0.05) from 8 to 48 h (P < 0.001). Plasma P concentration decreased from 4.7±0.5 ng/ml (0 h) to 0.6±0.2 ng/ml (24 h). The TAMXV and CL volume decreased at 8 h (P < 0.05) and further decreased (P < 0.001) from 12 h to 24 h after PG injection, indicating structural luteolysis. These changes were not detected in the early CL in which luteolysis did not occur. In the early CL, parallel increases in the CL volume, plasma P concentration and TAMX were observed. The present results indicate that PGF2 induces an acute blood flow increase followed by a decrease within the midcycle CL, but not in the early CL. This transitory increase may trigger the luteolytic cascade. The lack of intraluteal vascular response to PG injection in the early CL appears to directly correlate to the PG-resistant ability. Supported by JSPS grants.
KEY WORDS: corpus luteum, blood flow , prostaglandin F2 , luteolysis