PARENT SESSION

(T173) ADMINISTRATION OF A NITRIC OXIDE DONOR INTO THE CORPUS LUTEUM INCREASES LUTEAL BLOOD FLOW, STARTS THE CASCADE OF LUTEOLYSIS, AND SHORTENS THE ESTROUS CYCLE IN THE COW.

Shirasuna, Koumei¹, Watanabe, Sho¹, Yamamoto, Dai¹, Morota, Keiko¹, Asahi, Takayuki¹, Miyamoto, Akio¹, ¹ Obihiro University of Agr. & Vet. Med., Obihiro, Japan

ABSTRACT- Recently, we have shown that luteal blood flow (BF) temporarily but drastically increases prior to the onset of spontaneous luteolysis, and this is accompanied by upregulation of mRNA expression for endothelial nitric oxide synthase (eNOS). Thus, we hypothesized that this acute increase of luteal BF is due to vasodilation of arterioles surrounding the CL by the action of nitric oxide (NO), and has a key role to start the cascade of CL regression in the cow. Thus, we examined if administration of NO donor into the CL is capable of inducing luteal BF increase followed by the occurrence of spontaneous luteolysis in the cow. A NO donor (S-nitroso-N-acetylpenicillamine [SNAP]; 5 mg/500l; n=5) or vehicle (n=5) was directly injected twice at 0 and 4 h (first injection = 0 h) into the CL on Day 14 of the estrous cycle. The CL volume and BF area within the CL (>2mm/sec) were observed by using transrectal color Doppler ultrasonography. Serial luteal mini-biopsies were applied at 0, 0.5, 4, 12, 24, 48 and 72 h, and mRNA expressions were quantified by real-time PCR. Plasma concentrations of progesterone (P) were determined by EIA. After 30 min of the first NO donor injection, the luteal BF area increased to 180 % and maintained at high levels up to 8 h. Plasma P level started to decrease at 36-48 h, and the CL volume at 48-60 h. The expression of endothelin-1 mRNA within the CL started to increase from 12 h when luteal BF returned to the basal level, followed by the decrease in StAR and 3HSD mRNA expressions from 24 to 48 h. Consequently, administration of NO donor shortened the cycle length compared with that of vehicle (17.5 ± 0.6 days vs. 21.3 ± 0.7 days). These results support our hypothesis that an acute increase in luteal BF induced by NO is essential to start luteolytic cascade that involves endothelin system within the CL in the cow. Supported by JSPS and the 21^st Century COE Program (A-1).

KEY WORDS: corpus luteum, luteolysis, nitric oxide, luteal blood flow