Prostaglandin F 2a (PGF 2a ) released from the uterus causes alterations in luteal blood flow, reduces progesterone secretion, and induces luteolysis in the bovine corpus luteum (CL). We have recently discovered that luteal blood flow in the periphery of the mature CL acutely increases coincidently with pulsatile increases in a metabolite of PGF 2a (PGFM). In this study, we characterized changes in regional luteal blood flow together with regional alterations in endothelial nitric oxide synthase (eNOS) expression during spontaneous luteolysis and in response to PGF 2a . Smooth muscle actin-positive blood vessels larger than 20 mm were observed mainly in the periphery of mature CL. PGF 2a receptor was localized to luteal cells and large blood vessels in the periphery of mid-CL. PGF 2a acutely stimulated eNOS expression in the periphery but not in the center of mature CL. Injection of the NO donor S-nitroso-N-acetylpenicillamine into CL induced an acute increase in luteal blood flow and shortened the estrous cycle. In contrast, injection of the NOS inhibitor L-NAME into CL completely suppressed the acute increase in luteal blood flow induced by PGF 2a and delayed the onset of luteolysis. In conclusion, PGF 2a has a siterestricted action depending on not only luteal phase but also the region in the CL. PGF 2a stimulates eNOS expression, vasodilation of blood vessels, and increased luteal blood flow in periphery of mature CL. Furthermore, the increased blood flow is mediated by NO, suggesting that the acute increase in peripheral blood flow to CL is one of the first physiological indicators of NO action in response to PGF 2a .

Angiogenesis, changes in blood flow, and extracellular matrix remodeling are the processes associated with the development and demise of the bovine corpus luteum (CL) during the estrous cycle. APJ (putative receptor protein related to angiotensin type 1 receptor) is a G-proteincoupled receptor, and its ligand, apelin, has been identified as a novel regulator of blood pressure and as an angiogenic factor. We hypothesized that the apelin-APJ system is involved in luteal function. This study investigated whether apelin-APJ exists in bovine CL and determined their expression profiles and localization during luteal phase and prostaglandin F 2a (PGF 2a )-induced luteolysis. During the luteal phase, apelin mRNA expression increased from early to late CL and decreased in regressed CL. APJ mRNA expression increased from early to mid-CL and remained elevated in late and regressed CL. Apelin and APJ proteins were immunohistochemically detected only in the smooth muscle cells of intraluteal arterioles during the luteal phase. PGF 2a stimulated apelin and APJ mRNA expression at 0.5-2 and 2 h respectively, and then the mRNA expression of apelin-APJ was inhibited from 4 h during PGF 2a -induced luteolysis. Additionally, apelin mRNA and protein were stimulated at 1 h after PGF 2a injection only in the periphery of mid-but not early CL. The present study indicated that the apelin-APJ was localized in the smooth muscle cells of intraluteal arterioles, and responded to PGF 2a at the periphery of mid-CL in the cow. Thus, the apelin-APJ system may be involved in the maturation of CL and the luteolytic cascade as a regulator of intraluteal arterioles in cow.

Abstract. We have shown that luteal blood flow increases in the peripheral vasculature of the mature corpus luteum (CL) prior to the onset of luteolysis in response to prostaglandin F2α (PGF2α) in the cow, but this phenomenon does not occur in the early CL. We therefore hypothesize that this acute increase of luteal blood flow occurs by vasodilation of large blood vessels due to local release of nitric oxide (NO). In the present study, we characterized the CL vasculature together with endothelial NO synthase (eNOS) expression during the estrous cycle in the cow. Immunohistochemistry was used to quantify the number of arteriolovenous vessels (surrounded with smooth muscle actin-positive smooth muscle cells), capillaries (with von Willebrand Factor-positive endothelial cells) and eNOS protein. The arteriolovenous vessels existed more in the periphery of the matured CL (mid, late and regressing CL) than in the center region. In the early CL, there were as many arteriolovenous vessels in the periphery as in the center, while more capillaries existed in the center than in the periphery of the mid and late CL. Also, eNOS protein existed in the periphery more than in the center of the matured CL. These results indicate that the early CL has a homogeneous vascular and eNOS distribution. In contrast, the matured CL is a heterogeneous organ having a higher vascular and eNOS distribution in the periphery than in the center. In conclusion, the distribution of arteriolovenous vessels and eNOS in the matured CL was higher in the periphery than in the center of the CL. Thus, this suggests that this structural change from the early (homogeneous) to the mid (heterogeneous) luteal phase is related to the difference in the CL response of blood flow increase due to PGF2α, which is only observed in the mature CL. Key words: Blood vessels, Corpus luteum, eNOS, Luteolysis (J. Reprod. Dev. 56: [124][125][126][127][128][129][130] 2010) rich vascular network is established within the corpus luteum (CL) in order to enable it to function as an endocrine gland. Although the CL is constituted of several types of cells, such as luteal cells, endothelial cells and smooth muscle cells, vascular endothelial cells account for up to 50% of the bovine mature CL [1,2]. Moreover, endothelin-1 (EDN1) and angiotensin II (Ang II) secreted by endothelial cells in the CL have been shown to inhibit progesterone secretion from luteal cells [3][4][5]. These findings indicate that luteal blood vessels have a key role in regulation of luteal function.Functionally, luteal blood vessels can be divided roughly into two types of blood vessels. One of these types is an arteriolovenous vessels, i.e., arteriola and venula vessels, which have a smooth muscle layer and receive the vasorelaxant effect. In general, the diameter of an arteriola is about 40 μm (from 20 μm to 100 μm), and the arteriola has an important role in creation of peripheral resistance for local blood flow. In the circulation of blood, the arteriola connects to microcapillary vessels followed by ...

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