Expression and localization of apelin and its receptor APJ in the bovine corpus luteum during the estrous cycle and prostaglandin F_2α-induced luteolysis

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-protein-coupled 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_2α. (PGF_2α)-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_2x 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_2x-induced luteolysis. Additionally, apelin mRNA and protein were stimulated at 1 h after PGF_2α 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_2α 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.