Relative changes in mRNA expression of angiopoietins and receptors tie in bovine corpus luteum during estrous cycle and prostaglandin F _2α-induced luteolysis: A possible mechanism for the initiation of luteal regression

Local angiogenesis and angiolysis in the corpus luteum (CL) relate to the luteal function. Recent studies indicate that angiopoietins (ANPT) and their receptors Tie regulate remodeling of microvasculature. We therefore examined 1) the relative changes in the expression of mRNA for ANPT-1, ANPT-2, Tie1 and Tie2 in bovine CL by semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) during the estrous cycle and prostaglandin F _2α (PGF_2α)-induced luteolysis, and 2) the effect of ANPT-2 on progesterone (P₄) release from CL at the late stage of the estrous cycle by an in vitro microdialysis system (MDS). The CLs were classified into 4 stages (early: Day 2-5, n=7, mid: Day 8-12, n=15, late: Day 15-17, n=9, regressing: Day >18, n=19). The levels of ANPT-1 mRNA in early and regressing CL were lower than those in mid and late CL, whereas ANPT-2 mRNA expression did not change during the estrous cycle. The Tie2 mRNA expression decreased as the CL aged. During PGF_2α-induced luteolysis, ANPT-2 mRNA expression was acutely and temporally increased at 2 h after PGF_2α injection. The expression of ANPT-1 mRNA was decreased from 4 h after PGF_2α injection and kept low levels. In the experiment with the in vitro MDS, an infusion of ANPT-2 (100 ng/ml) acutely inhibited P₄ release from late CL. Overall, results suggest that decrease of ANPT-1 mRNA is a basic mechanism of vascular remodeling in CL. In addition, ANPT-2 might play a role in regulation of P₄ secretion in CL during luteolysis.