Changes in prostaglandin secretion by the regressing bovine corpus luteum

Secretion of prostaglandins (PGs) by the regressing corpus luteum (CL) was investigated in the cow. Six cows were implanted with microcapillary dialysis membranes of a microdialysis system (MDS) into the CL during Days 8-9 (Day 0=estrus), and a prostaglandin (PG) F_2α analogue (Estrumate) was injected intramuscularly (i.m.) to induce luteolysis. Acute increases in intraluteal release of PGF_2α and PGE₂ were observed during the first 4h, followed by decreases over the next 8h. Intraluteal release of both PGs gradually increased again during the period 48-72h. Concentrations of PGF_2α in ovarian venous plasma (OVP) were 4-13 times higher than those of jugular venous plasma (JVP) (P<0.001) during the period of the experiment, and increased from 24h after treatment with Estrumate (P<0.05). Cyclooxygenase (COX)-2 mRNA expression increased (P<0.05) at 2 and 24h after treatment with Estrumate. The results indicated that local release of PGF_2α and PGE₂, and COX-2 mRNA expression were increased by Estrumate in the regressing CL at the later stages of luteolysis. Thus, luteal secretion of PGs may be involved in the local mechanism for structural rather than functional luteolysis.