Expression of cyclooxygenase 1 and 2 in the canine corpus luteum during diestrus

In the dog, unlike most other domestic animal species, corpus luteum (CL) life span is not affected by hysterectomy. Only in pregnant dogs, during the immediate prepartum decline of progesterone, does PGF2α clearly seem to act as an endogenous luteolytic agent. Whether endogenous PGF2α plays a role in the slow regression of the corpora lutea of the nonpregnant cycle is not known. To test for possible paracrine/autocrine effects of locally produced PGF2α, luteal expression of the key rate-limiting enzymes in prostaglandin biosynthesis, i.e. cyclooxygenase 1 and 2 (Cox1 and Cox2), was examined in dogs during diestrus, including the periods of CL formation, as well as early and late CL regression. Corpora lutea were collected by ovariohysterectomy from nonpregnant bitches 5, 15, 25, 35, 45 and 65 days after ovulation. On the mRNA-level, expression of Cox1 and Cox2 was tested by qualitative and quantitative, Real Time (Taq Man) RT-PCR; on the protein level, expression of Cox2 was studied by immunohistochemistry. The mRNA for Cox1 and Cox2 were detected at all stages of diestrus. Expression of Cox1 was lowest on Day 5 (ovulation = Day 0) and higher and nearly constant thereafter. Expression of Cox2-mRNA was distinctly cycle related and highest on Day 5; it decreased by Day 15 and remained constantly low until Day 65. Immunohistochemistry localized expression of Cox2 in the cytoplasm of luteal cells. Staining was restricted to Days 5 and 15, with stronger signals on Day 5. These data suggested that increased expression of Cox2 is associated with luteal growth and development and not luteal regression. Furthermore, the expression of Cox1 more likely reflected activity of a housekeeping gene.