PROSTAGLANDIN FORMING CYCLOOXYGENASE OF OVINE UTERUS - RELATIONSHIP TO LUTEAL FUNCTION

The activity of the prostaglandin forming cyclooxygenase was measured in uterine microsomes prepared from 26 ewes distributed over Days 3, 8, 11, 13, 14 and 15 (estrus = Day 0) of an estrous cycle. The specific enzyme activity was higher on Days 13-15 than earlier in the cycle (P<0.01), reaching a maximum on Day 14 (P<0.05). Immunochemical equivalence points of the cyclooxygenase measured using anticyclooxygenase serum remained the same during the estrous cycle, indicating that increases in enzyme activity on Days 13-15 result from increases in the concentration of cyclooxygenase protein. In a second experiment, cyclooxygenase activity was measured in microsomal preparations of myometrium, intercaruncular endometrium and caruncles dissected from the uteri of 21 ewes distributed over Days 3, 11 and 14 of an estrous cycle. A 3-fold increase in the specific activity of the cyclooxygenase in caruncular microsomes occurred on Day 14 of the cycle, but no cyclical changes were observed in the activity of the cyclooxygenase in microsomes from other uterine regions. Thus, both temporal and anatomical correlations exist between the changes in the uterine cyclooxygenase activity and the previously reported changes in the secretion of the uterine luteolysin PGF2(α). The authors suggest that the onset of luteolysis depends on an increase in the concentration of the caruncular cyclooxygenase and, thus, on the efficiency with which arachidonic acid is converted to PGH2, the endoperoxide precursor of PGF2(α). An immunocytofluorescence procedure was used to establish that cyclooxygenase containing cells in the uterine caruncles include the luminal epithelium, vascular endothelium and caruncular stroma. One of these cell types, possibly the caruncular stromal cells, is the site of the cyclical changes in cyclooxygenase activity and may be the luteolysin producing cell of the ovine uterus.