The effect of ovarian steroids on oxytocin-stimulated secretion and synthesis of prostaglandins in bovine myometrial cells.

The aim was to study whether bovine myometrial cells have an enzymatic system able to produce prostaglandins (PGs) and whether PGs synthesis is regulated by steroids in a similar manner as endometrial cells. In Experiment 1, immunohistochemical studies localized proteins for cyclooxygenase 2 (COX2), PGE synthase (PGES) and PGF2alpha synthase (PGFS) in myometrial and endometrial (as positive control) slices from days 14 to 16 of the estrous cycle. Enzymatically isolated myometrial cells (2.5 x 10(5)/ml) were cultured for 96 h to attach them to the bottom of the culture well. In Experiment 2, cells were preincubated for 30 min with progesterone (P4; 10(-5) M), and thereafter incubated for 4 or 6h with arachidonic acid (AA; 10(-5) M, as positive control), oxytocin (OT; 10(-7) M), and OT+P4. In medium, PGE and PGFM (PGF2alpha metabolite) were increased (P<0.05) by AA treatment after 4 and 6h, but by OT only after 6h of incubation. Progesterone did not affect (P>0.05) basal secretion of both PGs, but it diminished (P<0.05) the stimulatory effect of OT on PGF2alpha and PGE secretion after 6h of incubation. The amount of enzyme protein for COX2, PGES and PGFS analyzed by Western Blot was affected (P>0.05) by any of the factors added to the culture medium. In Experiment 3, myometrial cells were preincubated with P4 (10(-5)M) and pregnenolone (P5; 10(-5)M) for 30 min, and then incubated for 6h with OT (10(-7) M) and OT plus each of these steroids used. Expression of mRNA for COX2, but not PGFS and PGES, was found in the cells stimulated with OT. Neither P4 nor P5 affected expression of the studied genes; however, both steroids diminished (P<0.05) OT-stimulated mRNA expression of COX2. The data suggest that: (a) myometrial cells are able to synthesize both PGF2alpha and PGE and (b) synthesis of these PGs may be regulated by steroids through a transcription-independent manner, which modulated the effect of OT on COX2 mRNA expression.