The rat pregnant myometrium when chopped and incubated in Krebs solution (25% w/v) released an anti-aggregatory material closely resembling prostacyclin (PGI2). The activity was stable at pH 12 and was conclusively identified as from detection of the hydrolysis product 6-oxo-PGF1alpha by gas chromatography-mass spectrometry. Optimum incubation conditions were found to be 15 minute incubation at 20°C at pH8. PGI2 Production could be stimulated in a dose-dependent manner by addition of arachidonic acid or phospholipase A2. Output of PGI2 was inhibited by pretreatment with mepacrine, indomethacin, eicosatetraynoic acid, 15-hydroperoxyarachidonic acid, tranylcypromine and soybean lipoxidase enzyme. Studies on synthesis by different uterine fractions during pregnancy showed that myometrial PGI2 output was low up to day 12 of pregnancy, but then increased progressively to reach a maximum on day 22 (day of delivery). 48 hours post-partum synthesis had markedly decreased. Decidual PGI2 output was much lower being first detectable on day l4 and reaching a maximum on day 22. PGI2 generation by the implantation sites was very low. Maximum production occurred on day 5 but had decreased sharply on day 6 and remained very low even at parturition. A lipoxygenase enzyme contained in blood platelets trapped within the decidual vasculature produced a hydroperoxy acid which inhibited decidual PGI2 synthesis. If decidua was incubated with myometrial tissue, the myometrial PGI2 synthesis was also reduced. Uterine perfusion or pretreatment the decidua with eicosatetraynoic acid abolished the decidual inhibition of myometrial production. The oxytocic activity of PGI2 was compared with other PGs using strips of rat pregnant uterus treated with indomethacin to suppress spontaneous contractility. PGI2 elicited uterine contractions and was 80-times more potent than 6-OXO-PGF1alpha but less active than or PGF2 or PGF2alpha. Threshold concentrations of significantly potentiated threshold doses of oxytocin but not those of PGF2alpha. Several uterine stimulant drugs (oxytocin, bradykinin, angiotensin, ergometrine, carbachol and 5-hydroxytryptamine) significantly stimulated myometrial PGI2 release. Mepacrine inhibited the actions of these oxytocic drugs. PGF2alpha did not increase myometrial PGI2 release. The uterine relaxant drugs relaxin and histamine inhibited myometrial PGI2 output. Relaxin inhibition was reversed by arachidonic acid. Furthermore, relaxin inhibited oxytocin-induced release of PGI2, suggesting inhibition of phospholipase A2.
|Date of Award||1981|