The mechanisms of action of progesterone. : L AndersonDr MBchB, SM NelsonProf MRCOG, WMartinProf PHD and JE NormanProf MRCOG. 1Department of Obstetrics and Gynaecology, University of Glasgow
Introduction: Current evidence suggests that prolonged treatment with progesterone therapy may reduce the incidence of premature delivery. The metabolite 17 alpha hydroxyprogesterone (17OHP) has also been shown to reduce the preterm birth rate. However, progesterone is not uniformly effective in preventing preterm labour and at present its principal mode of action on myometrium is unknown.Weaimed to determine some of the specific mechanisms of action of progesterone.
Methods: The project was approved by North Glasgow University Hospitals Research Ethics Committee ref no.05/S0705/18. All patients gave written informed consent to participate.Women within the STOPPIT (a double blind randomised placebo controlled study of progesterone for the prevention of preterm birth in twins) were given vaginal progesterone, 90mg daily from 24-34 weeks gestation, or placebo.Myometrial biopsies were obtained from either STOPPIT participants undergoing caesareandelivery or other patients undergoing routine LUSCS,samples were dividedfor contractility and mRNA analysis. Myometrial strips were cut and suspended under resting tension. Once stable contractile activity developed,concentration-response curves were carried out in response to oxytocin, levcromakalim, nifedipine and ritodrine to ascertain any reduction in effect by progesterone on oxytocics or enhancement of tocolytic effects.We also assessed ex-vivo,the inherent contractility of the placebo versus progesterone groups.Real time RT-PCR(Abi,Taqman) was used to quantitate endogenous inhibitors of uterine contractility (cGRPt, ep2), cytokines (IL6, IL8, IL1β) and gap junction components (connexin-26 and -43) and expressed relative to 18s.Statistical analysis was performed by ANOVA or paired t-test.
Results:Progesterone exerted a concentration-dependent inhibitory effect on amplitude of myometrial contractionsin vitro.We collected 12 placebo and 6 progesterone samples from STOPPIT participants. Baseline contractile amplitude measured 55±4mN, frequency 8.1±0.7h-1 and 48±6mN, 8.1±0.5h-1 in placebo and progesterone groups respectively (p>0.05). There was no significant difference between placebo and progesterone groups in amplitude or frequency of contractions following activation with oxytocin, or inhibition with levcromakalim, nifedipine, or ritodrine when tested ex vivoorin vitro.RT-PCR analysis showed no difference in cytokine production betweenplacebo and progesterone groups. When analysis was restricted to non-labouring samples there was alower mRNA density in cGRPt, ep2, CX26, CX43and IL6 (p≤0.05) within the progesterone group compared with placebo. Prolonged administration of progesterone inhibitedgap junction component CX 43 in myometriumin vitro however in 24 hours or less itdoes not appear to modify cytokine release, nitric oxide synthetase or calcitonin gene related peptide.
Conclusions: Prolonged maternal administration of progesterone inhibits gap junction components in humans. However, ex-vivo assessment of the functional impact of this does not demonstrate a long-term detrimental impact on myometrial function with down regulation of endogenous inhibitors suggesting a functional and compensatory recovery after progesterone administration, occurs. We have clearly demonstrated that progesterone can affect myometrial contractility in vitro however its mechanism of action both in vitro and in vivo warrants further investigation. This may well be part of the mechanism of which progesterone contributes to prevention of preterm labour.