Our in vitro study demonstrates that oxytocin pretreatment results in attenuation of myometrial contractility with subsequent administration of oxytocin, but not with ergonovine or carboprost. Furthermore, our results demonstrate that the combination of oxytocin with either ergonovine or carboprost is likely to be more effective under such circumstances. However, in oxytocin-naïve myometrium, oxytocin is still the most effective drug when compared with ergonovine or carboprost or their combinations. Our study findings are novel because dose-response profiles of various uterotonics or their combinations have not been investigated in either clinical or laboratory settings.
The observed attenuated response of oxytocin-induced contractility in myometrial strips pretreated with oxytocin confirms the previously established oxytocin-receptor desensitization phenomenon. In a previous study published by our group, oxytocin-induced contractility was found to be attenuated in a concentration- and time-dependent manner when the myometrial strips were pretreated with oxytocin 10−10, 10–8, or 10–5 M for 2, 4, 6, or 12 hours.18 The effect was predominantly seen with pretreatment with oxytocin 10–5 M for at least 2 hours or 10–8 M for at least 4 hours. This established desensitization model was used as a basis for studying various uterotonic drugs in the current study.
Our results are also in keeping with 2 of our previous studies.16,17 The first was conducted on isolated myometrial strips of pregnant rats.16 We demonstrated that the contractility induced by ergonovine or prostaglandin F2α remains unaffected by prior oxytocin exposure, indicating that the mechanism involved in desensitization does not interfere with these 2 uterotonics acting via different signaling pathways. The second was conducted on myometrial strips obtained from nonlaboring women or those with augmented labor. Ergonovine, prostaglandin F2α, and misoprostol produced similar contractions in both subsets, whereas oxytocin resulted in poor contractility in women with augmented labor.17
The molecular mechanism of oxytocin-induced desensitization is well understood; however, more research is needed in this area to understand the implications of this phenomenon. As a G-protein–coupled receptor, the oxytocin receptor undergoes rapid desensitization characterized by decreased cellular responsiveness and impaired signal transduction with continuous or repeated homologous stimulation. The phenomenon likely involves receptor phosphorylation, sequestration, internalization, and either degradation by lysosomes or recycling into the cell membrane.12,19 This is characterized by downregulation of oxytocin receptors and messenger RNA, as well as decrease in oxytocin binding.12,13 The clinical implications of the desensitization phenomenon can be significant, leading to failed induction of labor, failure to progress in labor requiring CD, and poor uterine responsiveness to oxytocin after delivery leading to uterine atony and PPH. With the current practice of administering increasingly larger doses of oxytocin over prolonged periods during induction or augmentation of labor, this phenomenon is highly relevant.
High-dose aggressive oxytocin protocols or prolonged administration of oxytocin for labor induction or augmentation are associated with a higher incidence of PPH related to uterine atony compared with low-dose regimens.15,20 In a previous study, we found that women with oxytocin-augmented labor require 9-fold greater doses of oxytocin (90% effective dose = 2.99 IU; 95% CI, 2.32–3.67 IU) to produce adequate uterine contractions during CD compared with the dose effective in nonlaboring women (90% effective dose = 0.35 IU; 95% CI, 0.18–0.52 IU).21,22 These clinical findings likely indicate differences in oxytocin receptor distribution or function, signaling pathways, and sensitivity to oxytocin in laboring and nonlaboring women, and clearly warrant the need for different management protocols in these settings.
There is limited clinical evidence in the literature on the comparative effects of individual uterotonic drugs or their combinations in women with or without augmented labor to suggest clear guidance to their use. In a 2013 Cochrane review of 5 trials (n = 2226) by Westhoff et al.,8 prophylactic oxytocin was found to be superior to ergot alkaloids in preventing PPH greater than 500 mL (relative risk 0.76; 95% CI, 0.61–0.94 mL); however, in subgroup analysis in which only randomized trials with low risk of methodologic bias were analyzed, this benefit did not persist. The authors suggested a lack of high-quality evidence supporting a benefit of prophylactic oxytocin over ergot alkaloids. The use of oxytocin was, however, associated with fewer side effects such as nausea and vomiting, making oxytocin the more desirable option for routine use to prevent PPH. The authors also found no benefit in the combination of oxytocin and ergometrine versus ergometrine alone (5 trials; n = 2891) in preventing PPH greater than 500 mL (relative risk 0.90; 95% CI, 0.34–2.41).8 Another review of 6 trials (n = 9332) by McDonald et al. found a small reduction in the risk of PPH (blood loss between 500 and 1000 mL) with a combination of oxytocin 5 IU and ergometrine 0.5 mg when compared with oxytocin 5 or 10 IU (odds ratio 0.82; 95% CI, 0.71–0.95). This difference was greater with the lower dose of oxytocin, but was not demonstrated for estimated blood loss of >1000 mL. Vomiting, nausea, and hypertension were more in the oxytocin–ergometrine combination.9 Tunçalp et al.,10 in their review of 13 trials comparing injectable prostaglandins with oxytocin or ergot alkaloids, found that the reporting of primary outcomes such as a blood loss of 1000 mL or more and the use of additional uterotonics was insufficient to give any reliable estimates. Because of the insufficient information in the existing clinical trials, our study findings have the potential to help design studies to better tailor prophylactic uterotonic regimens to specific clinical situations.
In our study, we found that among the 3 uterotonics, oxytocin is the most effective drug if the myometrium is not preexposed to oxytocin. Even when various combination groups (i.e., high-dose or low-dose oxytocin with ergonovine or carboprost) were compared with oxytocin alone, the effect seemed to be mainly driven by the presence of oxytocin during the dose-response period. The response with HDOx-ergonovine or HDOx-carboprost combination was similar to that of oxytocin alone, which implies that at a certain high concentration of oxytocin, most receptors are saturated with oxytocin and a maximal possible contractile response is produced exclusively with oxytocin. Under such circumstances, there is no benefit of adding ergonovine or carboprost to oxytocin to further enhance contractility. By contrast, poor contractility seen with the addition of low-dose oxytocin to ergonovine or carboprost compared with oxytocin alone suggests that the amount of oxytocin in these combination groups was not sufficient for the available oxytocin receptors to induce effective myometrial contractility. Based on our results, it appears to be desirable to increase the dose of oxytocin until maximal response is produced, especially in nonlaboring women undergoing CDs. However, one should be cautious, because higher doses may be associated with side effects.
In the oxytocin-pretreated myometrium, among the 3 individual uterotonics, ergonovine produces the most effective contractions followed by oxytocin and then carboprost. This relative difference is not due to improved contractility of ergonovine, but rather to the reduced efficacy of oxytocin in the desensitized myometrium. However, it is unlikely that ergonovine will be used as a primary sole agent for prophylaxis of PPH in women exposed to oxytocin during labor, considering its greater potential for side effects compared with oxytocin. Nevertheless, it is evident that in the myometrium previously exposed to oxytocin, contractility induced by oxytocin alone is not adequate; it is lower than all combination groups, and the addition of ergonovine or carboprost is required to produce a synergistic response. This can be explained by the desensitization phenomenon, resulting in attenuation of oxytocin-induced contractility response, thereby rendering the response produced by other uterotonic drugs superior. It is likely that with oxytocin pretreatment, the downregulation of receptors limits the binding of oxytocin to functioning myometrial oxytocin receptors and results in attenuated response, regardless of the administered dose. This implies that ergonovine or carboprost should be considered either prophylactically or in the event of poor uterine responsiveness to oxytocin, especially if the uterus is preexposed to oxytocin during labor.
There are some limitations to our study, including the in vitro study design, which may not exactly replicate clinical findings. Unlike our previous report, we did not observe a typical increase in contractility response with the increasing doses of uterotonics. The reason for this finding is unclear and may be related to the variable number of myocytes within a sample or to the variability in the genetic make-up of different individuals. The concentrations used in the study were chosen based on our previous research and standard concentrations used in most dose-response studies in the literature.11,14,16–18 There is insufficient information in the literature regarding the plasma levels of uterotonics after parenteral administration in the setting of PPH. The reported levels of oxytocin vary significantly during pregnancy, labor, and postpartum from 10–12 to 10–8 M and may not accurately reflect local myometrial concentration.23–26 The serum levels of ergonovine after oral administration were found to be in the range of 10–6 to 10–18 M; however, these were measured after variable clinical doses in a population in which gender and pregnancy status were not clearly described.27–29 The approximate peak serum levels of carboprost after IM administration of 250 μg range from 2718 to 3097 pg/mL (10–8 M) in term pregnant women undergoing vaginal deliveries.30 Our cumulative dose response in the range of 10−10 to 10−5 M likely reflects the levels reached after parenteral administration of these drugs postpartum. However, it should be noted that in vitro concentrations may not reflect the in vivo plasma levels and perhaps represent an overestimation in the experimental settings. It is unclear whether the addition of higher concentrations of ergonovine and carboprost to oxytocin than those used in our study will have any additive effect. However, clinically, the use of ergonovine and carboprost is likely to be associated with more side effects.
In summary, we have confirmed that oxytocin pretreatment attenuates contractility induced by oxytocin, but not by ergonovine and carboprost. In oxytocin-naïve myometrium, oxytocin is the most effective drug and there is no benefit to adding ergonovine or carboprost once the maximal dose of oxytocin is given. In oxytocin-pretreated myometrium, however, a combination of oxytocin with ergonovine or carboprost produces a superior response than oxytocin alone. On the basis of our findings, we suggest the choice of uterotonic drug for prevention of PPH should be based on labor type. However, further in vivo studies are warranted to confirm these findings and to determine whether these differences observed in vitro are clinically significant.
Name: Mrinalini Balki, MBBS, MD.
Contribution: This author is responsible for the design and conduct of the study, data collection and analysis, and writing and revision of the manuscript.
Attestation: Mrinalini Balki attests to the integrity of the original data and the analysis reported in this manuscript, and is the archival author responsible for maintaining the study records.
Name: Magda Erik-Soussi, MSc.
Contribution: This author helped in the design and conduct of the study, data collection and analysis, and manuscript revision.
Attestation: Magda Erik-Soussi attests to having reviewed the original data and the analysis reported in this manuscript and approved the final manuscript.
Name: Nivetha Ramachandran, PhD.
Contribution: This author helped in the conduct of the study, data collection and analysis, and manuscript preparation.
Attestation: Nivetha Ramachandran attests to the integrity of the original data and the analysis reported in this manuscript, and approved the final manuscript.
Name: John Kingdom, MD.
Contribution: This author helped in the study design and conduct, data analysis, and manuscript revision.
Attestation: John Kingdom attests to having reviewed the original data and the analysis reported in this manuscript, and approved the final manuscript.
Name: Jose C. A. Carvalho, MD, PhD.
Contribution: This author helped in the design and conduct of the study, data collection and analysis, and manuscript preparation.
Attestation: Jose C. A. Carvalho attests to having reviewed the original data and the analysis reported in this manuscript, and approved the final manuscript.
This manuscript was handled by: Cynthia A. Wong, MD.
The authors acknowledge Cedric Manlhiot, PhD(c), Clinical Research Program Manager, Labatt Family Heart Centre, The Hospital for Sick Children, Toronto, for statistical analysis of this study. The authors also thank Ms. Kristi Downey (Research Coordinator) for assisting with patient recruitment, and Dr. Stephen Lye (Associate Director, Research) and Dr. Lee Adamson (Senior Investigator) from the Samuel Lunenfeld Research Institute, Toronto, for their continued guidance and support for our research.
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. Accessed October 16, 2014