Objective
The possibility exists that the vehicle for 17-alpha-hydroxyprogesterone caproate, castor oil, exerts an effect on human uterine contractility. The aim of this study was to evaluate its effects on contractility of myometrial preparations that were obtained during pregnancy.
Study Design
Myometrial strips were suspended under isometric conditions. Contractility was induced with oxytocin. Strips were incubated in castor oil or physiologic salt solution and suspended for a further oxytocin challenge. Contractile integrals were compared between both groups.
Results
Strips that were exposed to castor oil demonstrated increased contractile activity that was elicited by oxytocin (mean contractility value, 165.53% ± 17.03%; n = 8; P = .004), compared with control strips (mean contractility value, 72.57% ± 7.48%; n = 8; P = .003). There was a significant increase in contractile activity of the castor oil–exposed strips, compared with those that were exposed to physiologic salt solution (n = 8; P < .001).
Conclusion
Exposure of human myometrial preparations to castor oil results in enhanced oxytocin-induced contractility.
Preterm delivery results in significant neonatal morbidity and death. Despite the ever increasing research output that is directed at understanding mechanisms of preterm labor and the use of therapeutic interventions to prevent or arrest the process, the results have been less than satisfactory. Although the use of progesterone as a potential therapeutic agent for preterm labor was first outlined >30 years ago, closer scrutiny of its potential benefits in recent years has resulted in renewed enthusiasm for its use as a preventative agent against preterm birth in women with a singleton pregnancy who have had a previous spontaneous preterm birth or a shortened cervical length. However, there remain many unanswered questions in relation to the mode of action of progesterone, its pharmacokinetics, its safety profile, and the ultimate benefits for perinatal death and morbidity.
See related editorial, page 409
For Editors’ Commentary, see Table of Contents
The most commonly used synthetic progesterone as a treatment for preterm labor is 17 alpha-hydroxyprogesterone (17P), which is administered by the intramuscular route. In many reports, this administration has used castor oil as a vehicle for the 17P, with the vehicle also serving as the placebo in the control group. This has led to a valid scientific and clinical query over whether the vehicle, castor oil, could have exerted an effect on uterine contractility independently and hence influenced outcome in the placebo group. Although castor oil frequently has been suggested as a compound that may assist in priming the cervix and hence be useful for induction of labor, there is little to support this clinically. Whether castor oil exerts an influence on myometrial contractility is another question, and the data that are available in relation to this matter are also minimal. One study that used tocographic techniques suggested that the maternal administration of castor oil significantly increased the contractile activity of the uterus in pregnancy. The purpose of this study was to evaluate the effects of exposure to castor oil on uterine contractile activity in isolated human myometrial preparations that were obtained during pregnancy and to compare the findings with those that were observed in a control group.
Materials and Methods
Tissue collection
Biopsy specimens of human myometrial tissue were obtained at elective cesarean section delivery in the third trimester of pregnancy in the Department of Obstetrics and Gynaecology, Galway University Hospital, Ireland. Ethics committee approval for tissue collection was obtained from the Research Ethics Committee at University College Hospital Galway, and recruitment of patients was by written informed consent. The biopsy specimens were excised from the midline of the upper portion of the lower segment of the uterus. Immediately on collection, tissue biopsy specimens were placed in Krebs-Henseleit physiologic salt solution (PSS) at pH 7.4 that contained the following ingredients: 4.7 mmol/L potassium chloride, 118 mmol/L sodium chloride, 1.2 mmol/L magnesium sulfate, 1.2 mmol/L calcium chloride, 1.2 mmol/L potassium phosphate, 25 mmol/L sodium bicarbonate, and 11 mmol/L glucose (Sigma-Aldrich, Dublin, Ireland). Tissues were stored at 4°C and used within 12 hours of collection.
Tissue bath experiments
Longitudinal myometrial strips that measured approximately 2 × 2 × 10 mm were dissected free of uterine deciduas and serosa and mounted for isometric recording under 2 g of tension in organ tissue baths, as previously described. The tissue baths contained 10 mL of Krebs-Henseleit PSS that was maintained at 37°C, pH 7.4, and were gassed continuously with a mixture of 95% oxygen/5% carbon dioxide. Myometrial strips were allowed to equilibrate for a period of at least 1 hour, during which time the Krebs-Henseleit PSS was changed every 20 minutes. After equilibration, contractions were stimulated by bath exposure of the strips to oxytocin (0.5 nmol/L) for a period of 20 minutes. Strips were removed from the tissue bath and inserted in castor oil or PSS for a 30-minute period. The strips were then resuspended under isometric conditions and exposed to a further oxytocin challenge for a 20-minute period. When we planned the experimental design, it was deemed appropriate to avoid the addition of castor oil directly to the tissue bath because of its lipophilic nature.
The contractile activity that was elicited by the oxytocin exposure in all experiments was measured by the calculation of the integral for the selected area over the 20-minute period. The integral that was observed after incubation of myometrial preparations in castor oil or PSS was then expressed as a percentage of the integral that was measured for the 20-minute period after the initial oxytocin challenge. The mean contractility value (MCV) that represented the mean percentage contractility that was observed over a series of experiments after exposure to castor oil or PSS was calculated. The data acquisition system used was the PowerLab hardware unit (AD Instruments, Hastings, UK) and the associated software (Chart version 4.0; AD Instruments).
Drugs and solutions
Castor oil was purchased from Sigma-Aldrich. Fresh Krebs-Henseleit solution was made daily. A stock solution of oxytocin (1 mmol/L; Sigma-Aldrich) was prepared with deionized water. Serial dilutions were prepared in deionized water on the day of experiments.
Statistical analysis
A paired sample t test was used to compare the contractility that was observed before and after the specimen was bathed in castor oil or PSS. An independent sample t test was used to compare the contractility that was seen after castor oil exposure with that found after PSS exposure. A probability value of < .05 was considered to be statistically significant. We used the statistical package SPSS for Windows (version 11.0; SPSS Inc, Chicago, IL).
Results
Myometrial biopsy specimens were obtained from a total of 8 women between 38 and 39 weeks gestation. The mean maternal age was 35.28 years. The reasons for cesarean section were breech presentation and previous cesarean section. All cesarean section operations were carried out under regional anesthesia before the onset of labor.
After the period of equilibration, regular rhythmic contractions developed in all strips after bath exposure to oxytocin. In Figure 1 , a presentative recording of this myometrial contractility is demonstrated for the initial oxytocin exposure and for the final exposure after incubation in castor oil. The enhanced contractile performance after exposure to castor oil is observed. The integrals of contractile activity that were measured for both periods of exposure (ie, before and after castor oil) are shown in Table 1 . The mean contractility value that was measured after castor oil exposure was 165.53% ± 17.03% ( P = .004; n = 8; Table 2 ).
