OXYTOCIN PREEXPOSURE DOES NOT AFFECT OXYTOCIN INDUCED MYOMETRIAL CONTRACTIONS IN TERM PREGNANT WOMEN
Abstract Number: 4
Abstract Type: Original Research
Oxytocin (OT) receptors in human myometrial cell cultures undergo desensitization after exposure to OT, thereby reducing the ability of cells to respond to subsequent OT administration. This phenomenon was reproduced in an in-vitro study in pregnant rats, in which the OT-induced myometrial contractions were inhibited after preexposure to OT in a concentration-dependent manner. However, it is unclear if such a response is also seen in the human uterine muscle. The objective of our study was to investigate the in-vitro myometrial response in term pregnant women, after preexposure to different concentrations of OT for variable durations.
After REB approval, the study was done in 22 nonlaboring term pregnant women undergoing elective CS under spinal anesthesia. A small sliver of myometrium was excised from the lower uterine segment incision and dissected into 4 strips. After equilibration in the organ bath chambers containing physiological salt solution (PSS), 2 strips were exposed to either 10^-10M or 10^-8M OT (experimental gr), and the other two were kept in PSS (control gr) for 2h (n=24). All the strips were then subjected to dose-response testing with OT (10^-10 to 10^-5M). Similar experiments were done after the preexposure of separate myometrial strips to OT/PSS for 4h (n=24), 6h (n=24) and 12h (n=16) period. The amplitude and frequency of contractions during the dose-response period were analyzed using mixed linear modeling and compared among the groups.
There was no statistically significant difference in the amplitude (p=0.22) and frequency (p=0.16) of contractions during the dose-response period when the myometrial strips were preexposed to varying concentrations of OT (0, 10^-10 or 10^-8M) or for varying periods of time (2, 4, 6, 12h)(Table).
In the in-vitro preparation, the contractile response of the term pregnant uterus is not affected by preexposure to oxytocin at different concentrations or for different periods of time. This phenomenon differs from molecular biology studies that showed a time-dependent desensitization of the OT receptors following pre-exposure to OT. It also differs from the concentration-dependent desensitization observed in similar studies in rats. Further studies are warranted to understand the discrepancy between these data, and to improve strategies to optimize uterine contractility in obstetric patients.
1)Am J Obstet Gynecol 2003;188:497
2)Reprod Sci:in press