BACKGROUND: Prolonged exposure to oxytocin during augmentation of labor is a significant risk factor for uterine atony, resulting in the desensitization phenomenon, a decrease in the responsiveness of myometrium to further oxytocin. The importance of extracellular calcium is well established in spontaneous myometrial contractility; however, its significance is unknown in the context of desensitized myometrium. We aimed to investigate the effect of low, normal, and high extracellular calcium concentration on oxytocin-induced contractility in oxytocin-pretreated human myometrium in vitro. We hypothesized that extracellular normocalcemia would provide superior oxytocin-induced contractility in both naive and oxytocin-pretreated myometrium compared with hypocalcemia and hypercalcemia.
METHODS: Myometrial tissue was obtained from women undergoing elective cesarean deliveries and was dissected into longitudinal strips. Each strip was mounted in a single organ bath with physiological salt solution (PSS) under homeostatic conditions and then pretreated for 2 hours with either oxytocin 10 M or PSS (control). The tissue was then washed with PSS, and calcium concentrations were altered to reflect low (1.25 mM), normal (2.5 mM), or high (3.75 mM) levels, thereby providing 6 study groups. After equilibration in the desired calcium concentration, a dose-response testing to oxytocin 10 M to 10 M was performed. Contractile parameters were measured and compared among groups after square root transformation. The primary outcome was motility index (frequency × amplitude), and secondary outcomes included frequency, amplitude, and area under the curve.
RESULTS: One hundred seventy-four experiments were conducted from samples obtained from 36 women. In the control group, the mean motility index (√g·contractions/10 min) was significantly lower in the hypocalcemic group than in the normocalcemic group (estimated difference, -0.43; 95% confidence interval [CI], -0.82 to -0.04; P = 0.03). In addition, the mean frequency of contractions (√contractions/10 min) was significantly lower in the hypocalcemic (estimated difference, -0.27; 95% CI, -0.46 to -0.09; P = 0.002) and hypercalcemic groups (estimated difference, -0.18; 95% CI, -0.34 to -0.02; P = 0.03) compared with the normocalcemic group. In the oxytocin-pretreated group, there were no significant differences in the values of any of the contractility parameters of the hypocalcemic or hypercalcemic groups compared with the normocalcemic group (mean motility index [√g·contractions/10 min] estimated difference, 0.10; 95% CI, -0.23 to 0.43; P = 0.74 and -0.39; 95% CI, -1.10 to 0.32; P = 0.39, respectively).
CONCLUSIONS: In oxytocin-naive myometrium, normocalcemia provides superior oxytocin-induced contractility compared with hypocalcemic and hypercalcemic conditions. We were unable to draw conclusions regarding oxytocin-pretreated myometrium because of the small sample size relative to the large variability of the data. These observations warrant further investigations in laboratory and clinical settings.
