Myometrial contractile responsiveness to oxytocin after dexamethasone suppression of circadian uterine activity in pregnant rhesus macaques during late gestation.

OBJECTIVE

This study was designed to determine if dexamethasone alters myometrial responsiveness to oxytocin or oxytocin secretion.

STUDY DESIGN

Studies were conducted in rhesus macaques (n = 6), between 144 and 148 days' gestation (term 167 days). The first study was conducted at 9 AM and repeated 36 hours later at 9 PM. At 9 AM the following morning a continuous maternal dexamethasone infusion (15 micrograms/kg/hr given intravenously) was initiated, and the study was repeated at 9 PM, 60 hours later. Four doses of oxytocin (500, 1000, 2000, and 4000 pg/kg/min) were administered as 1-minute pulses every 5 minutes for 30 minutes. The number of contractions per pulse (contraction/pulse ratio) was used to determine differences in myometrial responsiveness.

RESULTS

Before dexamethasone infusion there was a circadian rhythm in uterine activity with peak contractile events between 8 and 10 PM (p < 0.01), whereas during infusion the rhythm was ablated. At oxytocin dose 1, the 9 AM contraction/pulse ratio (0.3 +/- 0.1) was lower than that for 9 PM (0.6 +/- 0.2) and for 60 hours later (0.6 +/- 0.1) (mean +/- SE, p < 0.05). Similar results were observed at dose 2, whereas no differences in the contraction/pulse ratio were noted at dose 3. Basal plasma oxytocin concentrations were unaffected by dexamethasone treatment, whereas plasma estradiol and cortisol concentrations were reduced compared with control values (p < 0.01).

CONCLUSIONS

(1) There is a differential sensitivity to oxytocin between morning and evening and (2) the dexamethasone-induced loss of the uterine contractile rhythm is not the result of a loss of myometrial sensitivity to oxytocin or to a suppression of plasma oxytocin concentrations.