COMPARISON OF THE MYOMETRIAL RESPONSE TO OXYTOCIN DURING DAYLIGHT WITH THE RESPONSE OBTAINED DURING THE EARLY HOURS OF DARKNESS IN THE FETECTOMIZED RHESUS-MONKEY AT 160-172 DAYS GESTATIONAL-AGE

Six rhesus monkeys, fetectomized at 122-129 days gestational age (dGA) , were studied to evaluate photoperiod-dependent differences in the my ometrial sensitivity to exogenous oxytocin in the absence of a live fe tus. One to two weeks before the study, at 150-162 dGA, sixteen matern al arterial samples were taken every 4 h to assess plasma concentratio ns as well as the presence of 24-h rhythmicity in plasma estradiol and cortisol. At 160-172 dGA, oxytocin was administered via the maternal inferior vena cava to elicit regular myometrial contractions. Each ani mal was studied twice: once during daylight and once during the early hours of darkness. Oxytocin was infused in a pulsatile fashion during the first minute out of every five over four consecutive 30-min interv als, during which the dose rate was increased after each 6-h oxytocin pulse (400, 800, 2000, and 4000 pg . kg-1 . min-1). The overall respon se of the myometrium of the fetectomized animals, evaluated as the num ber of contractions per pulse of oxytocin, was higher during the dayti me study (p = 0.006). The response to oxytocin in intact pregnant anim als is greatest during the early hours of darkness, Comparison of the overall myometrial response of the fetectomized monkeys with the respo nse of intact pregnant animals showed that the daytime responses were similar, but that the nighttime response of the fetectomized animals w as significantly lower (p < 0.0002). These contrasting findings may be due to differences in the maternal neuroendocrine environment dependi ng on the presence or absence of a fetal adrenal steroid contribution acting directly on the myometrium or acting indirectly through provisi on of precursors for estrogen formation. Estradiol concentrations were 30-50% lower than those reported for intact pregnant animals. No 24-h estradiol rhythm could be determined in any of the six animals, and p eak values occurred unrelated to the significant 24-h plasma cortisol rhythm. We hypothesize that in the intact pregnant animal, the fetal a drenal steroid output, highest at night, modulates the myometrial thre shold in such a way that during primate pregnancy, the myometrial resp onse to oxytocin is increased during darkness. Conversely, in the fete ctomized monkey, the myometrium is less responsive at night than durin g the day, because this fetal input is lacking.