The effects of photoperiod on the switching of myometrial contractility patterns of pregnant baboons: relationship to surgery and parturition.

Objectives: Pregnant baboons were studied to determine the precise time of the switch from myometrial contractures to contractions in relation to photoperiod after laparotomy and at parturition. We compared the patterns recorded in baboons to those we have previously reported in pregnant rhesus monkeys to determine fundamental primate characteristics.
Methods: Seven pregnant baboons (126-160 days' gestation) were instrumented with femoral arterial and venous catheters and electrodes for myometrial electromyogram. All animals were subjected to a 14-hour light:10-hour dark photoperiod. Myometrial activity was monitored using a computer-based data acquisition system. Onset time for all switches was noted and standardized against time of lights off. Animals were studied at three stages of pregnancy (stage 1, first 10 days after laparotomy; stage 2, more than 10 days after laparotomy and more than 10 days before cesarean; and stage 3, 10 days before cesarean section or vaginal delivery).
Results: All baboons demonstrated myometrial switches for a variable number of days preceding parturition. Onset of darkness was 0 hours. Average time of stage 1 switch onset was 2.17 +/- 0.60 hours and was not different from stage 3 switch onset, which was -1.00 +/- 0.27 hours.
Conclusions: Myometrial contractile patterns showed clear photoperiodicity in the switch from contractures to contractions in late pregnancy in the baboon. The relationship of the switch from contractures to contractions was not altered by surgical laparotomy. There was a significant difference in the time of switch in relation to photoperiod between pregnant rhesus monkeys and baboons. However, the fact that a significant photoperiod exists in both species indicates a fundamental similarity in the switch from contractures to contractions in primate pregnancy.