Alteration of 24-hour rhythms in myometrial activity in the chronically catheterized pregnant rhesus monkey after a 6-hour shift in the light-dark cycle.

We determined the effect of a 6-hour phase shift in the light-dark cycle on the 24-hour rhythm of myometrial activity with 15 chronically catheterized pregnant rhesus monkeys during the last third of gestation. Monkeys were housed indoors in constant temperature on a 14-hour-light/10-hour-dark photoperiod in two groups based on the time the lights were turned on (group A lights on at 6 AM and group B lights on at midnight). The power spectra were calculated for periods of 512 readings (1.138 hours) starting at the time the lights were turned on for continuous periods of 25.03 hours of data showing well-developed contraction-type activity at any time during that period. The power spectra data for contractures and contractions were analyzed separately from myometrial electromyographic data. During the 14 hours of light, myometrial activity was primarily composed of contractures. In all experimental animals the switch from contractures to contractions occurred around the time the lights were turned off. The proportion of contraction-type activity in each 1.138-hour analysis interval expressed as a percentage of the total contraction power for the whole 25.03-hour analysis period for all monkeys increased around the time the lights were turned off in both groups. The increase in contractions was observed 12 to 15 hours after the lights were turned on. No systematic change in the proportion of contracture activity was observed. Peak contraction activity was 22.6 +/- 0.60 hours (16.6 hours after lights on), and was at 18.3 +/- 1.4 hours (18.31 hours after lights on) in groups A and B, respectively. This difference was statistically significant (22.6 +/- 0.6 vs 18.3 +/- 1.4 hours, p less than 0.05). Seven of eight monkeys in group A and six of seven in group B had switched from contractures to contractions before lights were turned off. We have confirmed a 24-hour rhythm in myometrial activity in the pregnant rhesus monkey and provided new evidence to support the endogenous nature of this rhythm and its direct relationship to the light-dark cycle.