In control rats, the concentrations of catecholamines in the uterus were markedly modified between days 21 and 22 of gestation, just before parturition; there was a considerable decrease in the concentration of adrenaline (−73%), whereas the level of noradrenaline increased significantly.
Of rats ovariectomized on day 19, 59% underwent abnormal parturition, and the uterine concentration of catecholamines remained constant; on day 22 of gestation, the concentration of adrenaline was twice as high as that in control rats. Injections of progesterone during late pregnancy resulted in prolonged pregnancies in 100% of those rats injected. Changes in the uterine concentrations of catecholamines observed in control rats did not take place in the injected animals, and on day 22 of gestation the concentration of adrenaline was three times that in control rats. Injections of oestrogen in late pregnancy advanced littering in rats which normally produced late litters. In the uterus, a considerable decrease in the concentration of adrenaline was observed and this was similar to that observed in control rats, except that the reduction occurred earlier.
Sham treatments (sham ovariectomies or injections of solvent in which oestrogen or progesterone were administered) had no effect on the timing of parturition or the uterine concentration of catecholamines, compared with control rats.
The concentration of adrenaline in the uteri of pregnant rats was controlled by steroid hormones and it seems that a very low concentration was required for normal parturition.
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