Variations in the level of uterine norepinephrine during pregnancy in the guinea pig.

The uterine smooth musculature is innervated by short adrenergic neurons which are unique in that their transmitter content varies during pregnancy and under the influence of exogenous sex hormones. Uterine norepinephrine was measured fluorometrically at 25 stages throughout pregnancy in the guinea pig, which was chosen because one of the pregnant uterine horns often is devoid of fetuses and hence not subject to mechanical tension. During the initial 10 days of pregnancy, the total organ content of norepinephrine was almost doubled whether the horn contained conceptuses or not. In the horn with conceptuses, norepinephrine was subsequently reduced in the course of the remaining gestation time to reach near-zero levels at term (about 65 days post coitus). In the empty horn, the amine level was constantly elevated until the last two weeks, during which it fell to the same low values as in the contralateral horn. Cervical norepinephrine exhibited the same variations during pregnancy as the empty horn. It is suggested that the characteristic variations in uterine norepinephrine reflect a hormonal influence on the system of short adrenergic neurons, with the addition of mechanical factors in that horn containing fetuses.