Extensive sympathetic denervation of the uterus during pregnancy as evidenced by tyrosine hydroxylase determinations in the guinea pig.

The capacity for neuronal NE synthesis in the guinea pig uterus was studied during pregnancy and postpartum utilizing animals with bilateral or unilateral pregnancies. The activity of the NE-synthesizing enzyme TH was measured in various parts of the uterus. The submandibular gland was used for comparison. During advancing pregnancy the TH activity in the fetus-containing uterine horn was progressively reduced, reaching undetectable levels at term. This change was not seen in the submandibular gland. In parallel with previous fluorescence microscopic observations on uterine adrenergic innervation, the pronounced reduction in TH activity in the fetus-containing uterine horn during progressive pregnancy is suggested to be due to degeneration of the terminal network of adrenergic nerves. In the contralateral empty uterine horn of unilateral pregnancy, the TH activity was decreased by about 90% at term, despite evidence for a relatively intact adrenergic nerve plexus. During the puerperal period the TH activity increased very slowly in the previously empty as well as the fetus-containing horns. In the latter tissue, even at 6 months after delivery, the activity was only 14% of that in uterine horns of virgin animals. In view of the marked regional heterogeneity in the changes, also related to the position and size of the conceptus, it is assumed that they are caused mainly by local humoral and probably also mechanical factors within the uterus.