Pregnancy-induced alterations in the turnover rate of 3H-noradrenaline formed from 3H-tyrosine in guinea-pig uterus.

The rate of noradrenaline (NA) turnover in the non-pregnant and pregnant guinea-pig uterus was determined after the administration of the labelled precursor, tyrosine. Endogenous NA was determined fluorimetrically. In non-pregnant animals the turnover of 3H-NA was slower in the uterine horns (t1/2 = 10 h) and cervix (t1/2 = 9 h) than in the reference organ, heart (t1/2 = 6 h). During pregnancy (60-65 days post coitum) endogenous NA in the heart increased in pace with tissue weight gain; yet, in the uterus there was a drastic reduction, down to zero levels. 3H-NA turnover increased significantly in the cervix, no 3H-NA formation remained in the uterine horn (a situation resembling that following 6-hydroxydopamine induced sympathectomy of non-pregnant animals), and the turnover was unaltered in the heart. The observations confirm the general idea, illustrated by the results in the heart, of a functional adaptation with a constant neuron-target relationship during increased physiological demands. They also confirm that a very peculiar and complex neuron-target relationship develops in the uterus during pregnancy--involving functional and structural denervation of the adrenergic nerve plexus in the uterine horn, whereas an increased activation might explain the changes in neuronal NA and its turnover in the cervix.