The extraction of oestrogens, and the rate of secretion of oestrone and oestradiol-17beta by the uterus in the pregnant sheep.

Abstract

SUMMARY The sharp increase in oestrone and oestradiol-17β production during the 48 h before parturition in the ewe does not always result in the uterine venous concentration of these steroids being greater than that found in peripheral blood, even though the gravid uterus is considered to be the major source of enhanced oestrogen synthesis in late pregnancy. This anomaly has been partly resolved by the results from tracer kinetic studies. During the infusion of labelled oestradiol-17β or oestrone into a jugular vein to steady state, 53% of [3H]oestradiol-17β and 62% of [3H]oestrone in the peripheral circulation were extracted by a gravid uterine horn. The extraction of [3H]oestradiol-17α, a product of the infused steroids, was 53% and 45% during the infusion of [3H]oestrone and [3H]oestradiol-17β, respectively. Calculations of the secretion rate of these steroids by one gravid horn, after correcting for uterine extraction, gave values that represented a substantial proportion of the kinetically determined production rate. This confirms that an appreciable part of the oestrogen produced just before term is in fact derived from the gravid uterus. However, the possibility that other sources may contribute to oestrogen synthesis in late pregnancy cannot be excluded. The findings show that a direct measurement of arteriovenous difference and uterine blood flow is inadequate to determine oestrogen secretion rates by the gravid uterus.