THE KINETICS OF OESTRADIOL-17β METABOLISM IN THE SHEEP

Abstract

SUMMARY The metabolic clearance rate of oestradiol-17β from whole blood and its conversion to oestrone and oestradiol-17α were measured in pregnant and non-pregnant (cyclic, lactating and anoestrous) sheep using tracer kinetic techniques. The metabolic clearance rate was much the same in sheep in different reproductive states (mean in all animals (±s.e.m.) = 2·449 ± 0·155 1/min). The highest values were reached on days 143–145 of pregnancy (3·287 ± 0·3351/min). The mean conversion ratio of oestradiol-17β to oestrone was 17·0 ± 2·2%, and that of oestradiol-17β to oestradiol-17α was 14·1 ± 1·1%. The production rate of oestradiol-17β increased from 0·007 to 0·103 μg/min up to 3 days before parturition to 0·035–1·257 μg/min during the last 12 h of pregnancy. During the continuous infusion of [3H]oestradiol-17β and when a steady state had been reached, approximately 20–30% of the radioactivity in whole blood could be extracted with ether. Of this radioactivity more than 90% was attributable to labelled oestradiol-17β, oestrone and oestradiol-17α in jugular blood, though only about 70% could be accounted for as these oestrogens in uterine venous blood.