THE METABOLIC CLEARANCE RATE, PRODUCTION RATE AND CONVERSION RATIOS OF OESTRONE IN THE SHEEP

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Abstract
The metabolic clearance rate of oestrone from whole blood and its conversion to oestradiol-17β and oestradiol-17α were measured in pregnant and non-pregnant sheep using tracer kinetic techniques. The clearance rate of oestrone for all animals (3·435 ± 0·212 (s.e.m.) 1/min) was significantly greater than that of oestradiol-17β (2·449 ± 0·155 1/min). The mean conversion ratio of oestrone to oestradiol-17β was 23·8±1·6%, and that of oestrone to oestradiol-17α was 17·7 ± 2·5%. The transfer constant of oestrone to oestradiol-17β was estimated to be 17·5 ± 1·6% compared with that of oestradiol-17β to oestrone which was 23·2 ± 2·5%. The production of oestrone increased from values of 0·141 and 0·240 μg/min up to 2 days before parturition, to between 0·660 and 1·881 μg/min during the last 24 h of pregnancy; these values were appreciably greater than those of oestradiol-17β. During the continuous infusion of [3H]oestrone or [3H]oestradiol-17β and when a steady state had been reached, the uptake by erythrocytes of [3H]oestrone was 22·3%, that of [3H]oestradiol-17β, 6·6%, and that of [3H]oestradiol-17α, 22·0%. In three experiments on one sheep, the uptake of [3H]oestrone across the head was 11–16%.

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