DYNAMICS OF PROGESTERONE METABOLISM IN THE PSEUDOPREGNANT RAT

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Abstract
The characteristics of the disappearance of progesterone and 20α-dihydroprogesterone from blood were examined in rats made pseudopregnant by administration of gonadotrophins. Measurement of the disappearance of [3H]progesterone from blood indicated that progesterone metabolism can be represented by a two-compartment model in this animal preparation. The disappearance of [3H]progesterone from blood was described by two half-lives of 0·5 and 11·7 min. The metabolic clearance rate for this steroid was 2·9 litre/day, equivalent to a production rate of 3·9 μmol/day. The initial volume of distribution of the injected [3H]progesterone was 5·0 ml, a volume similar in size to the expected plasma volume. The total volume of distribution of [3H]progesterone (i.e. 'inner' and 'outer' pools) was in the range 24·8–35·4 ml. The disappearance of endogenous progesterone from the blood of pseudopregnant rats after vascular isolation of the ovaries showed two half-lives of 1·1 and 11·4 min respectively. The disappearance of a product of reductive metabolism of progesterone, 20α-dihydroprogesterone, was apparently uniphasic, with a half-life of 41·3 min.

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