The metabolic clearance rate and production rate of progesterone and the conversion of progesterone to 20 -hydroxypregn-4-en-3-one in the sheep.

Abstract

SUMMARY Tracer kinetic techniques were used to determine the metabolic clearance rate (MCR) and production rate (PR) of progesterone in the sheep during pregnancy, the oestrous cycle and the lactation anoestrus. The MCR from whole blood measured by the continuous infusion of [3H]progesterone was 3·483 ± 0·126 l/min in pregnancy, 2·752 ± 0·239 l/min in the oestrous cycle and 5·020 ± 0·566 l/min in lactation anoestrus. After correcting for body weight the respective values were 0·077, 0·077 and 0·133 l/min/kg. The PR ranged from 0·60 to 6·75 μ/min in the oestrous cycle and 6·66 to 72·14 μg/min in pregnancy. In the last 4 weeks of gestation the PR of progesterone was significantly greater in ewes bearing foetuses with birth weights of more than 4 kg than in ewes with smaller lambs. The conversion or isotope concentration ratio of progesterone to 20α hydroxypregn-4-en-3-one (20α-diHP) was 69·4±10·0% in non-pregnant and 77·2 ± 4·8% in pregnant sheep. The concentrations of [3H]progesterone and [3H]20α-diHP were closely correlated. There was an appreciable extraction (25·4 ± 3·0%) of progesterone and production (9·1 ± 2·0% of 20α-diHP by the head.