Use of deuterium-labelled tracers to establish endogenous rates of production and clearance of progesterone during the peri-implantation period in gilts

Abstract

Using a method based on the constant infusion of a deuterium-labelled tracer, production rates (PR) and metabolic clearance rates (MCR) of progesterone have been determined in gilts between Days 11 and 17 of gestation. This involved the development of a procedure to quantitate the tracer [(7,7,16- 2H 3)-progesterone] in plasma samples, and in the infusate, with the aid of gas chromatography-mass spectrometry. In a group of nine mated gilts, the average (± s.e.m.) progesterone PR was 274 (± 26) μmol/24 h, the average progesterone MCR being 3348 (± 312) 1 24 h . Declining peripheral plasma progesterone levels (PP) between Days 11 and 17 of gestation were associated with increasing progesterone MCR ( P < 0.05), and were not correlated with increasing PR. For comparison, progesterone PRs and MCRs were determined in five unmated gilts between Days 11 and 15 of oestrus, prior to the normal onset of luteolysis. The mean progesterone MCR in these animals (2735±275 1 24 h ) was similar to the respective pregnancy mean ( P > 0.1), but the PR (158±12 μmol/24 h) was lower ( P < 0.01). PP values, PR values and numbers of corpora lutea were not significantly correlated ( P > 0.1) in either mated or unmated groups of gilts. The results of this study support previous indications of luteotrophic effects of conceptus tissues after Day 10 of pregnancy in the sow, and provide quantitative bases for progesterone replacement in ovariectomized pigs between Days 11 and 17 after oestrus/coitus.