Progesterone, oestrogen and glucocorticoid receptors in the uterus and mammary glands of mares from mid- to late gestation.

Progesterone, oestrogen and glucocorticoid receptor concentrations in the uterus and mammary glands of pregnant mares during mid- to late gestation (from day 150 of gestation to term) were measured by binding assays to investigate the hormonal mechanisms involved in pregnancy maintenance and lactation. Uterine progesterone receptor concentrations did not increase significantly with increasing gestational age (from 67.8 +/- 13.7 to 126.1 +/- 48.7fmol mg(-1) protein), whereas oestrogen receptor concentrations were significantly higher in pregnant mares (271.7 +/- 28.9 fmol mg(-1) protein) than in non-pregnant control mares (54.9 +/- 8.1 fmol mg(-1) protein; P < 0.05). There was no correlation between progesterone and oestrogen receptor concentrations, and plasma progesterone and oestrone sulphate concentrations, respectively. In contrast, mammary gland progesterone and oestrogen receptor concentrations decreased significantly with gestational age (from 139.7 +/- 34.6 to 66.7 +/- 22.0 fmol mg(-1) protein and 225.2 +/- 13.3 to 87.6 +/- 14.4 fmol mg(-1) protein, respectively; P < 0.05). The dissociation constant (Kd value) of 16alpha-ethyl-21-hydroxy-19-nor-4-pregnene-3,20-dione (ORG2058) for progesterone receptors was 22 nmol l(-1). 5alpha-pregnane-3,20-dione (5alpha-DHP) had a high affinity for progesterone receptors, which was similar to that of progesterone, whereas other equine progestagens and 11-[4-(dimethylamino)phenyl]-17-hydroxy-17(1-propynyl)estra-4,9-diene-3-one (RU486) did not bind to progesterone receptors. Oestradiol bound the oestrogen receptors with a Kd value of 0.9 nmol l(-1), which was 10 times more potent than that of 3-hydroxy-1,3,5(10),7-oestratetraen-17-one (equilin). The concentration of glucocorticoid receptors (Kd value = 1.3 nmol l(-1)) was constant between the tissues and reproductive stages. In the present study, striking differences were observed between progesterone receptor expression in the uterus and the mammary glands during pregnancy, probably due to tissue-specific variations in 5alpha-DHP activity. This finding indicates that 5alpha-DHP has an important physiological role in equine gestation.