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

This study measures progesterone, oestrogen, and glucocorticoid receptor concentrations in the uterus and mammary glands of pregnant horses from mid to late pregnancy to investigate the hormones involved in maintaining pregnancy and producing milk. Findings suggest hormonal changes vary significantly between tissues, possibly due to varying activity of 5alpha-DHP, which reveals its crucial role in horse pregnancy.

Research Context and Objectives

• The study investigates the hormonal mechanisms involved in the maintenance of pregnancy and lactation in mares. Specifically, it focuses on the concentrations of progesterone, oestrogen, and glucocorticoid receptors in the uterus and mammary glands of the mares from the 150th day of the gestation period to the term.

Contrasting Uterus and Mammary Gland Receptors

• In the uterus, progesterone receptor concentrations did not significantly increase with the advancing gestational age. However, oestrogen receptor concentrations were found to be significantly higher in the pregnant mares.
• In the mammary glands, it was found that both progesterone and oestrogen receptor concentrations decreased significantly as gestation progressed.
• Unlike the general expectation, there was no correlation between progesterone and oestrogen receptor concentrations and the plasma concentrations of the corresponding hormones, namely progesterone and oestrone sulphate.

Receptor Affinity

• The study found that receptors show different affinity to various substances. A compound known as 5alpha-pregnane-3,20-dione (5alpha-DHP) showed a high affinity for progesterone receptors, similar to progesterone itself, while other equine progestagens and a substance known as RU486 did not bind to progesterone receptors.
• As for oestrogen receptors, they bound the oestradiol with a dissociation constant (Kd value) ten times more potent than equilin.

Role of 5alpha-DHP

• The most significant finding was the observation of striking differences in progesterone receptor expression between the uterus and the mammary glands during pregnancy. The study hypothesized that this was probably due to tissue-specific variations in the activity of 5alpha-DHP.
• Therefore, the research highlights the potentially important physiological role that 5alpha-DHP plays in equine gestation. The variations in receptor expression observed in different tissues during gestation could provide interesting insights into the complex hormonal regulation processes that occur during pregnancy in mares.