Effect of exogenous progesterone on its endogenous levels: biological half-life of progesterone and lack of progesterone binding in mares.

The research investigates how administering progesterone affects its own natural levels in the bodies of mares, how rapid progesterone tends to disappear from the blood, and whether blood in mares has specific proteins for progesterone binding. The study suggests that management of habitual abortion in mares might need to be reconsidered.

Research Method

• The study revolved around evaluating the rate of progesterone disappearance from the blood plasma of a mare that had its ovaries removed. This process appeared to occur quickly and in three distinct stages.
• The half-life of the first component of the disappearance was found to be between 2 and 5 minutes, while the second phase had a half-life of 20 minutes. The researchers didn’t measure the half-life of the last phase as it was slower than the first two.

Dosage Impact

• Progesterone was given to mares who had their ovaries removed and those that were deeply anestrous. The dosage varied between 150 and 300 mg every day.
• Upon administering progesterone at the rate of 150 or 300 mg/day, the maximum side levels remained between 6 to 8 ng/ml.
• With a daily dosage of 150 mg, physiological levels were attained in around 21 days, while 300 mg achieved the same in just 11 days.
• The progesterone levels within the blood rapidly decreased after ceasing the administered dosage.

Progesterone Binding in Mares

• The researchers noticed that the mares’ blood plasma seemed to lack progesterone-specific binding proteins. Such proteins would typically help keep progesterone within the body.
• Based on the absence of these proteins and the rapid decrease of progesterone levels post administration, the researchers proposed that the approach to habitual abortion in mares needed fresh evaluation.