title: Effect of exogenous progesterone on its endogenous levels: biological half-life of progesterone and lack of progesterone binding in mares.
abstract: The disappearance rate of progesterone from blood plasma of an ovariectomized mare was rapid and occurred in three phases. The half-life of the first component was 2-5 min and that of the second was 20 min; the half-life of the much slower third component was not measured. Progesterone, administered to ovariectomized and deeply anoestrous mares at the rate of 150 or 300 mg/day, maintained maximal peripheral levels of 6 to 8 ng/ml. Physiological levels were reached in 21 days using a dose of 150 mg and in 11 days with 300 mg daily. After withdrawal, circulating levels dropped rapidly. Blood plasma of mares appears to lack progesterone-specific binding proteins. It is concluded that the problem of habitual abortion and its treatment in mares need re-evaluation.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002369:Castration; D005260:Female; D006207:Half-Life; D006736:Horses / Q000378:metabolism*; D011374:Progesterone / Q000008:administration & dosage / Q000378:metabolism* / Q000494:pharmacology; D011485:Protein Binding
chemical_list: D011374:Progesterone