Effect of PGF-2 alpha on LH receptors in the equine corpus luteum

As quantified by Scatchard analysis, a 27 000 g crude luteal membrane fraction contained a single population of unoccupied LH receptors characterized by high affinity, ka = 0.647 +/- 0.158 X 10(11) M-1 and low binding capacity, Rt = 4.91 +/- 0.78 X 10(-11) M/mg membrane fraction. Acceptable hormonal specificity, reversibility, saturability, high affinity and tissue specificity indicated that the binding protein was a physiological receptor. To ensure that the methods used for Scatchard analysis were valid, hCG was characterized for specific activity and maximum bindability, non-specific binding was monitored, equilibrium binding assay conditions were optimized and the amount of hormone and receptor degradation was evaluated. Serum concentrations of LH and progesterone significantly increased within 1 h after PGF-2 alpha treatment (P less than 0.05). Serum and luteal progesterone concentrations were significantly reduced (P less than 0.05, P less than 0.01, respectively) by 3 h, long before a decline in luteal LH receptors was observed. A significant decline in receptor number (P less than 0.05) was detected by 36 h. This loss of receptors was associated with a decrease in luteal progesterone (P less than 0.05) and a significant increase in luteal cell degeneration, as judged by morphology. The affinity of the receptor for hCG did not differ at any of the times studied. Luteal weights remained unchanged. PGF-2 alpha treatment of the mare resulted in a rapid decline of serum and luteal progesterone before the loss of luteal LH receptors that presumably leads to the irreversible functional and structural demise of the equine CL.