Long-term characteristics of idiopathic persistent corpus luteum in the mare

Persistent CL (PCL; n = 10) in mares was studied daily from Day 20 (Day 0 = ovulation) to the ending ovulation. In addition, the 10 days before ovulation at the end of a PCL were compared with the end of an interovulatory interval (IOI; n = 28) during the same months. Concentration of P4, cross-sectional area of CL, and percentage of CL with Doppler signals of blood flow during PCLs remained constant from 64 to about 33 days before the end of luteolysis and then decreased linearly. Concentration of LH between Day 20 and beginning of the ovulatory LH surge increased linearly. A dominant follicle developed on average every 15 days throughout each PCL. Novel transient P4 depressions were detected with the P4 nadir at a day of maximal diameter of a dominant follicle. At the apparent beginning of luteolysis before the ending ovulation, P4 concentration in PCLs (5.0 ± 0.5 ng/mL) was less (P < 0.0001) than that in IOIs (9.3 ± 0.6 ng/mL). Concentration of LH began to increase 2 days before the end of luteolysis in each group, but concentration on the day of the ending ovulation in PCLs (3.7 ± 0.3 ng/mL) was less (P < 0.005) than that in IOIs (8.9 ± 1.8 ng/mL). In a separate survey of PCLs (n = 23) and IOIs (n = 352), frequency of PCL (6.1%) differed significantly among mares indicating repeatability. These original and critical comparisons between PCLs and IOIs should provide hypotheses for further study.