Comparative luteolytic activity of estradiol cyclopentylpropionate and prostaglandin F2α in diestrous cows

The effect of estradiol cyclopentylpropionate (EC) on corpus luteum (CL) function in diestrous cows was evaluated. Two doses of EC (4 and 20 mg) were given by intramuscular (IM) injection and one dose of EC (4 mg) was given by intrauterine (IU) infusion. Control cows were treated with physiologic sterile saline (PSS) IU or corn oil IU (negative controls) or prostaglandin F_2α (PGF_2α, 30 mg IM, positive control). A total of 24 cows, four per treatment, were treated on days 8 to 12 of the estrous cycle (day 0 equals day of estrus). Luteal function was monitored by serum progesterone through 96 hours after treatment. A decrease in serum progesterone from pretreatment diestrous concentrations to less than 1.0 ng/ml was considered indicative of luteolysis.Intrauterine injection of PSS and corn oil had no effect on luteal function. Neither IM nor IU administration of EC caused consistent or rapid luteolytic effects. Prostaglandin F_2α consistently induced rapid luteal regression. These results indicate that EC should not be considered luteolytic in the same sense as is PGF_2α.