The effect of Regu-Mate, a synthetic progestin, on early pregnancy maintenace, conceptus growth, and corpora lutea development in pregnant pony mares

Abstract

Summary In order to determine the effect of Regu-Mate on early pregnancy maintenance and ovarian function during pregnancy, pony mares were estrous synchronized and bred for use in two experiments. Beginning on Day 4 postovulation, Regu-Mate was fed (0.044 mg/kg body weight) daily to each mare through Day 85. In Experiment 1, each mare received prostaglandin F 2 α(PGF 2 α, 3 mg, IM) on Day 10 postovulation to regress the corpus luteum. Frequent palpation indicated that seven of the nine mares were pregnant at Day 35, three of these resorbed their fetus between Days 40 and 50. In the four mares in which pregnancy was maintained, accessory ovulations occurred between Days 32 and 85 with subsequent increases in plasma progesterone. Accessory ovulations varied in number from 1 to 4 per mare. In these four mares a single injection of PGF 2 α at Day 65 and Day 75 did not result in abortion and in mares with multiple ovulations, it did not appear to completely regress the corpora lutea, as indicated by a transient decrease in plasma progesterone. Repeated injections starting Day 85 of PGF 2 α (5 rag, IM, twice daily) did induce abortion within 2 to 3 days, even though mares were continued on Regu-Mate treatment. In Experiment 2 nine mares were bred and given Regu-Mate orally beginning Day 4 postovulation through Day 80. In addition, each treatment mare (n=5) received PGF 2 α (3 mg, IM) every five days postovulation starting on Day 10 to regress functional corpora lutea. Control mares (n=4) received an equivalent volume of sterile saline. Both groups were rectally palpated and examined by ultrasound three times per week to determine ovarian volume, follicle numbers, presence of corpora lutea, and conceptus size. Treatment did not affect (P>.05) conceptus growth rate in those mares that maintained pregnancy through Day 50. However, two treated mares aborted their fetus on Days 49 and 69. All mares had one or two accessory corpora lutea derived from ovulatory follicles. There was a tendency (P>.05) for more anovulatory accessory follicles becoming luteinized in treated mares (2.3/mare, range 1 to 5) than in control mares (0.75/mare, range 1 to 2). Following initial PGF 2 α on Day 10, luteolysis was rapid (within 2 days) and complete as measured by plasma progesterone ( 2 α (every 5 days).