The corpus luteum : source of oestrogen during early pregnancy in the mare

Thirty pregnant mares were assigned to 3 groups: Group 1 (n = 10) mares served as controls; Group 2 (n = 10) mares were treated with altrenogest (44 mg/day) from Day 16 to 80 and Group 3 (n = 10) mares were treated with a luteolytic dose of PGF2-alpha on Day 16 followed by altrenogest (44 mg/day) until Day 80. Concentrations of progesterone and chorionic gonadotrophin (CG) in plasma and oestrogen conjugate (OC) in urine were determined between Days 16 and 80 of gestation. In Group 3, complete luteolysis occurred in all 10 mares following administration of PGF2-alpha. Six of the 10 mares did not have an active CL from Day 16 until after Day 60 of pregnancy (Group 3a); the remaining 4 mares developed a new CL on Days 32, 40, 43 and 49 of pregnancy (Group 3b). In Groups 1 and 2, an increase in oestrogen secretion was observed between Days 35 and 40. In Group 3a, OC concentrations in the absence of an active CL were significantly lower than in Groups 1 and 2, and oestrogen secretion did not increase between Days 20 and 50. In Group 3b, OC concentrations remained low in the absence of an active CL. After the onset of CG secretion, OC concentrations increased to values similar to Groups 1 and 2 in conjunction with the development of a new CL. The results of this experiment demonstrate that the increase in oestrogen secretion observed between Days 35 and 50 of pregnancy requires the presence of an active CL. The close time relationship between the increase in CG and OC concentrations in the presence of an active CL suggests that CG stimulates luteal oestrogen synthesis