RAM-INDUCED SHORT LUTEAL PHASES - EFFECTS OF HYSTERECTOMY AND CELLULAR COMPOSITION OF THE CORPUS-LUTEUM

Two experiments were conducted on Prealpes ewes to test 2 complementary hypotheses which may explain the short lifespan of corpora lutea observed in some cases after ram-induced ovulation: i) the possible role of the uterus was tested by determining the effects of hysterectomy on the duration of luteal phases after the ram effect (RE); ii) the possible difference due to characteristics of follicles before ovulation was tested by determining the cellular composition and characteristics of corpora lutea (CL) induced by the RE compared to CL of the breeding season (BS) when ovulation is synchronized by FGA-impregnated sponges. In the first experiment, 9 ewes were hysterectomized (Hys) and introduced to rams at the same time as 1 0 control ewes. Plasma progesterone (P4) was analyzed each day for 17 consecutive d after the introduction of rams. The number of females ovulating was not different for the 2 groups (7/9 vs 9/10, respectively), but no Hys ewes experienced short cycles compared to 5 of the 9 control ewes (P = 0.029). The second experiment involved 16 ewes subjected to the RE in June, and 5 cyclic ewes in January. The ewes were ovariectomized 82 h after the preovulatory LH surge, the CL were separated, weighed and the luteal cells enzymatically dissociated to count the relative proportions of small (< 20 mu diameter) and large cells and assess in vitro P4 secretion both with and without stimulation with 1 00 ng ovine LH. Plasma P4 concentration increased significantly more slowly in RE than in BS ewes (P < 0.05). The weight of twin RE CL did not differ from that of twin FGA-BS CL (m +/- SD; 142-1 +/- 48.7 vs 139.1 +/- 23.0 mg). The percentage of large luteal cells was lower in RE than in FGA-BS CL (10.7 +/- 4.6 vs 25.0 +/- 6.6%; P < 0.001). The quantity of P4 secreted in vitro without LH stimulation was lower in RE than in FGA-BS CL. (7.3 +/- 6.6 vs 13.8 +/- 9.8 ng/105 luteal cells/3 h; P < 0.05). The incubation of luteal cells for 3 h with 100 ng oLH significantly increased P4 secretion in RE but not in FGA-BS CL (+73% vs +9%; P < 0.05). It is concluded that the uterus is essential for determining the lifespan of RE-induced CL and that the cellular composition and characteristics of the RE CL are very different from those of FGA-treated breeding season CL.