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 complementar y hypotheses which may explain the short lifespan of corpora lutea obs erved 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 poss ible difference due to characteristics of follicles before ovulation w as 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 intro duced to rams at the same time as 1 0 control ewes. Plasma progesteron e (P4) was analyzed each day for 17 consecutive d after the introducti on 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 shor t cycles compared to 5 of the 9 control ewes (P = 0.029). The second e xperiment involved 16 ewes subjected to the RE in June, and 5 cyclic e wes in January. The ewes were ovariectomized 82 h after the preovulato ry LH surge, the CL were separated, weighed and the luteal cells enzym atically dissociated to count the relative proportions of small (< 20 mu diameter) and large cells and assess in vitro P4 secretion both wit h and without stimulation with 1 00 ng ovine LH. Plasma P4 concentrati on 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 la rge 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.