Corpora lutea with a short life-span induced by rams in seasonally anovulatory ewes are prevented by progesterone delaying the preovulatory surge of LH

Abstract

Seasonally anovulatory Merino ewes isolated from rams were allocated to three treatments before the re-introduction of rams. Ten ewes received a single injection of progesterone (20 mg), 18 ewes received the injection of progesterone but had the ram-induced preovulatory surge of LH replaced by a series of injections of GnRH 24 h after the introduction of the rams, and 20 control ewes had no hormone treatment. Of the 48 ewes, 44 ovulated within 5 days of the introduction of rams and the treatments had no significant effect on the incidence of ovulation. The frequency of corpora lutea with a short life span (the interval between successive preovulatory surges of LH being 5.1 +/- 0.9 days) was 72% for control ewes and 58% for ewes treated with progesterone and GnRH, but such CL were prevented completely after the injection of progesterone alone (P less than 0.001). The injection of progesterone also delayed the preovulatory surge of LH (P less than 0.001). These results suggest that progesterone assures normality of corpora lutea by lengthening the period of gonadotrophin priming of follicles before ovulation.

Male-induced short oestrous and ovarian cycles in sheep and goats: a working hypothesis

“…20 mg per ewe) at exactly the same time as the introduction of males. This provokes an important delay in the post-introduction events: LH surge and subsequent ovulations are delayed from 24 to 72 h, depending on the experiments [5,17,56,57]. This was also demonstrated in goats [58,59].…”

“…Systemic plasma P4 concentrations were initially described as being roughly identical between females experiencing normal cycles and those showing short cycles up to D4-D5, after which luteolysis occurs in the latter group (ewes [17], goats [18], Fig. 2).…”

“…The concomitant suppression of short-lifespan CL and of enlargement of the interval, introduction of male-LH preovulatory surge, led to the proposition that the existence of these short-lifespan CL is due to an insufficient follicle maturation before ovulation [57]. This hypothesis is reinforced by the fact that an early induction of premature ovulations using GnRH injections in P4 treated ewes, restores short-lifespan CL [11,17]. The hypothesis of a longer duration of the follicular phase in females experiencing a normal lifespan CL duration has also been proposed based on a difference in the timing of the LH surge after introduction of rams in a limited number of ewes [5,17].…”

“…This hypothesis is reinforced by the fact that an early induction of premature ovulations using GnRH injections in P4 treated ewes, restores short-lifespan CL [11,17]. The hypothesis of a longer duration of the follicular phase in females experiencing a normal lifespan CL duration has also been proposed based on a difference in the timing of the LH surge after introduction of rams in a limited number of ewes [5,17]. But very few experiments or observations have been done.…”

Changes in LH pulse frequency and serum progesterone concentrations during the transition to breeding season in ewes

“…In most instances, these increases in progesterone only last 1-3 days and attain a maximum of about 1 ng/ml. Transient, low-amplitude increases in progesterone have also been observed before the first cycle at puberty in lambs (Foster & Ryan, 1979;Berardinelli et al, 1980;Keisler et ai, 1983;Foster et al, 1986), heifers (González-Padilla et al, 1975) and primates (Dierschke et al, 1974;Foster, 1977), after pulsatile administration of GnRH to anoestrous ewes (Legan et al, 1985a;Hunter et ai, 1986), after a ram-induced LH surge in anoestrous ewes (Pearce et al, 1985), at the end of post-partum anoestrus in cows (Pope et ai, 1969), and at onset of the breeding season in dairy goats (BonDurant et al, 1981). They are therefore a common phenomenon of the initiation of cyclicity in at least these 4 species.…”