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.
Two experiments involving 24 and 54 Australian Merino ewes were conducted in which the establishment of a cervical population of spermatozoa and several endocrinological events were studied after several regimens for the synchronization of oestrus. Intravaginal sponges impregnated with 500 mg (Exp. 1) or 200, 400 or 600 mg (Exp. 2) progesterone resulted in the maintenance of plasma progesterone concentrations of 1.5-4.9 ng/ml over a 12-day insertion period compared with 1.9-6.9 ng/ml during dioestrus in control ewes. In Exp. 1 basal concentrations of less than or equal to 0.25 ng/ml plasma were attained by 4 h after sponge withdrawal and this decline was much more rapid than in normal luteolysis. This was associated with fewer spermatozoa recovered from the cervix 2 h after insemination, and PMSG had no significant effect. In Exp. 2 injection of a supplementary dose of progesterone at sponge withdrawal resulted in a rapid increase in plasma progesterone concentrations followed by an equally rapid decrease and an attenuation of the rise in plasma oestradiol-17 beta, the LH surge, and the onset of oestrus. The numbers of spermatozoa recovered 4 h after insemination were not increased, and PMSG had no significant effect. Two factors were significant, namely the dose of progesterone in the sponge (600 mg greater than 400 or 200 mg, P less than 0.05) and stage of oestrus when inseminated (mid- or late oestrus greater than early). The data demonstrated that an adequate dose of progesterone/progestagen incorporated into intravaginal sponges and accurate timing of insemination relative to the LH surge are the most important factors involved in penetration of the cervix by spermatozoa.
The effects of season and of oestradiol and progesterone on the tonic secretion of LH were studied in ovariectomized Merino and Suffolk ewes, two breeds which differ markedly in the seasonal pattern of their reproductive activity. In the absence of exogenous steroids, the frequency of LH pulses was lower and the amplitude of the pulses was higher in anoestrus than in the breeding season for Merino and Suffolk ewes 30 days after ovariectomy. In long-term (190 days) ovariectomized ewes, this seasonal change in LH secretion was observed in Suffolk ewes only. During seasonal anoestrus, treatment of ewes with subcutaneous oestradiol-17 beta implants (3, 6 or 12 mm in length) decreased the frequency of LH pulses in a dose-dependent manner, with Suffolk ewes being far more sensitive to the inhibitory effects of oestradiol than Merino ewes. The lowest dose of oestradiol (3 mm) had no effect on the secretion of LH in Merino ewes, but reduced secretion in Suffolk ewes. Treatment of ewes with the highest dose of oestradiol (12 mm) completely abolished LH pulses in Suffolk ewes, whereas infrequent pulses remained evident in Merino ewes. During the breeding season, oestradiol alone had no effect on the pulsatile release of LH in either breed, but in combination with progesterone there was a significant reduction in LH pulse frequency. Progesterone effectively decreased LH secretion in both breeds in both seasons. It was concluded that differences between breeds in the 'depth' of anoestrus could be related to differences in the sensitivity of the hypothalamus to both negative feedback by oestradiol and the direct effects of photoperiod.
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