The influence of chlormadinone acetate treatment on the concentration of some steroids in the blood, on the ovarian activity, and on the sexual behaviour of the mare.

The effect of orally administered chlormadinone acetate (CAP), 10 mg daily for a period of 16 days, was investigated in the case of four mares with an irregular oestrous pattern accompanied by low ovarian activity (group 1), four mares which did not show oestrous symptoms at all and which had also low ovarian activity (group 2), and two ovariectomized and two ovario-hysterectomized mares (group 3). In all mares of group 1 and in the two ovariectomized mares of group 3 oestrus symptoms became apparent during treatment. Two mares of group 2 came into heat 8 and 11 days after the cessation of treatment. The two other mares of group 2 and the ovario-hysterectomized mares of group 3 showed no oestrous symptoms at all. In all except the two ovario-hysterectomized mares the progesterone concentration was found to increase in the peripheral blood from less than 1 ng/ml up to at least 4-6 ng/ml. In the mares which came into oestrus this progesterone increase occurred about three days before symptoms of heat became apparent. It is assumed that the uterus played an important role in the increase of the progesterone concentration in the blood during or after CAP treatment. A similar rise of the progesterone level was also found in the ovariectomized mares, indicating that this progesterone did not originate in the ovaries.

Some steroids in the blood of mares suffering from endometritis and post-partum mares as compared to normal cycling mares.

It appeared that in mares suffering from endometritis the concentrations of pregnenolone, of the compound "5.4", and of progesterone in the blood during the first 2 days of the heat period were significantly higher than in normal cycling mares, in which the concentrations of all steroids were less than or equal to 1 ng/ml. In five endometritis mares the "5.4" and progesterone concentrations decreased after the first days of the heat period, and at about two days before ovulation the values were comparable to those of normal cycling mares. Eight mares suffering from a severe endometritis did not ovulate, their oestrous period was prolonged, and the follicles became atretic. Possibly the relatively high progesterone content found in these mares, caused the reduction of the follicular growth, and this might explain the prolonged oestrous period which is observed in many mares suffering from endometritis. Often we observed in post-partum mares the same changes in concentration as in endometritis mares. An elevated level of these steroids was sometimes found in the blood a few days after foaling and, in four of eight post-partum mares, during the foal heat. Most probably this was caused by infection or stimulation of the uterus. The hypothesis is advanced that progesterone originating in the uterus can influence ovarian activity and that it may be one of the factors responsible for the anoestrous periods which can occur in mares with endometritis and in post-partum mares.

The influence of taking uterine biopsies on the concentration of some steroids in the blood and in the uterus, on the ovarian activity, and on the sexual behaviour of the mare.

It has been shown that stimulation of the uterus of mares by the daily taking of biopsies can result in the occurrence of oestrous symptoms. This is accompanied by some follicular growth and a progesterone content in the blood often higher than 1 ng/ml. The following observations suggest that this progesterone originates in the uterus and not in the ovaries: (1) no active corpora lutea appeared to be present in the ovaries after ovariectomy, (2) ovariectomized mares showed the same oestrous symptoms in similar experiments and even mating took place, (3) in the uterine biopsies the concentration of the compound referred to as "5.4", which is assumed to be easily convertible into progesterone, had already increased considerably in the second biopsy, (4) administration of stilboestrol reduced the rise of the progesterone level in the uterine biopsies as well as in the blood. The absence of oestradiol-17 beta in the ovarian follicles and the fact that ovariectomized mares also come into heat suggest that oestrogens cannot be held responsible for the oestrous symptoms in these mares. Our experiments demonstrate that the uterus can be involved in sexual behaviour and the formation of steroids.

Hormonal pattern in the blood of eight mares during the first weeks of pregnancy.

The hormonal pattern in the blood of eight mares was studied during the first 5 to 6 weeks of pregnancy; in two of the eight animals early embryonic death was established on D-28 and on D-40, respectively. Progesterone peaks were found in the blood around D-5, D-12 to D-10, and D-17 to D-18. It is suggested that the decrease of the progesterone concentrations in the blood during the intermediate periods is due to the action of oestrogens. During the period from D-18 to about D-34 the concentrations both of progesterone and of pregnenolone were low in the pregnant mares. Thereafter a small increase of the progesterone content was observed. Sometimes rather large amounts of the compound "5.4", an isomer of pregnenolone, were found, indicating the presence of follicles containing oestradiol-17beta or possibly a stimulated uterus. It is suggested that early embryonic death may be expected. a. if the progesterone concentration in the blood is low on D-17 to D-18; b. if there is no increase of the progesterone or the "5.4" concentrations during the period from D-34 to D-37. A relatively high pregnenolone level on these days indicates reduced conversion of pregnenolone into progesterone.