STEROIDS IN THE OVARIAN VENOUS BLOOD OF EWES BEFORE AND AFTER GONADOTROPHIC STIMULATION

Abstract

SUMMARY Ewes on the 9th and 15th days of the oestrous cycle were injected intravenously with follicle stimulating hormone, luteinizing hormone and prolactin of sheep pituitary origin, human chorionic gonadotrophin, pregnant mares' serum and an endometrial extract. The ovarian vein was cannulated, and progesterone measured chemically in the venous effluent. None of the above substances was capable of producing a pronounced or sustained alteration in the rate of progesterone secretion in the hour following injection. It was concluded that the ovine corpus luteum, if it responds at all to gonadotrophic stimulation, must do so in a manner very different from the testis, the response of which to luteinizing hormone is both immediate and sustained. The mean secretion rate of progesterone was approx. 150 μg./hr., thus being of the same order of magnitude as a previous indirect estimate of 113 μg./hr. There was no difference in the secretion rate between the 9th and 15th days of the cycle, apart from one animal at the 15th day in which the corpus luteum had ceased to secrete progesterone altogether. The turnover time of progesterone in the corpus luteum was 10 min., suggesting that the hormone is released from the gland as soon as it is formed. Pooled samples of ovarian vein plasma collected during the mid- and late luteal stages of the cycle contained, apart from progesterone, pregnenolone, 20α-hydroxypregn-4-en-3-one, oestrone and oestradiol-17β. In three samples of ovarian vein plasma collected from ewes in oestrus it was impossible to detect progesterone or 20α-hydroxypregn-4-en-3-one, even after chorionic gonadotrophin therapy. Oestradiol-17β and oestrone were present in concentrations similar to those found during the luteal phase of the cycle. Since the rate of ovarian vein blood flow is lower at oestrus than during the luteal phase, it follows that the oestrogen secretion rates are also lower at oestrus. The 24 hr. secretion rate of oestradiol-17β in two oestrous ewes was 3·3–7·4 μg. This is in close agreement with the median effective dose of 7 μg. of oestradiol-17β needed to induce oestrus in the spayed, progesterone-primed ewe.