Relationship between LH, FSH, and prolactin concentration and the secretion of androgens and estrogens by the preovulatory follicle in the ewe.

Abstract

The ovarian secretion of estradiol, androstenedione, testosterone, and progesterone and the concentration of LH, FSH, and prolactin were measured in the periovulatory period of five ewes with ovarian or utero-ovarian autotransplants. Samples of jugular and ovarian venous blood were collected every 1 or 2 h before and for 96 h after induction of luteal regression on Day 10 of the cycle by injection of 100 µg cloprostenol. Progesterone concentration in jugular vein plasma fell to less than 1 ng/ml in all ewes by 24 h, and coincidental to this decline there was a significant increase in the secretion of LH and prolactin and an associated rise in the ovarian venous concentration of estradiol, testosterone, and androstenedione. The rise in LH secretion was associated with a twofold increase in the frequency of episodic pulses each of which stimulated an increase in the secretion of estradiol. In contrast the concentration of FSH declined significantly in the 48 h after luteal regression. Shortly after the onset of estrus (48 ± 2.5 h), there was a marked rise in the concentration of prolactin, FSH, and LH which reached a peak at 61 ± 4 h. At the start of this preovulating LH surge, there was a further substantial stimulation in the secretion of estradiol, testosterone, and androstenedione, followed by a sharp fall within 3, 5, and 7 h, respectively. In the 24 h after the preovulatory surge, there was a marked decline in the concentration of LH and prolactin as well as all ovarian steroids. However, within 12 h there was a further rise in the concentration of FSH which reached a second peak at 23 h after the LH peak. These results suggest that the increased secretion of estradiol from the preovulatory follicle is due to stimulation by episodic pulses of LH which occur with increased frequency as the concentration of progesterone falls during luteal regression. The sustained rise in LH which occurs during the preovulatory surge stimulates and then markedly inhibits aromatase activity and eventually all steroid secretion from the follicle. Thus the final stages of development of the preovulatory follicle are determined not by FSH but by the pattern of secretion of LH.