Abstract
Ewes demonstrating an increased ovulation rate following immunization against androstenedione had increased plasma levels of luteinizing hormone (LH) and decreased follicle stimulating hormone (FSH) levels compared with untreated ewes during the late luteal and follicular phases. To assess whether this increased LH secretion could cause an increased ovulation rate, three groups of anoestrous ewes were supplemented with different regimens of exogenous LH mimicking the pattern of LH secretion either in androstenedione immunized or in control ewes. There was no treatment effect on ovulation rate, demonstrating that the increased LH secretion of androstenedione immunized ewes is not causing their increased ovulation rate. To assess whether ovarian sensitivity to gonadotrophins is altered by immunization, the ovarian response of steroid immune and control ewes to pregnant mare serum gonadotrophin (PMSG: 0, 750 or 1500 i.u.) or FSH (0, 14 or 28 mg) was compared. In this in vivo test, the ovarian response of immunized ewes was not increased over that of the controls. In an in vitro test, basal and FSH stimulated production of cAMP, androstenedione and oestradiol by follicles of immunized and control ewes were compared. There was no difference in basal and FSH induced secretion between follicles from control or immunized ewes. FSH induced an increase in cAMP (35%) and oestradiol (13%) and a decrease in androstenedione (30%) productions. It can be concluded that neither gonadotrophin secretion nor ovarian sensitivity to gonadotrophins is responsible for the high ovulation rate of androstenedione immune ewes.
Published Version
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