Circulating maternal immunoreactive inhibin levels during pregnancy in the rat: effects of oophorectomy, hypophysectomy, hemihysterectomy, delayed implantation and pseudopregnancy.

This study examines the source of inhibin in the maternal circulation of pregnant rats by measuring serum immunoactive inhibin levels following a range of experimental procedures. Ovariectomy at days 7, 13 or 19 of gestation, with maintenance of pregnancy by supplementation with progesterone and oestradiol dipropionate, led to a profound fall of serum inhibin levels in comparison with controls, demonstrating that the ovary is a major source of circulating inhibin. This conclusion was supported by the inhibition of the late rise (days 16-22) in serum inhibin in pregnant rats which were hypophysectomized on day 15 and maintained with oestrogen and progesterone supplementation. These data support the view that the rise in serum inhibin from days 16 to 22 is due to re-activation of follicular development in preparation for the post-partum oestrus. Reduction of fetal numbers by hemihysterectomy on days 7, 13 or 19 did not alter serum inhibin levels. Induction of delayed implantation by ovariectomy on day 3 and progesterone supplementation together with initiation of reimplantation by the addition of oestradiol dipropionate on day 7 or 11 did not significantly alter inhibin levels. The induction of pseudopregnancy by mating to vasectomized rats did not result in the maintenance of stable serum inhibin levels until oestrous cycles recommenced. Taken together, the studies have identified the ovary as the predominant source of circulating maternal inhibin levels throughout pregnancy in the rat.