LH release and luteal function in post-partum acyclic ewes after the pulsatile administration of LH-RH

Abstract

Summary. The administration of LH-RH in a pulsatile regimen (100 ng i.v./h for 48 h) to acyclic ewes 26\p=n-\30 days post partum increased plasma LH concentrations, and both the frequency and amplitude of plasma LH pulses. In 12/14 ewes these increases were followed by plasma LH surges similar to the preovulatory surges observed in 10 control cyclic ewes. Subsequent luteal function in the post-partum ewes was deficient. Plasma progesterone was detected in 7/12 post-partum ewes showing plasma LH surges. The concentrations were lower (1\m=.\3\ m=+-\ 0\m=.\2 ng/ml) and detected for shorter periods (3\p=n-\10 days) than in cyclic ewes (2\m=.\4\ m=+-\ 0\m=.\2 ng/ml, 12/15 days). In the post-partum ewes the increases in plasma LH concentrations before the LH surge were higher but of shorter duration than in the cyclic ewes. The inadequate luteal function in the post-partum ewes could therefore have been due to inappropriate LH stimulation of the ovary before the LH surge.

Corpora lutea induced by gonadotrophin-releasing hormone treatment of anoestrous Welsh Mountain ewes: reduced sensitivity to luteinizing hormone in vivo and to chorionic gonadotrophin in vitro

“…Transient inadequate luteal function occurs naturally in sheep, for example, in early postpartum (Wright et al 1983, Braden et al 1989, at puberty and during the transition from anoestrus to the breeding season (Legan et al 1985), and following ram-induced ovulation during anoestrus (Martin et al 1986). Induction of ovulation in seasonally anoestrous ewes by the administration of luteinizing hormone (LH) or gonadotrophin-releasing hormone (GnRH; Crighton et al 1973) or GnRH agonist generates inadequate corpora lutea (CL), and such CL have been widely used as a model to study naturally occurring inadequate luteal function.…”

“…However, the incidence of adequate luteal phases during anoestrus was increased significantly by progesterone pretreatment prior to induction of ovulation with GnRH (McLeod et al 1982b, Legan et al 1985. Progesterone prolonged the period between GnRH injection and the onset of the LH surge Haresign 1984, Southee et al 1988a), lengthening the final phase of follicular maturation, but had no effect on the onset, amplitude or duration of the LH surge (Wright et al 1983, Legan et al 1985.…”

“…A number of different mechanisms have been suggested to be responsible for the formation of inadequate luteal phases in sheep, including defective maturation of the preovulatory follicle (Keisler & Keisler 1989, Hunter et al 1986, White et al 1987, Southee et al 1988a, Khalid et al 1997, Lund et al 1999, Bartlewski et al 2001, perhaps due to an inappropriate pattern of LH stimulation of the follicle prior to the LH surge (McLeod et al 1982a, Wright et al 1983, attenuation of the LH surge (Bartlewski et al 2004), inadequate luteinization (Atkinson 1988, Hunter et al 1988, inappropriate luteal support of the new CL (Hunter et al 1988) and/or increased susceptibility to luteolytic stimuli (Hunter et al 1989, Beard & Hunter 1996 arising from the uterus (Southee et al 1988b, Hu et al 1991, though see Rahmanian and Murdoch 1987).…”

Differences in the reproductive endocrine status of ewes in the early post-partum period and during seasonal anoestrus

“…It is generally accepted that the LH pulse frequency during seasonal anoestrus is less than at any time during the oestrous cycle (Yuthasastrakosol, Palmer & Howland, 1975;Scaramuzzi & Baird, 1977) which may be a contributory cause of acyclicity (Karsch, 1980). We have previously suggested that post-partum acyclicity in ewes lambing during the mating season is also due to an inability to generate LH pulses of sufficient frequency to cause ovulation (Wright et al, 1983a;Wright, Clarke & Findlay, 1983b Stores of Gn-RH in the stalk/median eminence of post-partum ewes are similar in quantity to those in seasonally anoestrous ewes. The amount of Gn-RH in the median eminence region is probably a reliable indicator of Gn-RH available for release into the hypothalamo-pituitary portal blood since this is where most of the Gn-RH is stored (Polkowska, Dubois & Domanski, 1980).…”

Induction of plasma LH surges and normal luteal function in acyclic post-partum ewes by the pulsatile administration of LH-RH

“…Although the administration of LH-RH in a pulsatile regimen (100 ng i.v. every hour for 48 h) to acyclic postpartum ewes resulted in plasma LH surges similar to preovulatory LH surges in cyclic ewes, subsequent luteal function was deficient (Wright et al, 1983b Hormone assays. Plasma LH levels were measured in a double-antibody radioimmunoassay (Wright et al, 1980).…”