Peptides interact in gonadotrophin regulation

The regulation of luteinizing hormone (LH) activity is vital to normal reproductive functioning of the female. Although gonadotrophin-releasing hormone (GnRH) has a prominent role in the regulation of LH it is now believed that other peptides are also involved. Among these peptides is oxytocin. The addition of oxytocin to cultures of pituitary cells from female rats elicited a concentration-dependent secretion of LH. This secretion was enhanced in an oestrogenised environment and was inhibited by progesterone and testosterone. Oxytocin administered to female rats at pro-oestrus advanced the endogenous LH surge that occurs on the evening of pro-oestrus. Conversely oxytocin receptor antagonist suppressed the production of the LH surge in a dose-dependent manner, indicating that endogenous oxytocin is a crucial component of LH regulation. In the human female, oxytocin administered during the late follicular phase advanced the onset of the midcycle LH surge. Oxytocin added to rat pituitary cells in vitro induced LH synthesis. Furthermore rats administered oxytocin on pro-oestrus had higher LH pituitary content following development of the LH surge than did rats administered saline. Thus oxytocin promoted synthesis and replacement in the pituitary of LH released into the circulation. Incubation of pituitary pieces with oxytocin plus GnRH induced secretion of amounts of LH greater than the sum of the amounts released by oxytocin and GnRH separately. Additionally the increased LH levels observed in the peripheral circulation of pentobarbitone-anaesthetised rats administered GnRH were enhanced if the rats received oxytocin prior to the GnRH. Thus oxytocin synergised with GnRH in stimulating LH release. Addition of diBucAMP reduced the oxytocin-mediated augmentation and dideoxyadenosine enhanced the augmentation, suggesting that oxytocin worked most efficiently in a milieu low in cAMP activity. The use of a cell immunoblot assay revealed that individual cells responded differently to oxytocin and to GnRH and that the two peptides could act on the same cell. Perifusion studies performed on hemipituitaries demonstrated that a LH response could be determined by the presence of three peptides, oxytocin, neuropeptide Y and GnRH. Hence oxytocin is potentially involved also in multiple interactions during the process of LH regulation. LH regulation is therefore apparently the result of a community of peptides acting in a co-operative network.     

Response of mouse ovaries in vivo and in organ culture to pregnant mare's serum gonadotrophin and human chorionic gonadotrophin

The effect of various doses of pregnant mare's serum gonadotropin (PMSG) and human chorionic gonadotropin (HCG) on the yield of oocytes undergoing preovulatory maturation in organ culture was studied in mice. The mice received ip injections of .5, 1, 3, or 5 IU PMSG and 48 hours later, 0, 1, 2, or 4 IU HCC or their ovaries were cultured with 0, .2, .4, or .8 IU HCG/ml. 3-5 IU PMSG by 1-2 IU HCG induced maximum preovulatory response both in vivo and in culture. The proportion of large antral follicles with oocytes at Metaphase 2 was lower in culture than in the intact animal. However, the number of preantral and small Graafian follicles increased to a higher level in vivo and thus the overall total number of oocytes that progressed beyond the germinal vesicle stage was higher in organ culture.     

Morphine, naloxone and the gonadotrophin surge in ewes

Possible endogenous opioid peptide regulation of the preovulatory gonadotrophin surge was examined in ewes during the breeding season. Intact ewes (n = 54) were synchronized by treatment for 12 days with intravaginal sponges releasing medroxyprogesterone acetate. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion prior to and during the gonadotrophin surge were not affected by naloxone (0.33 mg/kg body wt per h) administered from the time of medroxyprogesterone acetate withdrawal until 30 h after the onset of oestrus (n = 6). Morphine was administered in 4 patterns: (i) 0.25 mg morphine/kg body wt per h from medroxy-progesterone acetate withdrawal until 30 h after the onset of oestrus (n = 6), (ii) 0.25 mg morphine/kg body wt per h from 24 to 48 h after medroxyprogesterone acetate withdrawal (n = 6), (iii) 0.50 mg morphine/kg body wt per h from 24 to 36 h after medroxyprogesterone acetate withdrawal (n = 6) and (iv) 0.50 mg morphine/kg body wt per h from 18 to 30 h after medroxyprogesterone acetate withdrawal (n = 6). Oestrus and the gonadotrophin surge were delayed, but not blocked, in all cases of morphine administration (P less than 0.05). Inconsistent effects of morphine on circulating oestradiol and gonadotrophin concentrations prior to the gonadotrophin surge suggest that the delays are not due to reduced gonadotrophic support of ovarian oestradiol output. Morphine may reduce responsiveness of central behavioural and gonadotrophin surge-generating centres to the oestradiol signal. The absence of effects of naloxone on gonadotrophin secretion suggest that suppression of LH secretion by opioid peptide activity is reduced after the end of the luteal phase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Purification and characterization of guinea-pig chorionic gonadotrophin

A human chorionic gonadotrophin-like protein (GF-1, 1.0 g) from the placentae of 50 guinea-pigs killed at Day 26 of gestation was purified by pH and ammonium salt fractionation followed by column chromatography on DEAE-Sephadex and filtration on Sephadex G-100. Relative to the Second International hCG standard (MRC 61/6) GF-1 had an immunological potency of 21 000 i.u./mg as measured in a specific hCG-beta radioimmunoassay and, using the ovarian ascorbic acid depletion assay, an apparent biological potency of 24 064 i.u./mg. Isoelectric focusing yielded 6 bands between pH 4.4 and 5.7 and the material comprised two non-covalently linked subunits. The Stokes' radii were 3.40 nm for the native preparation, and 2.38 nm and 3.15 nm for GF-1-alpha and GF-1-beta subunits respectively. The guinea-pig placenta therefore produces a chorionic gonadotrophin which on purification has physicochemical, biological and immunological properties similar to those of hCG.     

Purification and properties of baboon chorionic gonadotrophin

Baboon CG from the urine and placenta (Days 33-45 of gestation) was purified by ammonium acetate/alcohol extraction followed by ionic exchange, gel filtration and affinity chromatography. The CG activity was monitored using a double-antibody radioimmunoassay utilizing anti-baboon CG and hCG both as the labelled ligand and standard. Biological activity was measured by the rat luteal cell radioreceptor assay. The purified preparation exhibited heterogeneity in terms of its behaviour during ionic exchange chromatography and isoelectric focussing. Like hCG, baboon CG was made up of two non-covalently linked subunits: the Stokes' radii were 36.5, 29.0 and 22.0 X 10(-10) m for native CG, the beta subunit and the alpha subunit. The material focussed between pH 4.2 and 5.5. Relative to the second international hCG standard the biological potency of the purified urinary baboon CG was 4058 i.u. whilst the immunological activity was 4364 i.u. It is concluded that baboon and human CG are very similar with respect to their physicochemical properties and that baboon CG can be purified by the methods that have been developed for hCG.     

Use of equine chorionic gonadotrophin in female mink

This study was comprised of three trials to determine the effects of equine chrionic gonadotrophin (eCG) on induction of sexual receptivity in female mink that had failed to mate by late in the breeding season. In the first trial one ovary was removed from unmated mink, which were then injected with 100 IU eCG. This treatment induced ovarian activity, including ovulation in the remaining ovary. In the second experiment, mink that had not been observed to mate were treated with 100 IU eCG or saline, resulting in mating of 10 11 of the eCG-treated animals, compared to 5 11 controls. Litter sizes were larger in mink in the control group, suggesting that eCG interfered with some phase of the reproductive process. In the third trial, 226 mink that had failed to mate until late in the breeding season were treated with 100 IU eCG. Of the 191 that subsequently mated, 99 produced litters, but litter sizes were reduced slightly from those observed in the remainder of the herd that bred without hormone treatment prior to March 20. Neonatal kit loss per female whelping was greater in mink treated with eCG. It is concluded that eCG treatment will induce mating in mink that refuse to mate, but this treatment results in reduced whelping success and greater neonatal kit loss. Its utility may be restricted to salvage situations where large numbers of mink fail to mate.     

Evidence that gonadotrophin surge-attenuating factor exists in man

This review article summarizes the evidence provided by in-vivo and in-vitro studies suggesting that the human ovary produces a nonsteroidal factor distinct from inhibin which participates in the control of gonadotrophin secretion from the pituitary. This factor has been called gonadotrophin surge-attenuating factor (GnSAF) and is defined as attenuating the endogenous surge in luteinizing hormone (LH) in superovulated women by reducing the pituitary response to LH-releasing hormone. In-vivo bioactivity of GnSAF has been detected during the follicular phase of superovulated cycles; in-vitro studies have shown activity of this factor in human follicular fluid. From a physiological point of view, a hypothesis is proposed that GnSAF attenuates the amplitude of the positive effect of oestradiol on gonadotrophin secretion during the follicular phase of the human menstrual cycle and therefore plays an important role in controlling ovulation. If GnSAF is isolated, it may have several clinical applications including contraception.     

Indomethacin and gonadotrophin release from rat pituitaries.

The influence of low (5 mcM) and high (200 mcM) concentrations of indomethacin on gonadotropin release from rat pituitaries was studied in vitro. Low concentrations significantly (p less than .05) reduced the release of luteinizing hormone (LH) in comparison with controls, whereas high concentrations significantly (p less than .05) increased the rate of release. The release of follicle stimulating hormone (FSH) was not affected. When pituitary tissue was stimulated by LH-releasing hormone (LH-RH), the high concentration of indomethacin significantly (p less than .05) increased LH release and produced a nearly significant (p less than .01) increase in FSH. The low concentration was without effect. The effect of the addition of prostaglandins (PGs) alone and in combination with indomethacin was also investigated. PGE-1 significantly (p less than .05) increased the release of LH. However, there was no significant (p greater than .1) interaction between the 2 drugs. The effects on the release of FSH were similar. The addition of PGE-2 or PGF-2alpha slightly increased the release of LH and FSH (p greater than .1). It is suggested that high concentrations of indomethacin probably do not inhibit PG synthesis, but may inhibit cyclic nucleotide phosphodiesterase.