Secretion of a gonadotrophin-releasing hormone- (GnRH-)like factor by the rabbit fetal placenta in vitro

Previous studies have shown that the rabbit fetal placenta contains both biological and immunological gonadotrophin-releasing hormone- (GnRH-) like activity. The purposes of this study were to determine whether (1) the fetal placenta secretes a GnRH-like factor, and (2) this GnRH-like factor is secreted into the uterine vein. In experiment 1, fetal placentae were collected from day-18 pregnant rabbits. The placentae were divided in half, and one-half was used for measurement of GnRH; the other half was cut into small pieces and placed into a perifusion system. Following a 2-h stabilization period, tissue was challenged with two 60 mm K⁺ injections spaced 90 min apart. Six replicates were run with either liver (n = 4) or spleen (n = 2) as control tissues. Basal levels of GnRH secreted (nmol/g protein/30 min) were 1.2 ± 0.3 for placental tissue, 0.3 ± 0.1 for liver tissue and non-detectable for spleen tissue. Following the first K⁺ injection there was no response from liver or spleen tissue, while placental tissue showed a 78 per cent increase after 90 min. There was no response to the second injection. The concentration of GnRH in the placenta was 6.8 ± 0.5 nmol/g protein. In experiment 2, plasma samples were obtained from the marginal ear vein of five oestrous does and five day-7 pseudopregnant does, and from the marginal ear vein and uterine vein of six day-18 pregnant does. These samples were assayed for GnRH and found to contain 7.1 ± 1.2, 6.3 ± 2.1, 10.2 ± 2.5 and 9.8 ± 2.2 pmol/l, respectively. These data indicate (1) that the rabbit fetal placenta secretes an immunological GnRH-like factor in vitro, and (2) that this GnRH-like factor is not secreted into the uterine vein.