Corticotropin releasing hormone-binding protein (CRH-BP): plasma levels decrease during the third trimester of normal human pregnancy.

In pregnancy, maternal plasma corticotropin releasing hormone (CRH) concentrations rise substantially in the third trimester and fall rapidly post-partum. A binding protein (BP) specific for CRH exists in the human circulation which inactivates CRH, thus possibly explaining why maternal ACTH does not rise outside normal limits throughout gestation. We here describe the measurement of CRH-BP directly in plasma during human pregnancy using a radioimmunoassay that is not affected by the presence of the high plasma levels of CRH that occur at this time. In 119 healthy non-pregnant individuals, mean CRH-BP levels were 4.46 nmol/L +/- 1.0 (SD), with a wide range of 1.81-7.24 nmol/L. Plasma CRH-BP in 34 pregnant women randomly sampled during the first and second trimesters also averaged 4.46 nmol/L +/- 1.54, with individual values ranging from 1.59-7.51 nmol/L and there was no correlation of CRH-BP levels with gestational age. In a group of 14 women sampled sequentially throughout the third trimester, plasma CRH-BP averaged 4.56 nmol/L +/- 1.70 at 30-35 weeks gestation and fell dramatically to 1.84 nmol/L +/- 0.43 at weeks 38-40 (P < 0.001). The post partum recovery in CRH-BP levels occurred within 48 hours of delivery. These results indicate that there is an increase in the availability of free, potentially bioactive CRH at term to stimulate the release of ACTH from the maternal pituitary and/or to act at a peripheral, non-pituitary CRH receptor(s).

Plasma measurements of corticotrophin-releasing hormone-binding protein in normal and abnormal human pregnancy.

Using corticotrophin-releasing hormone-binding protein (CRHBP) purified from human plasma and a 25 amino acid peptide corresponding to the C-terminus of CRHBP we have been able to produce rabbit polyclonal antisera specific for CRHBP. This has allowed the development of a radioimmunoassay which is able to detect CRHBP specifically in human plasma regardless of the presence of endogenous CRH. We have used this assay to estimate the level of CRHBP in non-pregnant human plasma to be approximately 20 nmol/l with a range of 9.1-40.6 nmol/l. We have also examined sequential plasma samples taken from 84 normal pregnant women at fortnightly intervals from 16 weeks gestation through to term. Four women were also sampled during labour and the first week postpartum. The median plasma level of CRHBP at week 16 of normal pregnancy was 21.59 nmol/l, levels rose slightly during the early part of the third trimester (26.76 nmol/l at week 30, (P < 0.01) and fell markedly towards term (19.72 nmol/l, P < 0.01) with only 8.70 nmol/l at labour. CRHBP levels returned to normal non-pregnant levels within 48 h of parturition suggesting a role for the fetoplacental unit in CRHBP production. In eight pregnancies complicated by diabetes, CRHBP levels at each gestational age were similar to those recorded for normal pregnancy. However, in pregnancies complicated by pre-term labour (n = 9) and pre-eclampsia (n = 7), plasma CRHBP levels were significantly reduced (P < 0.01).

Corticotrophin-releasing hormone-binding protein in human fetal plasma.

During pregnancy maternal plasma corticotrophin-releasing hormone (CRH) levels rise 1000-fold whilst fetal plasma levels are often 100-fold higher than the concentrations seen in normal non-pregnant human plasma. Despite these high CRH levels neither the maternal nor fetal pituitary releases excessive amounts of ACTH. A specific CRH-binding protein (CRHBP) exists in the maternal circulation which is able to bind and inactivate the ACTH releasing activity of CRH. In this study we have used a specific CRHBP radioimmunoassay to determine the level of CRHBP in fetal and maternal plasma samples. Fetal samples were collected by cordocentesis between 20 and 33 weeks gestation and matched maternal samples were taken by venepuncture at the same time. In a second study, plasma samples were collected from 8 women at fortnightly intervals from week 20 to term, at labour and post-partum. A fetal sample, taken from the umbilical vein, was collected immediately post-delivery. The mean maternal CRHBP concentration for the samples collected between 20 and 33 weeks (n = 23) was 8.12 nmol/l and the fetal level was 8.62 nmol/l. Data from the second study showed that at term the maternal CRHBP concentration decreased significantly (P < 0.025) to 6.32 nmol/l. The fetal CRHBP level also decreased significantly (P < 0.001) at term to a level of 5.84 nmol/l. The CRHBP in both fetal and maternal plasma was shown to be functional by 125I-CRH binding and gel permeation chromatography. The capacity of maternal and fetal plasma to bind 125I-CRH decreased at term in agreement with the quantitation of plasma CRHBP by radioimmunoassay.

Corticotrophin-releasing hormone and corticotrophin-releasing hormone binding protein in normal and pre-eclamptic human pregnancies.

OBJECTIVE: To measure the plasma levels of corticotrophin-releasing hormone and corticotrophin-releasing hormone binding protein in normal pregnancy and in pregnancies complicated by pre-eclampsia. SETTING: John Radcliffe Hospital, Oxford and St Thomas's Hospital, London. SUBJECTS: One hundred and twenty pregnant women sampled prospectively throughout gestation, of whom 91 experienced a normal pregnancy and eight developed pre-eclampsia; in a second study, 10 women with severe pre-eclampsia, presenting at a range of gestational ages, were sampled once and compared with appropriately matched normal pregnant women. MAIN OUTCOME MEASURE: Plasma levels of corticotrophin-releasing hormone determined by immunoradiometric assay. Plasma levels of corticotrophin-releasing hormone binding protein measured by direct radioimmunoassay. RESULTS: In the prospective study, plasma samples from women with pre-eclampsia exhibited higher (390.2 versus 292.7 pmol/l at 36 weeks) levels of corticotrophin-releasing hormone and significantly lower (5.24 versus 8.14 nmol/l at 36 weeks, P < 0.002) levels of corticotrophin-releasing hormone binding protein than normal controls. In the second, single time point study a significant elevation in CRH (P < 0.002) and reduction in CRH-BP (P < 0.001) was found in pre-eclamptic pregnancies compared with controls. CONCLUSIONS: In human pregnancies complicated by pre-eclampsia there is an elevated level of corticotrophin releasing hormone whilst there is less corticotrophin-releasing hormone binding protein; therefore there is a net increase in free potentially bioactive hormone which may play a role in the pathology of the disease.