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Gillian D. Bryant and W. A. Chamley

Relaxin is clasically a hormone of pregnancy and is considered to be responsible for the relaxation of the interpubic ligament during pregnancy in small mammals (Hall, 1960). In pigs, the relaxin bioactivity in corpora lutea and blood from pregnant and post-partum animals has been measured (Anderson, Ford, Melampy & Cox, 1973), and a radioimmunoassay for porcine relaxin has been used for measurement of relaxin during pregnancy and parturition (Sherwood, Chang, Bevier & Dziuk, 1975): relaxin levels rose from 3 days before parturition and reached a peak 14 hr before birth. However, the ovaries are required throughout gestation in this species (Belt, Anderson, Cavazos & Melampy, 1971). The pregnant sheep, like women, is not dependent upon a functional corpus luteum for the whole of gestation, but no measurements of relaxin (either bioactivity or immunoactivity) have yet been reported in these species.

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Gillian D. Bryant and W. A. Chamley

Department of Anatomy & Reproductive Biology, School of Medicine, University of Hawaii, Honolulu, Hawaii 96822, U.S.A., and Reproductive Research Section, University of Melbourne, S.S. Cameron Laboratory, Werribee 3030, Australia

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WAYNE A. CHAMLEY, T. STELMASIAK and GILLIAN D. BRYANT

Summary.

Plasma relaxin immunoactivity was measured every 2 hr during 4-day periods in a series of sheep to cover the 17-day period of the ovine oestrous cycle. The immunoactivity fluctuated considerably throughout each 4-day period, and a large between-animal variation was found. A marked episodic release, occurring at approximately 12.00 and 24.00 hours, was identified and shown to occur more regularly either at certain times of the cycle or in certain animals. Relaxin immunoactivity was high throughout the late pro-oestrous phase of the cycle (Days 15 and 16), and at 24 hr after the onset of the LH peak, coincident with the approximate time when ovulation occurs. Bursts of relaxin activity were found on Days 8 to 9 in one ewe, and Days 10 to 11 and 13 to 14 in another. There was no significant correlation between prolactin levels and relaxin immunoactivity in one ewe studied throughout the oestrous period.

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J. P. McMurtry, G. L. Floersheim and Gillian D. Bryant-Greenwood

Summary. A biologically-active succinylated porcine relaxin was iodinated by a modification of the Bolton—Hunter method. Fibroblasts cultured from the mouse pubic symphysis and human skin were used to investigate relaxin binding sites. 125I-labelled relaxin binding to both cell types was time- and temperature-dependent. An accelerated rate of labelled hormone degradation (90%) was observed when both cell types were incubated at 37°C.

Specific relaxin binding sites on the mouse and human cells were observed as other peptides, such as insulin, epidermal growth factor, glucagon, hFSH and human prolactin, failed to inhibit relaxin binding. Further results indicate that porcine relaxin is mitogenic to these specific fibroblasts because increasing concentrations (10−9 to 10−6 m) of this hormone stimulated cell growth in vitro. These data suggest that the effect of relaxin at the target tissue level is mitotic in nature.

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Gillian D. Bryant, J. F. Sassin, E. D. Weitzman, S. Kapen and A. Frantz

Summary. Relaxin was secreted episodically in all 6 human subjects studied. A 24-hr rhythm was detected in the pooled data, with maximum secretion in the early–mid-morning hours and a nadir in the early evening.

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Anoma D. Gooneratne, Gillian Bryant-Greenwood, Felicity Maule Walker, Helen M. Nottage and P. E. Hartmann

Summary. The pre-partum plasma concentrations of progesterone, relaxin, PGF-2α and 13,14-dihydro-15-keto PGF-2α (PGFM) were determined in 6 sows treated with meclofenamic acid (5·0 mg/kg body wt daily, from Day 109 to Day 113 of pregnancy), and 2 sows which farrowed prematurely. The inhibition of PG synthesis with meclofenamic acid did not prevent either the decline in plasma progesterone or the release of relaxin. Three distinct stages in the progressive changes of the above hormones were apparent before parturition: (1) the gradual decline in the concentrations of progesterone from about 7 days pre partum to reach <4·5 ng/ml at farrowing; (2) a surge in relaxin levels (>100 ng/ml) by 2 days pre partum; and (3) sharp increases in the concentrations of PGF-2α and PGFM within 24 h of parturition. Maximal concentrations of PGF-2α (>0·5 ng/ml) and PGFM (>20·0 ng/ml) occurred during farrowing. It is suggested that PG from an extra-ovarian source may not be responsible for initiating functional regression of the corpora lutea in the sow.

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J. P. McMurtry, S. C. M. Kwok and Gillian D. Bryant-Greenwood

Summary. Various tissues from the mouse, rat and guinea-pig were used to examine the binding of a biologically active, esterified and 125I-labelled porcine relaxin. Binding to mouse symphysial homogenates was time- and temperature-dependent. Other peptide hormones did not compete with relaxin for binding. Mouse uterine tissue displayed similar binding characteristics. Fractionated mammary tissue from 15and 20-day-pregnant rats exhibited significant relaxin binding activity, as did homogenates of the guinea-pig pubic symphysis and cervix. Under the conditions used, no relaxin receptors were noted in the liver, spleen or heart from any of the species investigated.

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Kelly Yamasato, Pai-Jong Stacy Tsai, James Davis, Sandra Y Yamamoto and Gillian D Bryant-Greenwood

Relaxin, a systemic and placental hormone, has potential roles in fetoplacental growth. Human placenta expresses two RLN genes, RLNH1 and RLNH2. Maternal obesity is common and is associated with abnormal fetal growth. Our aims were to relate systemic and cord blood RLNH2, placental RLNs and their receptor (RXFP1) with fetoplacental growth in context of maternal body mass index, and associations with insulin-like growth factor 2 (IGF2) and vascular endothelial growth factor A (VEGFA) in the same placentas. Systemic, cord blood and placental samples were collected prior to term labor, divided by prepregnancy body mass index: underweight/normal (N = 25) and overweight/obese (N = 44). Blood RLNH2 was measured by ELISA; placental RLNH2, RLNH1, RXFP1, IGF2 and VEGFA were measured by quantitative immunohistochemistry and mRNAs were measured by quantitative reverse transcription PCR. Birthweight increased with systemic RLNH2 only in underweight/normal women (P = 0.036). Syncytiotrophoblast RLNH2 was increased in overweight/obese patients (P = 0.017) and was associated with placental weight in all subjects (P = 0.038). RLNH1 had no associations with birthweight or placental weight, but was associated with increased trophoblast and endothelial IGF2 and VEGFA, due to female fetal sex. Thus, while systemic RLNH2 may be involved in birthweight regulation in underweight/normal women, placental RLNH2 in all subjects may be involved in placental weight. A strong association of trophoblast IGF2 with birthweight and placental weight in overweight/obese women suggests its importance. However, an association of only RLNH1 with placental IGF2 and VEGFA was dependent upon female fetal sex. These results suggest that both systemic and placental RLNs may be associated with fetoplacental growth.

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Salei'a Afele, Gillian D. Bryant-Greenwood, W. A. Chamley and Elizabeth M. Dax

Summary. One sow bled at 30–60-min intervals for 48 h at 5 and 4 days before parturition had mean ± s.e.m. relaxin levels of 5·0 ± 0·48 ng/ml and 5·5 ± 0·44 ng/ml for each 24-h period respectively. This sow and another were bled at frequent intervals during parturition; both showed considerable fluctuations in their relaxin levels but no consistent peaks in relation to each birth. Mean levels during parturition were 10·7 ± 0·46 ng/ml and 13·4 ± 0·81 ng/ml respectively, both significantly higher than the levels at 4 and 5 days before birth.

Relaxin levels in two lactating sows rose acutely during nursing, showing a 3-fold rise in one animal and an 8-fold rise in the other. Results from a third sow during an extended period of nuzzling and sucking by the piglets showed multiple peaks of relaxin immunoactivity associated with each nuzzling/sucking stimulus.