RELAXIN AND RELATED PEPTIDES: FIFTH INTERNATIONAL CONFERENCE

Conceptus Numbers Do Not Affect BloodConcentrations of Relaxin in the Rabbit

Phillip Fieldsa and Michael Fieldsb

aDepartment of Cell Biology and Neuroscience, University of South Alabama College ofMedicine, Mobile, Alabama 36688, USA

bAnimal Sciences Department, University of Florida, Gainesville, Florida 32611, USA

On day 18 of pregnancy, conceptuses were surgically removed in order that rabbits wouldbe left with seven, five, three, or one conceptus. During pregnancy, blood concentrationsof relaxin were determined by radioimmunoassay. There was no difference betweenrelaxin concentrations in rabbits carrying one versus seven conceptuses. The resultsindicate that conceptus number does not influence the blood concentration of relaxin inthe rabbit.

Key words: rabbit; relaxin; conceptus

Introduction

Bioactive relaxin was purified from rab-bit placentas and localized in pregnant rab-bit uterus and placenta.1,2 The ovary is not asource of relaxin in the rabbit.2–4 Blood concen-trations of relaxin increase to a peak by midges-tation and decline following parturition.3

Relaxin mRNA was detected in the rabbitplacenta and uterus, and the gene productsequence was identical to SQ10.4 SQ10 is aprotein secreted by rabbit tracheobronchial ep-ithelial cells in response to trauma and is in-volved in the repair process.5 The amino acidsequence of relaxin from the rabbit placenta isidentical to SQ10.6 These results suggest thatSQ10 is rabbit relaxin. The current study eval-uated the effects of conceptus number on theblood concentration of relaxin in the rabbit.

Methods

On day 18 of pregnancy, rabbits were anes-thetized, the abdominal cavity opened, and

Address for correspondence: Phillip Fields, University of South Al-abama College of Medicine, Department of Cell Biology and Neuro-science, 307 University Blvd., MSB 1201, Mobile, AL 36688. Voice:+251-460-6766; fax: +251-460-6771. pfields@usouthal.edu

conceptuses removed in order to limit the num-ber of implantation sites to seven, five, three,or one. Blood was collected from an ear veinon days 5, 10, 15, 20, 25, 30, 32, and 35of pregnancy. Relaxin concentrations were de-termined by a heterologous relaxin radioim-munoassay using porcine relaxin antiserum aspreviously reported.3 Repeated-measurementsanalysis was used to regress relaxin concen-tration against experimental group, days ofpregnancy, and interactions of the two (SASInstitute, Cary, NC, USA).

Results and Discussion

Relaxin has been localized in the syncytiotro-phoblast of the rabbit placenta.1 Therefore, itis assumed that placental relaxin is secreteddirectly into the maternal circulation. Unex-pectedly, there was no significant difference inthe blood concentrations of relaxin in rabbitscarrying one versus seven conceptuses. Relaxinhas been localized in the endometrium of preg-nant and pseudopregnant rabbits.2 In addition,bioactive relaxin has been reported in the bloodof pseudopregnant rabbits.7 Since there is noplacental tissue in pseudopregnant rabbits, thisbioactive relaxin would have originated from

Relaxin and Related Peptides: Fifth International Conference: Ann. N.Y. Acad. Sci. 1160: 186–187 (2009).doi: 10.1111/j.1749-6632.2008.03811.x C© 2009 New York Academy of Sciences.

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Fields & Fields: Conceptus Numbers and Rabbit Relaxin 187

the uterus. In those rabbits with fewer im-plantation sites (1 vs. 7), the additional surfacearea of the endometrium may have contributedto maintaining the blood concentrations ofrelaxin.

Conflicts of Interest

The authors declare no conflicts of interest.

References

1. Eldridge, R.K. & P.A. Fields. 1985. Rabbit placentalrelaxin: Purification and immunocytochemical local-ization. Endocrinology 117: 2512–2519.

2. Lee, V.H. & P.A. Fields. 1990. Rabbit endometrial re-laxin: Immunohistochemical localization during pre-

implantation, pregnancy and lactation. Biol. Reprod.

42: 737–745.3. Lee, V.H. & P.A. Fields. 1991. Rabbit relaxin: Influ-

ence of pregnancy and ovariectomy during pregnancyon the plasma profile. Biol. Reprod. 45: 209–214.

4. Fields, P., S. Kondo, L. Tashima, et al. 1995. Ex-pression of SQ10 (a preprorelaxin-like gene) in thepregnant rabbit placenta and uterus. Biol. Reprod. 53:1139–1145.

5. Jetten, A.M., S.H. Bernacki, E.E. Floyd, et al. 1992.Expression of a preprorelaxin-like gene during squa-mous differentiation of rabbit tracheabronchial ep-ithelial cells and its suppression by retinoic acid. Cell

Growth Diff. 3: 549–556.6. Fields, P.A., V.H. Lee, A. Jetten, et al. 1999. B-chain se-

quence and in situ hybridization of the rabbit placentalrelaxin-like gene product. Biol. Reprod. 61: 527–532.

7. Zarrow, M.X. & B. Rosenberg. 1953. Sources of re-laxin in the rabbit. Endocrinology 53: 593–598.