Relaxin has been described by some researchers as a "fickle and elusive hormone" (Steinetz, 2000). While much has been discovered about relaxin, there is still much work to be done. Recent research, such as the collaborative work presented at the Relaxin conference (Geoffery W et al, 2000) show that relaxin is not only a pregnancy hormone, but has a wider range of collagen-related functions in the body.
Relaxin has been implicated in connective tissue remodeling, specifically by inhibiting collagen synthesis, while promoting matrix metalloproteinases, which degrade collagen. (Samuel S et al, 1998)
One of the best ways to determine the overall function of a hormone is to knock out its expression using transgenic animals. Transgenic rats have been used to study the effects of relaxin in areas such as the heart. High affinity relaxin receptors have been found in the aorta and heart chambers of both male and female rats (Osheroff L, Ho W., 1993). One study showed that knock out mice had a 30% increase in atrial weight, and moderate increases in the weight of other organs such as the lungs and liver (Samuel S et al, 2003). Increase in atrial weight caused by collagen build up resulted in impeded blood flood into the chambers, and reduced elasticity. While the rats did not show increased blood pressure, or heart rate, it is clear that relaxin acts to remodel collagen in body tissues.
During pregnancy, Relaxin and relaxin receptors have been found in areas such as the nipple, cervix, vagina and pubic symphysis. Knockout mice have been used to show that relaxin causes an increase in the length of pubic symphysis during the second half of pregnancy. However, it was shown that relaxin deficient mice can still deliver newborns, and the duration of gestation is unaffected. Similar results exist for the cervix and vagina of female rats. These studies have shown that relaxin increases the overall length or width, but is not necessary for a successful delivery (Ling Zhao et al, 1999). Interestingly, milk secretion seems to be dependant on collagen remodelling by relaxin. Knockout mice that delivered newborn babies were unable to feed their newborns with milk. While they produced milk, development of the mammary gland was not sufficient to secrete it (Zhao et al, 2000).
From the perspective of male physiology, relaxin has been shown to affect development of the male reproductive tract. Knock out mice showed increased weight in prostate glands, testis, epididymus and seminal vesicles. This lead to decreased growth and fertility, however the males still produced viable sperm. (Samuel CS,2003)
While the effects of relaxin are present in several areas of the body, the exact purpose of the hormone has been difficult to determine, as it has effects on collagen production in many tissues, but is not normally essential to these tissues. Despite this, it is produced in a wide variety of tissues including the heart, brain, corpus luteum, endometrium and ovaries. While the role of relaxin in many body tissues remains ambiguous, the development in the mammary gland does in fact seem dependant on relaxin.
REFERENCES
Samuel C.S., Coghlan J.P., Bateman J.F. (1998) Effects of relaxin, pregnancy and parturition on collagen metabolism in the rat pubic symphysis. J Endocrinol 159:117–125
Osheroff P.L., Ho W.H. (1993) Expression of relaxin mRNA and relaxin receptors in postnatal and adult rat brains and hearts. Localization and developmental patterns. J Biol Chem 268:15193–15199.
Xiao-Jun Du, Chrishan S Samuel, Xiao-Ming Gao, Ling Zhao, Laura J Parry, Geoffry W Tregear (2003) “ Increased myocardial collagen and ventricular diastolic dysfunction in relaxin deficient mice: a gender-specific phenotype” Cardiovasc Res (2003) 57 (2): 395-404.
Ling Zhao, Peter J. Roche, Jenny M. Gunnersen, Vicki E. Hammond, Geoffrey W. Tregear, E. Marelyn Wintour and Felix Becks (2000) "Mice without a Functional Relaxin Gene Are Unable to Deliver Milk to Their Pups Endocrinology" Vol. 140, No. 1 445-453
Zhao L., Samuel C.S., Tregear G.W., Beck F., Wintour E.M. (2000) Collagen studies in late pregnant relaxin null mice. Biol Reprod 63:697–703
Geoffery W Tregear, Richard Ivell, Ross A Bathgate, John D Wade “Relaxin 2000: The proceedings of the third international conference on relaxin & related peptides” Kluwer Academic Publishers
Samuel CS, Tian H, Zhao L, Amento EP, (2003) "Relaxin is a key mediator of prostate growth and male reproductive tract developement" Lab Invest, Jul;83(7):1055-67.
