The physiological effects of relaxin during pregnancy: studies in rats and pigs

For 50 years after its discovery in 1926, there was a general lack of interest in relaxin among both reproductive biologists and clinicians. A key reason for this lack of interest was the lack of information concerning relaxin's physiological importance during pregnancy in any species. Research conducted since the early 1980s has established that the hormone relaxin is essential during pregnancy in at least two species--rats and pigs. Two vital roles for relaxin during pregnancy have been identified. Relaxin promotes growth and softening of the uterine cervix and thereby enables rapid and safe delivery in both rats and pigs. Relaxin also promotes growth and development of the mammary apparatus in both species. Interestingly, the major effects of relaxin on mammary growth and development are targeted on the nipple in the rat, whereas they are targeted on the glandular parenchyma in the pig. Relaxin-dependent growth of the nipple in rats is required for normal lactational performance. Although likely, it remains to be established that relaxin's profound effects upon mammary gland development in pigs are required for normal lactational performance. The fact that relaxin has effects upon cervical and mammary gland development during pregnancy in both rats and pigs encourages the view that relaxin may have similar effects during pregnancy in other species. Nevertheless, one must keep in mind that there is great diversity in the physiology of relaxin among species (reviewed by Sherwood 1988). This diversity includes not only relaxin's source, regulation of synthesis and secretion, and secretory profiles during pregnancy, but also its biological effects. It seems essentially certain that relaxin's effects during pregnancy differ among species. For example, transformation of the pubic joint cartilage to a flexible and elastic interpubic ligament occurs during pregnancy in several species, including guinea pigs, mice, and bats. This pelvic adaptation, which is nearly certainly relaxin dependent, does not occur in species such as rats and sheep. It is possible that relaxin may have little or no physiological significance during pregnancy in some species. Although considerable progress has been made toward an understanding of the physiological role(s) of relaxin in pregnant rats and pigs, many fundamental questions remain unanswered.(ABSTRACT TRUNCATED AT 400 WORDS)