SOURCES AND BIOLOGICAL ACTIONS OF RELAXIN IN PIGS

Although the major source of relaxin in pigs is the corpus luteum of p regnancy, there is now evidence for relaxin gene expression and transl ation into protein in the theca interna cells of the preovulatory foll icle, the corpus luteum of the cycle and the uterus. The theca interna cells retain their ability to express the relaxin gene and protein fo llowing ovulation. During the early stages of development of the corpu s luteum, the theca-derived small lutein cells are the source of the r elaxin transcript. As the corpus luteum becomes fully functional, ther e is a switch in the site of relaxin synthesis h om small theca-derive d lutein cells to large granulosa-derived cells. In the absence of lut eolysis, this switch is accompanied by a dramatic rise in relaxin synt hesis. Relaxin has been identified in boar seminal plasma and can main tain or increase sperm motility. However, a source of relaxin in the b oar has not been identified. Relaxin is an important regulator of uter ine function during pregnancy acting systemically to suppress myometri al activity and promote cervical dilation at parturition. The changes in thecal relaxin production during follicle development and its abili ty to promote growth and changes in proteolytic enzyme activity of gra nulosa cells in vitro have led to the concept of: an autocrine or para crine role for relaxin within the follicle. Uterotrophic effects of re laxin have been reported in rodents and swine and support the hypothes is that relaxin promotes uterine growth and expansion in early pregnan cy to accommodate the growing fetuses. Mammotrophic effects of relaxin in rodents have now been extended to pigs, with evidence that relaxin is necessary for normal mammary parenchymal development in late pregn ancy. In most instances the mechanisms responsible for, and the physio logical significance of, these diverse biological effects remain to be elucidated.