EVIDENCE FOR A DUAL SOURCE OF RELAXIN IN THE PREGNANT RAT - IMMUNOLOCALIZATION IN THE CORPORA-LUTEA AND ENDOMETRIUM

Uterine tissue was collected from intact pregnant rats on days 4, 10-15, 18, and 22 of pregnancy and day 2 postpartum and from rats on day 22 of pregnancy after ovariectomy (day 9) and progesterone-estrogen treatment. Placental, cervical, mammary, and pituitary tissues were collected from intact pregnant rats on day 22. Ovarian tissue was collected from intact pregnant rats on days 15, 20, and 22. These tissues were evaluated for relaxin immunostaining at the light microscope level using an antiserum to purified rat relaxin and the avidin biotin immunostaining technique. Since the corpus luteum is the major source of relaxin in the rat, this tissue was used as a positive control. Relaxin immunostaining in the corpus luteum was observed in a discrete region of the cytoplasm. This pattern of staining corresponded with the discrete clustering of relaxin-containing secretory granules found in the rat luteal cells. Relaxin was not observed in the day 22 placenta, cervix, mammary gland, pituitary gland, or day 4-13 pregnant uterus. Relaxin immunostaining was detected in endometrial epithelial cells on days 14-22 of pregnancy and day 2 postpartum. Relaxin immunostaining was more evident at implantation sites than between implantation sites. Antimesometrial epithelial cells at implantation sites contained relaxin immunostaining. The mesometrial surface, which consists of the placenta and decidualized endometrium, did not contain relaxin immunostaining. Relaxin immunostaining was also present in the endometrial epithelial cells of the day 22 pregnant rats that had been ovariectomized on day 9 and given subsequent replacement therapy with ovarian steroids. The presence of relaxin immunostaining in the ovariectomized rat indicates the endometrium is not sequestering relaxin secreted from the corpus luteum. These data provide evidence that the pregnant rat endometrium is a source of relaxin in addition to the corpus luteum. The appearance of relaxin in endometrial epithelial cells after implantation and its prominence at implantation sites may indicate that locally secreted conceptus factors stimulate endometrial relaxin synthesis.