- Changes in size and function during pregnancy are unique to the uterine artery. The aim of this study was to determine the interleukin (IL)-6 activity of the uterine artery wall tissue in pregnant rats. A total of 18 Charles River white rats (nine virgin and nine in midpregnancy) were used for the study. Bilateral uterine arteries were obtained, together with reference tissues from aorta and uterus. IL-6 production was measured as optical density (OD)/mg protein, in control culture media, and in the presence of stimulants including IL-1, tumour necrosis factor alpha and lipopolysaccharide. Polyclonal rabbit anti-human IL-6 antibodies were used to assess IL-6 activity. In control culture medium, uterine artery tissue samples from virgin rats produced significantly higher concentrations of IL-6 than samples obtained from pregnancy animals (1.8 +/- 0.3 versus 0.9 +/- 0.25 OD/mg protein respectively (mean +/- SE, P = 0.001). Stimulation by lipopolysaccharide increased IL-6 activity of the uterine artery wall. In comparison with the uterine artery, the aorta produced higher activities of IL-6, and its production in virgin animal samples was higher than during pregnancy. Stimulants increased IL-6 production by both aorta and uterus tissues. Neutralization of IL-6 activity was obtained in a range of 77-93% in all samples. The lower level of IL-6 activity during pregnancy in the uterine artery and in reference tissues including aorta and uterus, may be related to acceptance of pregnancy by maternal tissues.