Given current controversies regarding anti- and pro-inflammatory effects of estrogen, there is a need to explore relationships between gonadal hormones and inflammation using appropriate animal models. It has been proposed that rats are not appropriate for such research since, contrary to the effect of estrogen in humans, earlier animal studies had reported that estrogen downregulates serum C-reactive protein (rCRP) levels in the rat. With these considerations in mind, we re-examined the effects of estrogen withdrawal and replacement on CRP expression and complement activation in the rat. F-344 rats underwent bilateral ovariectomy or sham surgery at 9-10 months of age. Four months later, ovariectomized rats were treated with traditional high-dose 17beta-estradiol (Hi-E2) capsules, lower-dose (Lo-E2) 17beta-estradiol capsules, or placebo capsules for 7 days prior to sacrifice. Levels of plasma rat C-reactive protein (rCRP) were significantly lower in ovariectomized vs. sham-operated animals (415.5 +/- 10.6 vs. 626.6 +/- 23.0 mg/L, p < 0.001). Estrogen replacement significantly raised rCRP levels in ovariectomized animals (690.0 +/- 28.0 mg/L in Lo-E2 and 735.5 +/- 35.8 mg/L in Hi-E2, respectively, p < 0.001). Plasma rCRP levels correlated significantly with both hepatic rCRP (r = 0.79, p < 0.001) and serum estradiol (r = 0.70, p < 0.001) levels. However, no significant differences were observed in indices of complement activation (C4b/c) or CRP-complement complex generation (rCRP-C3 complex). In the mature female rat, ovariectomy reduces and estrogen replacement raises rCRP. Effects of estrogen on plasma rCRP induction are mediated, at least in part, through hepatic mechanisms and do not appear to require or be associated with complement activation.

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