Proinflammatory role for AT₁receptors in the rat synovium

Experimental arthritis was induced under anaesthesia (halothane/O₂/N₂O) by intra-articular and peri-articular injection of Freund's complete adjuvant in adult male Wistar rats (350 ± 20 g body weight, mean ± SEM, n = 6). Knee joint swelling was measured using callipers and expressed as the percentage change from pre-induction values. The anti-inflammatory potential of losartan (Merck & Co., USA) was tested both prophylactically and therapeutically. Prophylactically, losartan (15 mg/kg; subcutaneous) was given 1 hour prior to induction of inflammation and every 48 hours thereafter. Therapeutically, 12 days after the induction of inflammation, losartan (15 mg/kg; subcutaneous) treatment was started and dosing continued every 48 hours. The mechanism of action of losartan was investigated using EXP 3174 (Merck & Co.), given prophylactically (15 mg/kg; subcutaneous, maintenance every 48 hours) 1 hour prior to induction of inflammation (Freund's complete adjuvant).

U937 human monocytes at a logarithmic phase of growth were harvested and adjusted to 10⁶ cells/ml in the presence of 10 nM phorbol 12-myristate 13-acetate for 3 days to differentiate into adherent macrophages. Cells were then quiesced for 24 hours before experiments commenced. Cells were then stimulated with lipopolysaccharide (50 ng/ml) incubated in the presence of medium (control), Ang II (10^-9 mol) or losartan (10^-9 mol to 10^-6 mol). Supernatants were collected and tumour necrosis factor alpha (TNF-α) levels measured by ELISA.

Prophylactic administration of losartan resulted in a significant reduction in knee joint swelling over 14 days (two-way analysis of variance [ANOVA], P < 0.00001; n = 6). Therapeutic losartan treatment reduced established knee joint swelling compared with vehicle control (two-way ANOVA, P < 0.00001; n = 7). Prophylactic administration of EXP-3174 also resulted in a reduced percentage increase in knee joint diameter, again found to be significant (two-way ANOVA, P < 0.00001; n = 6–8). Losartan inhibited TNF-α production from U937 cells in a dose-dependent manner (cf. LPS-treated cells; n = 3).

The anti-inflammatory benefit of both prophylactic and therapeutic losartan treatment suggests both an important role for Ang II in arthritis, and a novel therapeutic application for AT1 receptor antagonists. This anti-inflammatory action may be due to inhibition of TNF-α release from macrophages.