Proinflammatory role for AT₁receptors in the rat synovium

The increasing recognition that angiotensin II (Ang II) has proinflammatory roles suggests it may therefore contribute to chronic inflammatory conditions such as rheumatoid arthritis (RA), but this requires investigation. Walsh and colleagues demonstrated the presence and upregulation of AT₁ receptors [1] and angiotensin-converting enzyme [2] in the synovium of RA patients. The aim of this study was to elucidate the role of Ang II in rat knee joint pathophysiology and to investigate the anti-inflammatory potential of AT₁ receptor antagonist losartan in models of arthritis. The second phase of this study was to determine the mechanism of any anti-inflammatory action of losartan. In vivo losartan has been reported to undergo hepatic first-pass metabolism [3], yielding two metabolites, EXP-3174 (the active metabolite) and EXP-3179. The latter is structurally similar to the non-selective cyclooxygenase inhibitor indomethacin. Since the anti-inflammatory benefit of cyclooxygenase inhibitors is well documented, we aimed to determine whether any anti-inflammatory benefit associated with losartan was attributable to AT₁ receptor antagonism or inhibitory action on the prostanoid system.

In vivoexperiments

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).

In vitroexperiments

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.