Divergent roles of nitrergic and prostanoid pathways in chronic joint inflammation

Abstract

Background: Nitrergic and prostanoid pathways have both been implicated in inflammatory processes.

Objective: To investigate their respective contributions in a rat model of chronic arthritis.

Methods: Male Wistar rats (n = 4-6/group) received either an intra-articular injection of 2% carrageenan/4% kaolin (C/K) or intra- and periarticular injections of Freund's complete adjuvant (FCA; 10 mg/ml M tuberculosis). Joint diameter, urinary nitric oxide metabolites (NO(x)), and prostaglandin E(2) (PGE(2)) levels were measured as indices of the inflammatory process. A prophylactic and therapeutic (day 5) dose ranging study of an inducible nitric oxide synthase inhibitor, L-N-(1-iminoethyl)-lysine (L-NIL), and a cyclo-oxygenase-2 (COX-2) inhibitor, SC-236, was performed with the drugs given subcutaneously. Submaximal doses were identified and used for combination studies. Appropriate vehicle controls were included.

Results: L-NIL and SC-236 dose dependently inhibited C/K induced acute joint swelling, the magnitude being greatest when they were given in combination. Both prophylactic and therapeutic administration of SC-236 in the FCA induced model of chronic arthritis produced a dose dependent reduction in all the measures assessed. However, although L-NIL demonstrated similar dose dependent inhibition of urinary NO(x) and PGE(2) levels, joint swelling was significantly exacerbated in this model. Co-administration of the inhibitors nullified the benefits of SC-236.

Conclusion: Whereas COX-2 derived prostaglandins are proinflammatory in both acute and chronic joint inflammation, NO seems to have divergent roles, being anti-inflammatory in chronic and proinflammatory in acute joint inflammation.

Figure 1

Acute inflammation: knee joint swelling (24 hours) induced by intra-articular injection of carrageenan and kaolin, expressed as a percentage change in knee joint diameter compared with preinjection values. L-NIL and SC-236 given in combination resulted in greater inhibition of knee joint swelling than either drug alone (p<0.01, two way ANOVA).

Figure 2

Chronic inflammation—prophylactic administration: (A) knee joint swelling; (B) urinary NO_x; and (C) PGE₂, expressed as a percentage change compared with preinjection values in response to intra-articular injection of FCA. There was significant joint swelling and increased NO_x and PGE₂ production in the vehicle, L-NIL (43 mg/kg), and SC-236 (5.6 mg/kg) prophylactically treated groups. A non-inflamed, untreated group is included for comparison. Values represent means (SEM).

Figure 3

Chronic inflammation—histology: (A) normal (untreated) knee joint; (B) 10 days after induction of FCA in SC-236 treated animals; (C) 10 days after induction of FCA in L-NIL treated animals. Sections stained with haematoxylin, safranin O, and fast green.

Figure 4

Chronic inflammation—therapeutic administration: (A) knee joint swelling; (B) urinary NO_x; and (C) PGE₂, expressed as a percentage change compared with pre-injection values in response to intra-articular injection of FCA. There was significant joint swelling and increased NO_x and PGE₂ production in vehicle, L-NIL (43 mg/kg), and SC-236 (5.6 mg/kg) therapeutically treated groups. A non-inflamed, untreated group is included for comparison. Values represent means (SEM).

Figure 5

Chronic inflammation—combined administration: Knee joint swelling, expressed as a percentage change compared with preinjection values in response to intra-articular injection of FCA. Combined administration of L-NIL (43 mg/kg) and SC-236 (5.6 mg/kg), (A) prophylactically or (B) therapeutically at days 9 and 14, respectively, as compared with untreated and vehicle treated controls. Data for independent administration of each of these drugs (from figs 2 and 3) are also included for comparison. Values represent means (SEM), n = 6/group. ***p<0.001 compared with untreated controls; ‡‡‡p<0.001 compared with vehicle treated inflamed animals; ††† p<0.001 compared with combination treatment inflamed animals (Bonferroni post hoc tests).