Role of nitric oxide synthase isozymes in endotoxin-induced uveitis

The authors previously reported that in vitro treatment with N(G)-nitro L-arginine (L-NNA), an inhibitor of nitric oxide synthase (NOS), reduces aqueous humor (AH) protein and cellular infiltration in endotoxin-induced uveitis in the rat eye. The objective of the current study was to determine the role(s) of respective major forms (constitutive and inducible) of NOS by comparing the effects of relatively selective inhibitors of these NOS isozymes.

N(G)-nitro L-arginine (L-NNA), a relatively selective inhibitor for constitutive NOS (c-NOS), and N-iminoethyl L-ornithine (L-NIO), a more selective inhibitor for inducible NOS (i-NOS), were administered in vivo. Male Lewis rats were footpad injected with bacterial lipopolysaccharide (LPS, 200 microgram) and were injected intraperitoneally at 0 hours, 6 hours, or both, after LPS injection with 10 mg of NIO, NNA, or saline as a control. Nitric oxide synthase activity in the ocular tissue and AH protein and cell content were determined at various times after treatment with LPS.

After in vivo treatment, L-NIO was found to be a more potent inhibitor than L-NNA for ocular i-NOS (87% versus 43% inhibition), and L-NNA was more potent than L-NIO for ocular c-NOS (81% versus 39%). Two injections of L-NNA, one at time 0 and one 6 hours after LPS injection, inhibited the AH protein increase by 71%, but L-NIO did so by only 30%. L-NNA inhibited cellular infiltration by 86%, whereas L-NIO had no significant effect on cellular infiltration. A significant inhibition of cellular infiltration and AH protein increase also was observed with a single injection of 10 mg of L-NNA but not of L-NIO when the inhibitors were given simultaneously with LPS. Thus, reduction of uveitis symptoms correlates with the inhibition of c-NOS.

The constitutive form of NOS in ocular tissue, presumably in vascular endothelial cells, appears to play a critical role at the onset of the development of endotoxin-induced uveitis.