Attenuation of the induction of nitric oxide synthase by endogenous glucocorticoids accounts for endotoxin tolerance in vivo

An enhanced formation of nitric oxide (NO) due to induction of a calcium- independent (inducible) NO synthase (iNOS) contributes importantly to the cardiovascular failure caused by bacterial endotoxin. Repeated challenges with small doses of endotoxin result in tolerance to both peripheral vascular failure and death caused by subsequent injection of a higher dose of endotoxin. Here we investigate whether tolerance to endotoxin is associated with a lack of induction of iNOS in vivo and whether endogenous glucocorticoids play a role in the development of endotoxin tolerance. In anesthetized rats, i.v. administration of Escherichia coli endotoxin [lipopolysaccharide (LPS); 2 mg · kg^-1] resulted in a prolonged decrease in mean arterial blood pressure (MAP) and hyporeactivity to the contractile responses elicited by norepinephrine (NE; 10 nM) in aortic rings ex vivo. Hyporeactivity to NE was partially reversed by N(G)-nitro-L-arginine methyl ester (0.3 mM) in vitro, suggesting that an enhanced formation of NO contributes to this hyporeactivity. There was a substantial increase in the activity of iNOS in the lung 3 h after i.v. injection of LPS (0.2 ± 0.1 to 6.6 ± 0.6 pmol · mg^-1 · min^-1; n = 5; P < 0.01). Rats injected i.p. with LPS (0.5 mg · kg^-1) for 4 consecutive days became tolerant to an i.v. injection of LPS (2 mg · kg^-1) in that both hypotension and vascular hyporeactivity to NE were significantly attenuated. Moreover, in these endotoxin-tolerant rats, the induction of iNOS by LPS in the lung was attenuated by 63% ± 6%. Injection of LPS caused a 9-fold increase in plasma corticosterone (CCS) levels within 2 h and CCS levels remained significantly elevated 6 and 24 h after LPS. Animals rendered tolerant to endotoxin by administration of a low dose of LPS (0.5 mg · kg^-1, i.p.) for 4 days still had a 6-fold increase in plasma CCS levels 24 h after the last injection of LPS. When endotoxin-tolerant rats were treated with the glucocorticoid receptor antagonist RU 486 (50 mg · kg^-1, p.o. 3 h prior to LPS), there was a restoration of the effects of LPS (2 mg · kg^-1, i.v.) in causing hypotension, vascular hyporeactivity to NE, and iNOS induction in the lung. However, in control rats RU 486 enhanced neither the decrease in MAP nor the induction of iNOS in response to LPS (2 mg · kg^-1, i.v.). Thus, cardiovascular tolerance to endotoxin is accompanied and explained by reduced induction of iNOS in vivo due to the elevation of endogenous glucocorticoid levels.