Aminoguanidine attenuates the delayed circulatory failure and improves survival in rodent models of endotoxic shock

We have investigated the effects of aminoguanidine, a relatively selective inhibitor of the cytokine‐ inducible isoform of nitric oxide synthase (iNOS), on the delayed circulatory failure, vascular hyporeactivity to vasoconstrictor agents, and iNOS activity in a rat model of circulatory shock induced by bacterial endotoxin (E. coli lipopolysaccharide; LPS). In addition, we have evaluated the effect of aminoguanidine on the 24 h survival rate in a murine model of endotoxaemia. Male Wistar rats were anaesthetized and instrumented for the measurement of mean arterial blood pressure (MAP) and heart rate (HR). Injection of LPS (10 mg kg⁻¹, i.v.) resulted in a fall in MAP from 115 ± 4mmHg (time 0, control) to 79 ± 9mmHg at 180 min (P<0.05, n = 10). The pressor effect of noradrenaline (NA, 1 μg kg⁻¹, i.v.) was also significantly reduced at 60, 120 and 180 min after LPS injection. In contrast, animals pretreated with aminoguanidine (15 mg kg^_1, i.v., 20 min prior to LPS injection) maintained a significantly higher MAP (at 180 min, 102 ± 3mmHg, n= 10, P<0.05) when compared to rats given only LPS (LPS‐rats). Cumulative administration of aminoguanidine (15 mg kg⁻¹ and 45mgkg⁻¹) given 180 min after LPS caused a dose‐related increase in MAP and reversed the hypotension. Aminoguanidine also significantly alleviated the reduction of the pressor response to NA: indeed, at 180 min, the pressor response returned to normal in aminoguanidine pretreated LPS‐ rats. Thoracic aortae obtained from rats at 180 min after LPS showed a significant reduction in the contractile responses elicited by NA (10⁻⁹‐10⁻⁶ m). Pretreatment with aminoguanidine (15 mg kg⁻¹, i.v., at 20 min prior to LPS) significantly prevented this LPS‐induced hyporeactivity to NA ex vivo. Endotoxaemia for 180 min resulted in a significant increase in iNOS activity in the lung from 0.6 ± 0.2 pmol mg⁻¹ min⁻¹ (control, n = 4) to 4.8 ± 0.3 pmol mg⁻¹ min⁻¹ (P<0.05, n = 6). In LPS‐rats treated with aminoguanidine, iNOS activity in the lung was attenuated by 44 ± 5% (n = 6, P<0.05). Moreover, when added in vitro to lung homogenates obtained from LPS‐rats, aminoguanidine and N^ω‐nitro‐L‐arginine methyl ester (l‐NAME; 10⁻⁸ to 10⁻³ m) caused a concentration‐dependent inhibition of iNOS activity (n = 3–6, IC₅₀: 30 ± 12 and 11 ± 6μm, respectively P>0.05). In contrast, aminoguanidine was a less potent inhibitor than L‐NAME of the constitutive nitric oxide synthase in rat brain homogenates (n = 3–6, IC₅₀ is 140 ± 10 and 0.6 ± 0.1 μm, respectively, P<0.05). In addition, the inhibitory effect of aminoguanidine on iNOS activity showed a slower onset than that of L‐NAME (maximal inhibition at 90 min and 30 min, respectively). Treatment of conscious Swiss albino (T/O) mice with a high dose of endotoxin (60 mg kg⁻¹, i.p.) resulted in a survival rate of only 8% at 24 h (n=12). However, therapeutic application of aminoguanidine (15 mg kg⁻¹, i.p. at 2 h and 6 h after LPS) increased the 24 h survival rate to 75% (n = 8), whereas L‐NAME (3 mg kg⁻¹, i.p. at 2h and 6h after LPS) did not affect the survival rate (11%, n = 9). Thus, aminoguanidine inhibits iNOS activity and attenuates the delayed circulatory failure caused by endotoxic shock in the rat and improves survival in a murine model of endotoxaemia. Aminoguanidine, or novel, more potent selective inhibitors of iNOS may be useful in the therapy of septic shock. 1995 British Pharmacological Society