Nitric oxide‐mediated hyporeactivity to noradrenaline precedes the induction of nitric oxide synthase in endotoxin shock

Abstract

1 The role of an enhanced formation of nitric oxide (NO) and the relative importance of the constitutive and inducible NO synthase (NOS) for the development of immediate (within 60 min) and delayed (at 180 min) vascular hyporeactivity to noradrenaline was investigated in a model of circulatory shock induced by endotoxin (lipopolysaccharide; LPS) in the rat. 2 Male Wistar rats were anaesthetized and instrumented for the measurement of mean arterial blood pressure (MAP) and heart rate. In addition, the calcium-dependent and calcium-independent NOS activity was measured ex vivo by the conversion of [3H]-arginine to [HJ-citrulline in homogenates from several organs obtained from vehicle-and LPS-treated rats. 3 E. coli LPS (10mg kg-', i.v. bolus) caused a rapid (within 5 min) and sustained fall in MAP. At 30 and 60 min after LPS, pressor responses to noradrenaline (0.3, 1 or 3 jig kg-', i.v.) were significantly reduced. The pressor responses were restored by N0-nitro-L-arginine methyl ester (L-NAME, 1 mg kg-', i.v. at 60 min), a potent inhibitor of NO synthesis. In contrast, L-NAME did not potentiate the noradrenaline-induced pressor responses in control animals. 4 Dexamethasone (3 mg kg-', i.v., 60 min prior to LPS), a potent inhibitor of the induction of NOS, did not alter initial MAP or pressor responses to noradrenaline in control rats, but significantly attenuated the LPS-induced fall in MAP at 15 to 60 min after LPS. Dexamethasone did not influence the development of the LPS-induced immediate (within 60 min) hyporeactivity to noradrenaline. However, dexamethasone pretreatment prevented the hypotension and vascular hyporeactivity at 180 min. 5 At 60 min after LPS a moderate increase in the activity of a calcium-independent (inducible) NOS activity was detected in the aorta, but not in any of the other tissues studied. However, at 180 min after LPS, a significant NOS induction was observed in the lung, liver, spleen, mesentery, heart and aorta. This NOS induction was substantially prevented by pretreatment with dexamethasone. 6 These results suggest that the immediate hypotension and vascular hyporeactivity to noradrenaline in endotoxin shock is caused by an enhanced formation of NO due to activation of the constitutive enzyme. The delayed hypotension and vascular hyporeactivity, however, is due to enhanced NO formation by the LPS-induced enzyme.

Endotoxin triggers the expression of an inducible isoform of nitric oxide synthase and the formation of peroxynitrite in the rat aorta in vivo

“…Moreover, an increase in nitrotyrosine immunoreactivity was recently found in the lung of rats injected with bacterial endotoxin [18]. It is noteworthy in this contest that among many organs studied, the lung expresses the highest specific activity of iNOS in response to endotoxin injection in the rat [10]. Thus, it appears that endotoxin shock results in induction of iNOS, with subsequent formation of peroxynitrite.…”

“…Tissues were then homogenized in the buffer on ice using a Tissue Tearor 985-370 homogenizer (Biospec Products, Racine, WI). Conversion of [3H]L-arginine to [3H]L-citrulline was measured in the homogenates as described [10]. Briefly, cell homogenate (30 pl) was incubated in the presence of [3H]L-arginine (10pM, 5 kBq/tube), NADPH (1 mM), calmodulin (30 nM), tetrahydrobiopterin (5 gM) and calcium (2 mM) for 30 min at 22°C.…”

Effects of nitric oxide synthase inhibition combined with nitric oxide inhalation in a porcine model of endotoxin shock

“…Endotoxaemia also resulted in a rapid and sustained increase in plasma NO2-(breakdown product of NO) levels, which were significantly higher than in sham-operated controls between 30 and 180 min after the start of the endotoxin infusion. This increase in plasma NO2-is due to an enhanced formation of NO arising from both activation of the constitutive NOS (early phase) as well as the induction of NOS (delayed phase) (Szabo et al, 1993). The finding, however, that endotoxaemia was not associated with a significant rise in plasma nitrate levels is not entirely surprising, for NO2-is the predominant metabolite of NO (Ignarro et al, 1993).…”

“…Endotoxin leads to the induction of a calcium-independent nitric oxide (NO) synthase (NOS) (Szabo et al, 1993;JulouSchaeffer et al, 1991) resulting in an overproduction of the vasodilator. This excessive formation of NO contributes importantly to the severe hypotension seen in endotoxin shock (Thiemermann & Vane, 1990; see Thiemermann, 1994 for review).…”

Ketamine inhibits nitric oxide synthase in lipopolysaccharide-treated rat alveolar macrophages

“…6 Enhanced formation of NO following induction of iNOS contributes importantly to the circulatory failure and cardiovascular dysfunction, such as vasodilatation, severe hypotension and vascular hyporeactivity to vasoconstrictor agents, in various animal models of septic shock. 7 Ketamine, a phencyclidine derivative, has been recommended for the induction of anaesthesia and sedation in patients with circulatory shock because of its actions in increasing sympathetic nervous system activity, to maintain blood pressure, and in preserving cardiovascular function, s-n In addition, a recent study demonstrated that ketamine suppressed TNF-0t production induced by endotoxin in cultured macrophages and inhibited NO-dependent cGMP production in cultured cerebral neuron. 12,13 More recently, ketamine has been reported to inhibit accumulation of nitrite, one of the metabolites of NO, dose-dependently in LPS-activated macrophages.…”