Inhibition of exhaled nitric oxide production during sepsis does not prevent lung inflammation

Publisher Website
Google Scholar
Abstract
Objectives: Increases in exhaled nitric oxide have been demonstrated to originate from the lungs of rats after septic lung injury. The aim of this study was to investigate whether treatment with the nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME) would prevent lipopolysaccharide (LPS)-induced increases in exhaled nitric oxide and whether this would have an effect on septic lung inflammation. Design: Prospective, randomized, placebo-controlled animal laboratory investigation. Setting: University laboratory Subjects: Male, anesthetized, paralyzed, and mechanically ventilated Sprague-Dawley rats (n = 27). Interventions: Rats were mechanically ventilated with air filtered to remove nitric oxide (espiratory rate 40 breaths/min, tidal volume 3 mL, positive end-expiratory pressure 0, FIO2 0.21). They were then randomized to receive intravenous injections of either L-NAME (25 mg/kg/hr x 4 hrs) (n = 11) or saline (n = 10). Both groups were again randomized to receive either LPS (Salmonella typhosa: 20 mg/kg iv x 1 dose) or an equal volume of saline 5 mins later. Thereafter, exhaled gas was collected in polyethylene bags for measurements of nitric oxide concentration. After 4 hrs, the rats were killed and the lungs were preserved and examined histologically. To examine the effect of L-NAME and LPS on mean arterial blood pressure, six additional rats underwent the same ventilation protocol with cannulation of the right internal carotid artery so that systemic arterial pressures could be measured. Measurements and Main Results: Exhaled gas was collected and measurements of NO concentrations were made using chemiluminescence every 20 mins for 240 mins during ventilation. A total lung injury score was calculated by determining the extent of cellular Infiltrate, exudate and hemorrhage. Mean arterial pressure was recorded every 5 mins for 20 mins and then at 20-min periods for 120 mins. Exhaled nitric oxide concentrations increased in all the LPS-treated rats that did not receive L-NAME by 120 mins; a plateau was reached by 190 mins that was [approximately]4 times greater than control rats not treated with LPS (p <.001). In contrast, rats treated with L-NAME and LPS did not show an increase in exhaled NO. Administration of L-NAME induced a 10-min nonsustained increase in mean arterial pressure in two rats treated with L-NAME followed by LPS. This increase in mean arterial pressure was not seen in two placebo and two LPS-treated rats that did not receive L-NAME. Lung inflammation was significantly worse in the two groups of rats which received LPS compared with the two that did not. L-NAME did not cause lung inflammation in rats that did not receive LPS; however, LPS-treated rats that received L-NAME had more inflammatory interstitial infiltrate (p < .05) and a trend toward worse lung injury than did LPS-treated rats that did not receive L-NAME. Conclusion: We conclude that L-NAME can inhibit the increase in exhaled NO from the lungs of septic rats, but that this inhibition does not reduce lung inflammation, and may worsen it. (Crit Care Med 1998; 26:309-314) Since its discovery in 1987 and its identification as endothelium-derived relaxing factor, nitric oxide (NO) has been implicated in a wide variety of biological processes [1,2]. NO is known to function as a vasodilator [2,3]; the upregulation of the inducible form of nitric oxide synthase (iNOS) and subsequent accelerated production of NO have been implicated in the pathologic vasodilatation and hypotension associated with the sepsis syndrome [4,5]. Recently, nitric oxide synthase (NOS) inhibitors have been used clinically during sepsis to block production of NO to reverse septic shock [6,7]. However, the effects of NOS inhibitors on the lung are uncertain; L-NAME has been found to protect the lungs in oxidant-provoked pulmonary injury [8] and, conversely, to hasten lung injury in isolated perfused rat lungs [9]. We recently reported that rats treated with endotoxin exhibit dramatic increases in exhaled NO production from the lower respiratory tract [10]. We hypothesized that the NOS antagonist L-NAME might be able to block this increase in exhaled NO, and that blocking the production of NO, a mediator of the inflammatory response, might result in less parenchymal lung inflammation. The following randomized, placebo-controlled study was instituted to examine this hypothesis.