Effects of nitric oxide synthase inhibitors on systemic hypotension, cytokines and inducible nitric oxide synthase expression and lung injury following endotoxin administration in rats

Abstract

Endotoxin shock is characterized by systemic hypotension, hyporeactiveness to vasoconstrictors and acute lung edema. A nitric oxide synthase (NOS) inhibitor, NG-monomethyl-L-arginine (L-NMMA) has been shown to be effective in reversing acute lung injury. In the present study, we evaluated the effects of NOS blockade by different mechanisms on the endotoxin-induced changes. In anesthetized rats, lipopolysaccharide (LPS, Klebsiella pneumoniae) was administered intravenously in a dose of 10 mg/kg. LPS caused sustained systemic hypotension accompanied by an eightfold increase of exhaled NO during an observation period of 4 h. After the experiment, the lung weight was obtained and lung tissues were taken for the determination of mRNA expressions of inducible NOS (iNOS), interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha-(TNF-alpha). Histological examination of the lungs was also performed. In the control group injected with saline solution, mRNA expressions of iNOS, IL-1beta and TNF-alpha were absent. Four hours after LPS, the mRNA expressions of iNOS and IL-1beta were still significantly enhanced, but TNF-alpha was not discernibly expressed. LPS also caused a twofold increase in lung weight. Pathological examination revealed endothelial damage and interstitial edema. Various NOS inhibitors were given 1 h after LPS administration. These agents included Nomega-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg), a constitutive NOS and iNOS inhibitor; S, S'-1,4-phenylene-bis-(1,2-ethanedinyl) bis-isothiourea dihydrobromide (1,4-PBIT, 10 mg/kg), a relatively specific iNOS inhibitor, and dexamethasone (3 mg/kg), an inhibitor of iNOS expression. These NOS inhibitors all effectively reversed the systemic hypotension, reduced the exhaled NO concentration and prevented acute lung injury. The LPS-induced mRNA expressions of iNOS and IL-1beta were also significantly depressed by these NOS inhibitors. Our results suggest that NO production through the iNOS pathway is responsible for endotoxin-induced lung injury. Certain cytokines such as IL-1beta are possibly involved. These changes are minimized by NOS inhibitors through different mechanisms.

Nitric Oxide Stimulates Macrophage Inflammatory Protein-2 Expression in Sepsis

“…The parameters measured can also influence the results. Whereas in many studies survival is used as an endpoint, a recent report investigating lung injury (edema and histologic changes) in rats in response to endotoxin administration found that both a nonspecific NOS inhibitor and a specific iNOS inhibitor had equally protective effects (26). Whether the beneficial effects of L-NMMA we found at 6 h would translate into improved survival is not known.…”

“…LPS induces the production of large amounts of NO and superoxide in alveolar macrophages and lung epithelial, endothelial, and interstitial cells (22)(23)(24)(25). The primary mechanism by which this occurs is induction of inducible NOS (iNOS) mRNA expression in various organs, including lung (26,27), via activation of the transcription factor NF-B (28 -30). NF-B also regulates the transcription of numerous other genes involved in varied inflammatory and immune responses, including TNF-␣, ICAM-1, VCAM-1, and MIP-2 (31).…”

Cyclooxygenase-2 Inhibition Attenuates Lipopolysaccharide-Induced Cardiovascular Failure

“…This hypothesis was strengthened by findings that selective NOS-II inhibition 13-16,48 -51 and also nonselective NOS inhibition 15,52,53 prevents cardiovascular collapse. However, nonselective NOS inhibition was associated with marked side effects-such as provoked renal dysfunction, 15 increased number of bacteria in plasma and tissue with higher cytokine levels, 54 and accelerated and enhanced fall in blood pressure 52 -and overall did not improve 49,55 or even reduce 54 survival in experimental sepsis, possibly owing to detrimental effects of the inhibition of NO generation in the microcirculation.…”

Microcirculatory Dysfunction in the Brain Precedes Changes in Evoked Potentials in Endotoxin-Induced Sepsis Syndrome in Rats

“…We can only conclude that strong dilating factors occuring during sepsis progression interfere with cerebral autoregulation. The increase in cerebral blood flow velocity levels might be readily explained by changes in NO levels occurring in sepsis progression [25]. Administration of NO inhibitors prevented the occurrence of cerebral hyperemia [21].…”