| Objective: Lipopolysaccharide (LPS), which is the major component of gram-negative bacillary endotoxin, plays an important role in initiating inflammatory response and causing systemic inflammatory response syndrome (SIRS). Lung is one of the target organs primarily impaired in endotoxin infection. Acute lung injury (ALI) induced by LPS is an acute pulmonary inflammation response in the lung, in which the accumulation and activation of polymophonuclear neutrophil (PMN) and the release of oxygen free radical are the key link. However, the definite mechanism about ALI induced by LPS remains illuminated incompletely. Studies showed that nitric oxide (NO) can modulate the pulmonary function in physiologic states and participate the process of pulmonary disease. The enzymatic production of NO in tissues from L-arginine is catalyzed by nitric oxide synthase (NOS), including constitutive NOS (cNOS) and inducible NOS (iNOS). cNOS is expressed under physiological conditions, cNOS-derived NO can attenuate tissues injury in early ALI by protecting the integrity of blood vessel endothelium, reducing accumulation of inflammatory cells, down-regulating the express of TNF-αand so on. iNOS has been shown to be upregulated by endotoxin and proinflammatory cytokines such as interferon, interleukin, tumor necrosis factor in a number of cells, including macrophages, PMN and vascular smooth muscle cells. It has been demonstrated that iNOS-derived NO can react with superoxide anions to produce peroxynitrite, which is an oxidizer and can cause serious lung injury and initiate lipid peroxidation. So it suggested that cNOS-derived NO attenuated the injuries in early ALI, while iNOS-derived NO deteriorated that in late stage. Wan-mei et al also confirmed that NO is a 'double-edged sword'in the progress of ALI induced by LPS. Because of the protective role of NO at the early stage of ALI, inhaled low-dose NO has been used clinically for alleviating the injury of lung, but it can not easily be put forward as credible rationale for clinical use. The reason is that ALI is a complicated pathological process induced by so many factors, so it is not enough to reveal the underlying mechanism of ALI only by studying NO. Recently, hydrogen sulfide (H2S), a new small endogenous gaseous molecular, caught our more attention. The enzymatic production of H2S in mammalian tissues from L-cysteine is catalyzed by one of pyridoxal-5'-phosphate-dependent enzymes, including cystathionine?-synthase (CBS), cystathionine γ-lyase (CSE) and cysteine transferase. Similar to NO, H2S could regulate many physiological processes as an endogenous gasotransmitter. Recently, the studies about the performance of H2S in pathological conditions confirmed that decreased H2Slevel was the reason for rat hypertension and increased H2S level caused hypotension during endotoxic shock. It was also reported that H2S could effectively protect myocardial cells and reduce malondialdehyde (MDA) content from ischemic injury. Perhaps, this anti-oxidative effect achieved by scavenging peroxynitrite and increasing the synthesis of glutathione (GSH). However, the protective role of H2S in ALI induced by LPS and its relationship with NO were unknown. H2S exists as gaseous and HS-form at dynamic equili-brium with H2S. Thus, the balance between H2S and HS-keep the stability of H2S and pH in the body fluid. NaHS can dissociate to Na+ and HS-in solution, then HS-associates with H+ to produce H2S. So NaHS has been widely used for the studies of H2S. In this study, we established Sprague-Dawley (SD) rat model of ALI by instilling LPS intratracheally, and explored the protective effect of H2S on ALI by injecting NaHS intraperitoneally. In addition, we further investigated the relationship between NO and the protective effect of H2S. Methods: Sixty male SD rats were divided into five groups randomly (n=12 in each group): ①Control group: normal saline (NS) was instilled intratracheally (200μl/per rat); ②LPS group: LPS (100μg/200μl/per rat) was instilled intra -tracheally; ③PPG (propargylglycine, inhibitor of CSE)+LPS group: PPG (30μmol/0.5ml/100g) was injected intraperitoneally 10 min before LPS administration; ④NaHS+LPS group: NaHS (28μmol/0.5ml/Kg) was injected intraperitoneally 10 min beforeLPS administration; ⑤NaHS+NS group: NaHS (28μmol/ 0.5ml/Kg) was injected intraperitoneally 10 min before NS administration. Each parameter was observed respectively 4h and 8h after agent administration: Bronchoalveolar lavage (BAL) was done on 6 rats in each group to detect PMN number and protein content in bronchoalveolar lavage fluid (BALF). As for the other 6 rats in each group which didn't receive BAL, blood from carotid artery was collected to detect H2S and NO content; the lung coefficient was measured; H2S, NO and MDA contents as well as the activity of H2S synthase and iNOS in lung were detected; The change of lung tissue structure and index of quantitative assessment (IQA) were observed. Results: 1 Compared with control group, MDA content in the lung both increased 4h and 8h after LPS administration (44.02±3.93 nmol·ml-1 vs. 34.06±3.67 nmol·ml-1 and 54.72±7.58 nmol·ml-1 vs. 36.73±1.62 nmol·ml-1,both P﹤0.01). PMN number in BALF increased from 2.59±0.56 (×109/L) to 9.71±0.91 (×109/L ) (P﹤0.01) 4h after LPS administration, and increased to 35.09±5.07 (×109/L) (P﹤0.01) 8h after LPS administration. Compared with control group, protein content in BALF both increased 4h and 8h after LPS administration (336.01±24.33μg·ml-1 vs. 193.21±9.40μg·ml-1 and 517.81±21.91 μg·ml-1 vs. 249.45±16.07 μg·ml-1,both P﹤0.01). The lung coefficient also increased in LPS group (5.06±0.37 vs. 4.41±0.20 and 5.15±0.15 vs. 4.24±0.12,both P﹤0.01). The pathomorphological changes of lung tissues in LPS groupexpressed as widened septa of alveoli, diffuse infuse infiltration and migration of acute inflammation cells (PMN), accompanied by atrophied or damaged alveoli, and slight alveolar atelectasis or emphysema. 2 Compared with the same time points of LPS group, the lung coefficient, MDA content in lung, PMN number and protein content in BALF decreased in NaHS+LPS group (each P﹤0.05), but the above parameters did not change in PPG+LPS group (each P﹥0.05). The accumulation of PMN in lung septa and alveoli also reduced, accompanied by almost undamaged lung tissue structure and IQA in NaHS+LPS group. 3 Compared with control group, NO content in plasma increased 4h and 8h after LPS administration (59.43±4.75 μmol·L-1 vs. 50.59±5.13μmol·L-1 and 57.92±3.26 μmol·L-1 vs. 52.94±1.56 μmol·L-1,both P﹤0.05), and it correlated with MDA positively (r﹦0.913 and r﹦0.908,both P﹤0.05). Compared with control group, the activity of cNOS in the lung reduced 4h and 8h after LPS administration (0.2404±0.047 U·mgprot-1 vs. 0.2939±0.036 U·mgprot-1,0.2486±0.029 vs. 0.3069±0.019 U·mgprot-1,both P﹤0.05), but iNOS activity and NO content increased (both P﹤0.01). Compared with the same time points of LPS group, PPG did not bring any changes for above parameters; NO content of plasma in NaHS+LPS group decreased (4h:48.80±3.02 μmol·L-1; 8h:50.61±4.03μmol·L-1,both P﹤0.01). The cNOS activity of lung increased and NO content as well as iNOS activity decreased significantly (both P﹤0.01) 4h and 8h after LPS administration in NaHS+LPS group. 4 Compared with control group, in LPS group, H2S content of plasma decreased 4h and 8h after LPS administration (124.98±31.66 μmol·L-1 vs. 205.83±47.06μmol·L-1 and 133.02±43.87 μmol·L-1 vs. 210.83±41.27μmol·L-1,both P﹤0.01), and it correlated with NO and MDA of plasma negatively (r﹦-0.936; r﹦-0.894 and r﹦-0.803; r﹦-0.957, each P﹤0.05); the activity of H2S synthase and H2S content of lung increased (92.75±6.11 nmol·min-1·mg-1 vs. 77.72±7.19 nmol·min-1·mg-1 and 432.17±65.32 μmol·L-1 vs. 366.72±35.41 μmol·L-1,both P﹤0.05) 4h after LPS administration, then they decreased (52.31±12.88 nmol·min-1·mg-1 vs. 70.63±10.64 nmol·min-1·mg-1 and 308.58±18.97 μmol·L-1 vs. 369.56±13.43 μmol·L-1,both P﹤0.05) 8h after LPS administration. Compared with the same time points of LPS group, in PPG+LPS group, the activity of H2S synthase decreased 4h after LPS administration and there were no changes in others. In NaHS+LPS group, H2S content of plasma increased after LPS administration (4h:272.50±59.58μmol·L-1, 8h:247.60±45.25μmol·L-1, both P﹤0.01), the activity of H2S synthase and H2S content of lung decreased 4h after LPS administration (318.27±47.96 μmol·L-1, 75.66±11.95 nmol·min-1·mg-1, both P﹤0.05), but increased 8h after LPS administration (414.08±64.08 μmol·L-1, 90.12±17.33 nmol·min-1·mg-1, both P﹤0.05). Conclusions: 1 Instilling LPS intratracheally could cause...
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