Effect of Nitric Oxide Synthase Inhibitors in Vivo Tumor Necrosis Factor-induced

目前的研究是为了在活体内确定一氧化氮是否与细胞因子诱导的心肌抑制有关。根据体外数据，细胞因子对心脏的负致离子效应被认为是由一氧化氮介导的。一氧化氮合酶抑制剂N-G单甲基L精氨酸可阻断这种负性细胞因子致乳头肌电离效应。由于体外实验数据表明，一氧化氮合酶抑制剂可阻止肿瘤坏死因子(TNF)引起的心肌抑制的快速发作和逆转，因此我们研究了低剂量重组人肿瘤坏死因子对犬的心肌抑制作用。这种对肿瘤坏死因子的挑战会产生显著的、早期的和短暂的心肌抑制(24小时后消失)。令人惊讶的是，我们发现NMA并不能阻止肿瘤坏死因子对心脏功能的早期(长达6小时)有害影响。事实上，在这段时间内，肿瘤坏死因子和NMA对所有心脏和血流动力学参数的影响是相加的(即NMA不能阻断肿瘤坏死因子的影响)。但在注射肿瘤坏死因子后24小时，NMA确实改善了肿瘤坏死因子对酸碱失衡、平均动脉压和体循环阻力的影响。这些数据表明，肿瘤坏死因子诱导的心脏和血管异常的早期阶段可能与一氧化氮的产生无关。然而，肿瘤坏死因子的一些后期效应可能与一氧化氮的产生有关。鉴于NMA在输注肿瘤坏死因子24小时后的有益效果，我们评估了在较高剂量的肿瘤坏死因子导致更长时间的心肌抑制的情况下，一氧化氮抑制的效果。以前在犬身上进行的使用肿瘤坏死因子挑战的实验表明，这是一个合理的假设，即心脏损伤可能有两个阶段。在犬中，存在早期(小于8h)的心肌抑制机制和晚期(大于24h)的剂量依赖机制。据推测，当心肌抑制呈剂量依赖关系时，早期对一氧化氮合成的抑制可能不是有利的。因此，我们既研究了预防性治疗(治疗前)，也研究了NMA的治疗性治疗(在肿瘤坏死因子激发后的治疗后检测早期和晚期时间点)。NMA治疗降低了早期和晚期一氧化氮的产生。在早期，无论是治疗还是预防，NMA都不能预防肿瘤坏死因子的不良影响。然而，在24小时内，用产生一氧化氮的天然底物L-精氨酸逆转NMA后，预防性NMA改善了肿瘤坏死因子刺激所致的心功能下降。这些数据表明，肿瘤坏死因子对心脏功能具有双重作用。早期效应似乎是一氧化氮不依赖的，而后者的效应似乎是一氧化氮依赖的。在未来的研究中，我们计划在实际的细菌感染诱导的心肌抑制模型中证实这些发现。NMA正与细胞因子疗法一起用于癌症患者，以抑制他们的心血管毒性，并计划在艾滋病患者中进行同样的研究。这些研究将有助于确定这种方法的可取性。

The present investigation has been undertaken to determine, in vivo, if nitric oxide is responsible for cytokine-induced myocardial depression. The negative ionotropic effects of cytokines on the heart are believed to be mediated by nitric oxide, based on in vitro data. In isolated hamster cardiac papillary muscle, this negative ionotropic effect of cytokines can be blocked by N-G monomethyl-L-arginine (NMA), a nitric oxide synthase inhibitor. Because the in vitro data demonstrates that nitric oxide synthase inhibitors prevented tumor necrosis factor (TNF)-induced myocardial depression of rapid onset and reversal, we studied a low dose of recombinant human TNF challenge in canines. This TNF challenge produces significant, early, and short-lived myocardial depression (resolved by 24 h). Surprisingly, we found that NMA did not prevent the early (up to 6h) deleterious effects of TNF on cardiac function. In fact, during this time period, TNF and NMA effects on all cardiac and hemodynamic parameters were additive (i.e., NMA did not block TNF effects). However, 24 h after TNF infusion, NMA did ameliorate the effects of TNF on some parameters such as acid-base derangements and decreases in mean arterial pressure and systemic vascular resistance. These data suggest that the early phase of TNF-induced cardiac and vascular abnormalities may not be related to nitric oxide production. However, some of the later effects of TNF may be related to the production of nitric oxide. Given the suggestive finding of a beneficial effect of NMA 24 h post TNF infusion, we evaluated the effects of nitric oxide inhibition in the setting of higher doses of TNF causing longer-lasting myocardial depression. Previous experiments using TNF challenges in canines suggest that this is a reasonable hypothesis, i.e., there may be two phases of cardiac injury. In canines, there is an early (less than 8 h), dose-independent mechanism of myocardial depression and a late (greater than 24 h), dose-dependent mechanism of myocardial depression. It was hypothesized that the inhibition of nitric oxide synthesis might not be advantageous early when myocardial depression is dose dependent. Therefore, we studied both prophylactic treatment (pretreatment) or therapeutic treatment with NMA (post-treatment after TNF challenge examining both early- and late-time points). Treatment with NMA lowered measures of nitric oxide production in both early and late-time points. At early-time points, NMA given either therapeutically or prophylactically did not prevent the adverse affects of TNF. However at 24 hour, after reversal of the NMA with l-arginine, the natural substrate for nitric oxide production, prophylactic NMA ameliorated the decline in cardiac function seen with TNF challenge. These data suggest a dual effect of TNF on cardiac function. The early effect appears to be nitric oxide independent, while the later effect appears to be nitric oxide dependent. In future studies, we plan to confirm these findings in actual bacterial infection-induced myocardial depression models. NMA is being used with cytokine therapy for cancer patients to inhibit their cardiovascular toxicity and studies are planned in AIDS patients to do the same. These studies will help determine the advisability of this approach.