Cardioprotection, attenuated systemic inflammation, and survival benefit of beta1-adrenoceptor blockade in severe sepsis in rats.

OBJECTIVE: To explore the hypothesis that beta-1 adrenoreceptor blockade may be protective through the attenuation of sympathetic hyperactivity and catecholaminergic inflammatory effects on cardiac and hepatic function. DESIGN: Prospective, randomized, controlled study. SETTING: Animal laboratory in a university medical center. SUBJECTS: Male adult Wistar rats. INTERVENTIONS: Peripheral beta1-adrenoceptor blockade through daily intraperitoneal injection (metoprolol, 100 mg x kg(-1); atenolol, 6 mg x kg(-1)) or central nervous system beta1-adrenoceptor blockade (intracerebroventricular metoprolol, 25 microg) to achieve approximately 20% heart rate reduction in rats for 2 days before or after the induction of lethal endotoxemia, cecal ligation and puncture, or fecal peritonitis. MEASUREMENTS AND MAIN RESULTS: Peripheral beta1-adrenoceptor blockade established for 2 days before lethal endotoxemia markedly improved survival in both metoprolol-treated (n = 16; log rank test, p = .002) and atenolol-treated (n = 15; p = .03) rats. Overall mortality in cecal ligation and puncture was similar between metoprolol (40%; n = 10) and saline (50%; n = 10) pretreatment (p = .56), but the median time to death was increased by 33 hrs in metoprolol-treated rats (p = .03). Metoprolol pretreatment reduced hepatic expression of proinflammatory cytokines and lowered plasma interleukin-6 (both p < .05). Myocardial protein expression of interleukin-18 and monocyte chemoattractant protein-1, key mediators of cardiac dysfunction in sepsis, were also reduced (p < .05). Peripheral beta1-adrenoceptor blockade commenced 6 hrs after lethal endotoxemia or fecal peritonitis did not improve survival. However, arterial blood pressure was preserved and left ventricular contractility restored similar to that found in nonseptic controls. Central nervous system beta1-adrenoceptor blockade (metoprolol) did not reduce plasma cytokines or mortality, despite enhancing parasympathetic tone. CONCLUSIONS: Peripheral beta1-adrenoceptor blockade offers anti-inflammatory and cardioprotective effects, with mortality reduction if commenced before a septic insult. Its role in sepsis should be explored further.

The impact of inspired oxygen concentration on tissue oxygenation during progressive haemorrhage.

PURPOSE: Standard resuscitation practice for shock states mandates use of high flow, high concentration oxygen. However, this may induce microvascular constriction and potentially impair regional oxygen delivery. We thus investigated the impact of varying inspired oxygen concentrations in a rat model of progressive haemorrhage. METHODS: Tissue oxygen tension (the balance between local O2 supply and demand) was measured in four different organ beds (liver, renal cortex, muscle, bladder), with concurrent assessment of cardiorespiratory function and organ perfusion in a spontaneously breathing, anaesthetised rat model. 10% aliquots of circulating blood volume were removed at 15 min intervals until death. Different oxygen fractions in the gas mixture (0.15-1.0) were administered following 20% blood removal. A control group consisted of normovolaemic animals breathing varying oxygen fractions. RESULTS: Survival times following progressive haemorrhage were similar in animals breathing room air (98 +/- 10 min), 60% O2 (102 +/- 6 min) or 100% O2 (90 +/- 4 min), but significantly worse in those breathing 15% O2 (52 +/- 8 min, P < 0.01). Significant derangements of blood pressure, aortic blood flow and lactataemia were observed in both hypoxaemic and hyperoxaemic groups compared to normoxaemic animals. Breathing 100% O2 increased arterial PO2 sevenfold and tPO2 approximately threefold over baseline values during normovolaemia and mild haemorrhage (20% blood volume removal). However, with progressive haemorrhage, and despite maintained PaO2 values, tissue PO2 fell in line with the decrease in global oxygen delivery. CONCLUSION: Hypoxaemia and hyperoxaemia both compromised haemodynamics and biochemical markers of organ perfusion during severe, progressive haemorrhage. This may carry implications for resuscitation practice.

Tissue oxygen monitoring in rodent models of shock.

Tissue Po(2) (tPo(2)) reflects the balance between local O(2) supply and demand and, thus, could be a useful monitoring modality. However, the consistency and amplitude of the tPo(2) response in different organs during different cardiorespiratory insults is unknown. Therefore, we investigated the effects of endotoxemia, hemorrhage, and hypoxemia on tPo(2) measured in deep and peripheral organ beds. We compared arterial pressure, blood gas and lactate levels, descending aortic and renal blood flow, and tPo(2) in skeletal muscle, bladder epithelium, liver, and renal cortex during 1) LPS infusion (10 mg/kg), 2) sequential removal of 10% of circulating blood volume, and 3) reductions in inspired O(2) concentration in an anesthetized Wistar rat model with values measured in sham-operated animals. Different patterns were seen in each of the shock states, with condition-specific variations in the degree of acidemia, lactatemia, and tissue O(2) responses between organs. Endotoxemia resulted in a rise in bladder tPo(2) and an early fall in liver tPo(2) but no significant change in muscle and renal cortical tPo(2). Progressive hemorrhage, however, produced proportional declines in liver, muscle, and bladder tPo(2), but renal cortical tPo(2) was maintained until profound blood loss had occurred. By contrast, progressive hypoxemia resulted in proportional decreases in tPo(2) in all organ beds. This study highlights the heterogeneity of responses in different organ beds during different shock states that are likely related to local changes in O(2) supply and utilization. Whole body monitoring is not generally reflective of these changes.

Disruption of methylarginine metabolism impairs vascular homeostasis.

Asymmetric dimethylarginine (ADMA) and monomethyl arginine (L-NMMA) are endogenously produced amino acids that inhibit all three isoforms of nitric oxide synthase (NOS). ADMA accumulates in various disease states, including renal failure, diabetes and pulmonary hypertension, and its concentration in plasma is strongly predictive of premature cardiovascular disease and death. Both L-NMMA and ADMA are eliminated largely through active metabolism by dimethylarginine dimethylaminohydrolase (DDAH) and thus DDAH dysfunction may be a crucial unifying feature of increased cardiovascular risk. However, despite considerable interest in this pathway and in the role of ADMA as a cardiovascular risk factor, there is little evidence to support a causal role of ADMA in pathophysiology. Here we reveal the structure of human DDAH-1 and probe the function of DDAH-1 both by deleting the DDAH1 gene in mice and by using DDAH-specific inhibitors which, as we demonstrate by crystallography, bind to the active site of human DDAH-1. We show that loss of DDAH-1 activity leads to accumulation of ADMA and reduction in NO signaling. This in turn causes vascular pathophysiology, including endothelial dysfunction, increased systemic vascular resistance and elevated systemic and pulmonary blood pressure. Our results also suggest that DDAH inhibition could be harnessed therapeutically to reduce the vascular collapse associated with sepsis.

TREM-1 promotes survival during septic shock in mice.

Triggering receptor expressed on myeloid (TREM)-1 is integral to the inflammatory response occurring during septic shock, although its precise function has yet to be determined. Here we show that in vivo silencing of TREM-1 using siRNA duplexes in a fecal peritonitis mouse model resulted in a blunted inflammatory response and increased mortality. This was associated with impaired bacterial clearance related to marked inhibition of the neutrophil oxidative burst. By contrast, TREM-1-silenced mice were highly resistant to a lethal endotoxin challenge, while partial silencing of TREM-1 in the bacterial peritonitis model produced a significant survival benefit. These data highlight the crucial role of the TREM-1 pathway in mounting an adequate inflammatory and cytotoxic response to polymicrobial sepsis, and both the therapeutic promise and potential risks of its modulation.

Tissue oxygen and hemodynamics in renal medulla, cortex, and corticomedullary junction during hemorrhage-reperfusion.

Previous studies of intrarenal perfusion and tissue oxygenation have produced a wide range of results and have not matched tissue oxygen tension (tPo(2)) with concurrent changes in flow in three distinct regions. We thus used an anesthetized rat model of hemorrhage-reperfusion to address this question. Combined tpo(2)/laser-Doppler fiber-optic probes were simultaneously sited in cortical, corticomedullary (CMJ), and medullary regions of the left kidney. Total renal blood flow was measured in separate experiments. Recordings were made during exsanguination of 10 and 20% of estimated blood volume at 10-min intervals, followed by shed-blood resuscitation after a further 10 min. The decay in tpo(2) was then recorded following total cessation of blood flow, allowing estimation of local oxygen consumption. During exsanguination, tPo(2) was maintained in all intrarenal regions, despite significant falls in blood pressure and total renal blood flow. However, intrarenal flow was redistributed with reduced cortical, unchanged CMJ, and increased medullary blood flow. After resuscitation, significant rises above baseline were seen in blood pressure and in tpo(2) across all regions. Whereas cortical and medullary flows regained baseline values, CMJ flow fell. The ratio of tpo(2) to microvascular blood flow increased significantly in all regions during resuscitation, suggesting decreased oxygen consumption. On total cessation of blood flow, the cortex and CMJ showed significant increases in the oxygen decay half-life, consistent with decreased consumption. To our knowledge, this is the first quantitative demonstration of a markedly heterogeneous intrarenal cardiorespiratory response to a hemodynamic insult, with effects most marked at the corticomedullary junction.

Inflammation and endothelial function: direct vascular effects of human C-reactive protein on nitric oxide bioavailability.

BACKGROUND: Circulating concentrations of the sensitive inflammatory marker C-reactive protein (CRP) predict future cardiovascular events, and CRP is elevated during sepsis and inflammation, when vascular reactivity may be modulated. We therefore investigated the direct effect of CRP on vascular reactivity. METHODS AND RESULTS: The effects of isolated, pure human CRP on vasoreactivity and protein expression were studied in vascular rings and cells in vitro, and effects on blood pressure were studied in rats in vivo. The temporal relationship between changes in CRP concentration and brachial flow-mediated dilation was also studied in humans after vaccination with Salmonella typhi capsular polysaccharide, a model of inflammatory endothelial dysfunction. In contrast to some previous reports, highly purified and well-characterized human CRP specifically induced hyporeactivity to phenylephrine in rings of human internal mammary artery and rat aorta that was mediated through physiological antagonism by nitric oxide (NO). CRP did not alter endothelial NO synthase protein expression but increased protein expression of GTP cyclohydrolase-1, the rate-limiting enzyme in the synthesis of tetrahydrobiopterin, the NO synthase cofactor. In the vaccine model of inflammatory endothelial dysfunction in humans, increased CRP concentration coincided with the resolution rather than the development of endothelial dysfunction, consistent with the vitro findings; however, administration of human CRP to rats had no effect on blood pressure. CONCLUSIONS: Pure human CRP has specific, direct effects on vascular function in vitro via increased NO production; however, further clarification of the effect, if any, of CRP on vascular reactivity in humans in vivo will require clinical studies using specific inhibitors of CRP.

Nitrosyl heme production compared in endotoxemic and hemorrhagic shock.

We compared nitric oxide production and nitrosyl hemoglobin steady state concentrations during the early phases of endotoxemic and hemorrhagic shock of equivalent severity. Sprague-Dawley rats were randomly assigned to (1) sham-operated control, (2) hemorrhage, and (3) intravenous endotoxin. Electron paramagnetic resonance spectroscopy was used to measure NO in the vasculature (binding to hemoglobin) and in the liver (binding to cytochrome P450). Despite similar changes in cardiorespiratory variables and identical microvascular pO(2), nitrosyl hemoglobin concentrations were significantly higher in endotoxemic rats than in rats in hemorrhagic shock, suggesting increased rates of NO production. A substantial venous minus arterial concentration gradient was observed for nitrosyl hemoglobin. This increased in line with the plasma total nitrite + nitrate concentration. Nitrosyl hemoglobin formation is likely to occur predominantly in the venous pool, suggesting that removal of NO from hemoglobin in the presence of oxygen may be faster than previously thought. In the liver, an increase in intracellular heme-NO complexes was detected in endotoxemic rats compared with rats in hemorrhagic shock; this was associated with increased reduction of the mitochondrial respiratory chain and is suggestive of NO inhibition of mitochondrial respiration.

Mitochondrial dysfunction in a long-term rodent model of sepsis and organ failure.

Although sepsis is the major cause of mortality and morbidity in the critically ill, precise mechanism(s) causing multiorgan dysfunction remain unclear. Findings of impaired oxygen utilization in septic patients and animals implicate nitric oxide-mediated inhibition of the mitochondrial respiratory chain. We recently reported a relationship between skeletal muscle mitochondrial dysfunction, clinical severity, and poor outcome in patients with septic shock. We thus developed a long-term, fluid-resuscitated, fecal peritonitis model utilizing male Wistar rats that closely replicates human physiological, biochemical, and histological findings with a 40% mortality. As with humans, the severity of organ dysfunction and eventual poor outcome were associated with nitric oxide overproduction and increasing mitochondrial dysfunction (complex I inhibition and ATP depletion). This was seen in both vital (liver) and nonvital (skeletal muscle) organs. Likewise, histological evidence of cell death was lacking, suggesting the possibility of an adaptive programmed shutdown of cellular function. This study thus supports the hypothesis that multiorgan dysfunction induced by severe sepsis has a bioenergetic etiology. Despite the well-recognized limitations of laboratory models, we found clear parallels between this long-term model and human disease characteristics that will facilitate future translational research.

Inhibition of mitochondrial respiration during early stage sepsis.

It is known that nitric oxide (NO) is produced in response to a septic insult such as bacterial invasion and that overproduction of NO can have serious debilitating consequences. The mechanism by which NO causes damage at the cellular level is less clear. We have therefore studied the response to a septic insult in an anaesthetised spontaneously breathing Sprague-Dawley rat model. Six rats were given either an intravenous infusion of bacterial cell wall lipopolysaccharide (LPS, 5 mg/kg) or saline control over 1 hour. For electron paramagnetic resonance (EPR) studies, blood samples were collected every hour for a further two hours and liver tissue samples were collected postmortem. Measurement was also made of PaO2, blood pressure, base deficit, aortic and renal blood flow and hepatic microvascular pO2 (using porphyrin phosphoresence). Tissue samples were also collected for mitochondrial complex activity analysis. After the administration of LPS blood pressure, blood flow and microvascular PO2 were diminished and the base deficit increased. In addition a clear difference was observed by EPR between control and insulted blood and tissue samples. A large heam-nitrosyl signal is observed as well as an increase in the signal at g = 1.94, corresponding to the iron-sulphur centres of complex I becoming more reduced. However, no significant difference was observed for any of the mitochondrial complex activities. The effect of the NO produced was to depress the circulatory variables and increase base deficit, combined with a reduced oxygen consumption this implies an impairment of normal aerobic respiration. This was supported by increased iron-sulphur signals observed by EPR indicating a blockage in the mitochondrial redox chain with the subsequent accumulation of electrons. As no effect was observed in the mitochondrial complex activities this indicates that this inhibition is reversible in early stage sepsis. We conclude that nitric oxide produced in response to a septic insult can inhibit mitochondria causing an impairment of oxygen utilisation by aerobic respiration.

Angiotensin II blockade augments renal cortical microvascular pO2 indicating a novel, potentially renoprotective action.

BACKGROUND: The existence of tubulointerstitial damage in most cases of progressive human glomerular disease suggests that this compartment of the kidney is likely to be targeted by renoprotective agents which slow the progression of disease. Angiotensin-converting enzyme (ACE) inhibitors have become the cornerstone of renal protection. Since we have proposed that perturbation of the interstitial capillary circulation with consequent chronic hypoxia could be critical to the progressive nature of many renal diseases, we developed a dynamic method of measuring renal cortical pO(2) and sought to determine whether agents which block the renal effects of angiotensin II (AII) could affect interstitial microvascular oxygenation in the normal rat kidney. METHODS: Instrumented, anaesthetised adult male Sprague-Dawley rats were studied. Cortical microvascular pO(2 )was measured on the surface of the exposed kidney using protoporphyrin phosphorescence. Blood pressure and renal artery blood flow (Doppler flowmetry) were measured concurrently over a 180-min experimental period. Animals received non-hypotensive doses of enalaprilat (100 microg/kg i.v.) or candesartan (40 microg/kg i.v.) either at the beginning of the experimental period or after an initial decline in cortical microvascular pO(2). RESULTS: After a 30-min stabilisation period there was a slow decline in pO(2 )from 48.6 +/- 4.1 to 38.5 +/- 6.9 mm Hg in control animals over the 180-min experimental period. Administration of the ACE inhibitor, enalaprilat at the beginning of the experimental period, completely abrogated this decline and protected pO(2) levels throughout this period with no effect on blood pressure or renal blood flow. In separate experiments, administration of enalaprilat after microvascular pO(2) had fallen by 5 mm Hg, resulted in a rise in RBF and pO(2 )within 15 min with pO(2) remaining elevated for up to 60 min post-injection. The angiotensin II AT(1) receptor antagonist, candesartan, had a similar effect to enalaprilat, inducing a rapid and sustained elevation in cortical pO(2). CONCLUSIONS: These studies indicate that blockade of AII raises pO(2 )in the interstitial microvascular compartment of the normal rat kidney. This effect may contribute to the renoprotective action of ACE inhibitors and AII receptor antagonists in slowing the progression of chronic renal diseases.