Adenosine and nitric oxide regulate regional vascular resistance via interdependent and independent mechanisms during sepsis

Nicotine Improves Survivability, Hypotension, and Impaired Adenosinergic Renal Vasodilations in Endotoxic Rats: Role of α7-nAChRs/HO-1 Pathway

The nicotinic/cholinergic antiinflammatory pathway protects against acute kidney injury and other end-organ damages induced by endotoxemia. In this study, we tested the hypothesis that functional α7-nAChRs/heme oxygenase-1 (HO-1) pathway is imperative for the nicotine counteraction of hemodynamic and renovascular dysfunction caused by acute endotoxemia in rats. Renal vasodilations were induced by cumulative bolus injections of acetylcholine (ACh, 0.01 nmol-7.29 nmol) or ethylcarboxamidoadenosine (NECA, adenosine receptor agonist, 1.6 nmol-100 nmol) in isolated phenylephrine-preconstricted perfused kidneys. The data showed that 6-h treatment with lipopolysaccharide (LPS, 5 mg/kg i.p.) decreased systolic blood pressure and renal vasodilations caused by NECA but not Ach. The endotoxic insult also increased the mortality rate and elevated serum urea and creatinine. These LPS effects were sex-unrelated, except hypotension, and enhanced mortality which were more evident in male rodents, and abrogated after co-administration of nicotine (0.5, 1 mg/kg and 2 mg/kg) in a dose-dependent fashion. The advantageous effects of nicotine on NECA vasodilations, survivability, and kidney biomarkers in endotoxic male rats disappeared upon concurrent exposure to methyllycaconitine citrate (α7-nAChR blocker) or zinc protoporphyrin (HO-1 inhibitor) and were reproduced after treatment with bilirubin, but not hemin (HO-1 inducer) or tricarbonyldichlororuthenium (II) dimer (carbon monoxide-releasing molecule). Together, current biochemical and pharmacological evidence suggests key roles for α7-nAChRs and the bilirubin byproduct of the HO-1 signaling in the nicotine counteraction of renal dysfunction and reduced adenosinergic renal vasodilator capacity in endotoxic rats.

Effects of impaired microvascular flow regulation on metabolism-perfusion matching and organ function

Impaired tissue oxygen delivery is a major cause of organ damage and failure in critically ill patients, which can occur even when systemic parameters, including cardiac output and arterial hemoglobin saturation, are close to normal. This review addresses oxygen transport mechanisms at the microcirculatory scale, and how hypoxia may occur in spite of adequate convective oxygen supply. The structure of the microcirculation is intrinsically heterogeneous, with wide variations in vessel diameters and flow pathway lengths, and consequently also in blood flow rates and oxygen levels. The dynamic processes of structural adaptation and flow regulation continually adjust microvessel diameters to compensate for heterogeneity, redistributing flow according to metabolic needs to ensure adequate tissue oxygenation. A key role in flow regulation is played by conducted responses, which are generated and propagated by endothelial cells and signal upstream arterioles to dilate in response to local hypoxia. Several pathophysiological conditions can impair local flow regulation, causing hypoxia and tissue damage leading to organ failure. Therapeutic measures targeted to systemic parameters may not address or may even worsen tissue oxygenation at the microvascular level. Restoration of tissue oxygenation in critically ill patients may depend on restoration of endothelial cell function, including conducted responses.

Effects of cardiac preload reduction and dobutamine on hepatosplanchnic blood flow regulation in porcine endotoxemia

Insufficient cardiac preload and impaired contractility are frequent in early sepsis. We explored the effects of acute cardiac preload reduction and dobutamine on hepatic arterial (Qha) and portal venous (Qpv) blood flows during endotoxin infusion. We hypothesized that the hepatic arterial buffer response (HABR) is absent during preload reduction and reduced by dobutamine. In anesthetized pigs, endotoxin or vehicle (n = 12, each) was randomly infused for 18 h. HABR was tested sequentially by constricting superior mesenteric artery (SMA) or inferior vena cava (IVC). Afterward, dobutamine at 2.5, 5.0, and 10.0 μg/kg per minute or another vehicle (n = 6, each) was randomly administered in endotoxemic and control animals, and SMA was constricted during each dose. Systemic (cardiac output, thermodilution) and carotid, splanchnic, and renal blood flows (ultrasound Doppler) and blood pressures were measured before and during administration of each dobutamine dose. HABR was expressed as hepatic arterial pressure/flow ratio. Compared with controls, 18 h of endotoxin infusion was associated with decreased mean arterial blood pressure [49 ± 11 mmHg vs. 58 ± 8 mmHg (mean ± SD); P = 0.034], decreased renal blood flow, metabolic acidosis, and impaired HABR during SMA constriction [0.32 (0.18-1.32) mmHg/ml vs. 0.22 (0.08-0.60) mmHg/ml; P = 0.043]. IVC constriction resulted in decreased Qpv in both groups; whereas Qha remained unchanged in controls, it decreased after 18 h of endotoxemia (P = 0.031; constriction-time-group interaction). One control and four endotoxemic animals died during the subsequent 6 h. The maximal increase of cardiac output during dobutamine infusion was 47% (22-134%) in controls vs. 53% (37-85%) in endotoxemic animals. The maximal Qpv increase was significant only in controls [24% (12-47%) of baseline (P = 0.043) vs. 17% (-7-32%) in endotoxemia (P = 0.109)]. Dobutamine influenced neither Qha nor HABR. Our data suggest that acute cardiac preload reduction is associated with preferential hepatic arterial perfusion initially but not after established endotoxemia. Dobutamine had no effect on the HABR.

Regional haemodynamic responses to adenosine receptor activation vary across time following lipopolysaccharide treatment in conscious rats

BACKGROUND AND PURPOSE

Studies using adenosine receptor antagonists have shown that adenosine-mediated vasodilatations play an important role in the maintenance of regional perfusion during sepsis, but it is unclear whether vascular sensitivity to adenosine is affected. Here, we assessed regional haemodynamic responses to adenosine agonists and antagonists in normal and lipopolysaccharide (LPS)-treated rats to investigate a possible role for adenosine in the haemodynamic sequelae.

EXPERIMENTAL APPROACH

Male Sprague-Dawley rats were chronically instrumented with pulsed Doppler flow probes to measure regional haemodynamic responses to adenosine-receptor agonists (adenosine, 2-choloro-N6-cyclopentyladenosine (CCPA)) and antagonists (8-phenyltheophylline (8-PT), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX)), at selected time points in control and LPS-treated rats.

KEY RESULTS

The responses to 8-PT were consistent with endogenous adenosine causing bradycardia, and renal and hindquarters vasodilatation in control rats, whereas in LPS-treated rats, there was evidence for endogenous adenosine causing renal (at 1.5 h) and hindquarters (at 6 h) vasoconstriction. In control animals, exogenous adenosine caused hypotension, tachycardia and widespread vasodilatation, whereas in LPS-treated rats, the adenosine-induced renal (at 1.5 h) and hindquarters (at 6 h) vasodilatations were abolished. As enhanced A1 receptor-mediated vasoconstriction could explain the results in LPS-treated rats, vascular responsiveness to a selective A1-receptor agonist (CCPA) or antagonist (DPCPX) was assessed. There was no evidence for enhanced vasoconstrictor responsiveness to CCPA in LPS-treated rats, but DPCPX caused renal vasodilatation, consistent with endogenous adenosine mediating renal vasoconstriction under these conditions.

CONCLUSIONS AND IMPLICATIONS

The results show changes in adenosine receptor-mediated cardiovascular effects in endotoxaemia that may have implications for the use of adenosine-based therapies in sepsis.

THE EXTRACELLULAR CARDIAC PURINE METABOLOME IN SEPSIS

The total cardiac purine metabolome includes all of the adenine and guanine nucleoside and nucleosides and related molecules involved throughout the intracellular and extracellular compartments and various cell types in the heart. In considering purines as molecules involved in autocrine and paracrine communication, effective interstitial concentrations of the nucleoside adenosine, or purine metabolites, are of greatest interest. These molecules arise from the complex interactions between cardiac-specific cell types, including fibroblasts and myocytes, and noncardiac cells, such as tissue-resident macrophages and other immune cells that have vascular access. In the interstitial environment, adenosine can regulate vascular resistance, contractile function, and immunochemical interactions. The breakdown of purines can produce reactive oxygen species that also influence autocrine and paracrine interactions. A central enzyme in this paradigm, adenosine deaminase, is a pivotal molecule in regulating the balance between pro-inflammatory and anti-inflammatory signaling cascades. A new role for adenosine deaminase as an allosteric regulator of relevant membrane proteins has yet to be explored in the heart.

Adenosine A1 and A2 receptor agonists reduce endotoxin-induced cellular energy depletion and oedema formation in the lung

BACKGROUND AND OBJECTIVE

Tissue depletion of adenosine during endotoxaemia has previously been described in the lung. Therapeutic approaches to prevent adenosine depletion and the role of A1 and A2 receptor agonists, however, have not been investigated until now.

METHODS

In isolated and ventilated rabbit lungs, it was tested whether pretreatment with adenosine A1 agonist 2-chloro-N6-cyclopentyladenosine (CCPA; 10(-7) mol, n = 6) or A2 receptor agonist 5'-(N-cyclopropyl)-carboxyamido adenosine (CPCA; 10(-7) mol, n = 6) prior to injection of lipopolysaccharide (LPS) (500 pg mL-1) influenced pulmonary artery pressure (PAP), pulmonary energy content and oedema formation as compared with controls, solely infused with LPS (n = 6). Release rates of adenosine and uric acid were determined by high-performance liquid chromatography. Pulmonary tissue concentrations of high-energy phosphates were measured and the adenine nucleotide pool, adenosine 5'-triphosphate (ATP)/adenosine 5'-diphosphate (ADP) ratio and adenylate energy charge of the pulmonary tissue were calculated.

RESULTS

Administration of LPS induced increases in PAP within 2 h up to 20.8 +/- 2.9 mmHg (P < 0.01). While pretreatment with the A1 agonist merely decelerated pressure increase (13.8 +/- 1.1 mmHg, P < 0.05), the A2 agonist completely suppressed the pulmonary pressure reaction (9.6 +/- 1.0 mmHg, P < 0.01). Emergence of lung oedema after exclusive injection of LPS up to 12.0 +/- 2.9 g was absent after A1 (0.6 +/- 0.5 g) and A2 (-0.3 +/- 0.2 g) agonists. These observations were paralleled by increased adenosine release rates compared with LPS controls (P < 0.05). Moreover, tissue concentrations of ADP, ATP, guanosine 5'-diphosphate, guanosine 5'-triphosphate, nicotinamide-adenine-dinucleotide and creatine phosphate were significantly reduced after LPS. Consequently, the calculated tissue adenine nucleotide pool and the adenylate energy charge increased after adenosine receptor stimulation (P = 0.001).

CONCLUSIONS

Adenosine A1- and A2-receptor agonists reduced LPS-induced vasoconstriction and oedema formation by maintenance of tissue energy content. Thus, adenosine receptor stimulation, in particular of the A2 receptor, might be beneficial during acute lung injury.

Renal Vascular Resistance in Sepsis

AIMS

To assess changes in renal vascular resistance (RVR) in human and experimental sepsis and to identify determinants of RVR.

METHODS

We performed a systematic interrogation of two electronic reference libraries using specific search terms. Subjects were animals and patients involved in experimental and human studies of sepsis and septic acute renal failure, in which the RVR was assessed. We obtained all human and experimental articles reporting RVR during sepsis. We assessed the role of various factors that might influence the RVR during sepsis using statistical methods.

RESULTS

We found no human studies, in which the renal blood flow (and, therefore, the RVR) was measured with suitably accurate direct methods. Of the 137 animal studies identified, 52 reported a decreased RVR, 16 studies reported no change in RVR, and 69 studies reported an increased RVR. Consciousness of animals, duration of measurement, method of induction of sepsis, and fluid administration had no effect on the RVR. On the other hand, on univariate analysis, size of the animals (p < 0.001), technique of measurement (p = 0.017), recovery after surgery (p = 0.004), and cardiac output (p < 0.001) influenced the RVR. Multivariate analysis, however, showed that only cardiac output (p = 0.028) and size of the animals (p = 0.031) remained independent determinants of the RVR.

CONCLUSIONS

Changes in RVR during sepsis in humans are unknown. In experimental sepsis, several factors not directly related to sepsis per se appear to influence the RVR. A high cardiac output and the use of large animals predict a decreased RVR, while a decreased cardiac output and the use of small animals predict an increased RVR.

Renal blood flow in sepsis

Introduction

To assess changes in renal blood flow (RBF) in human and experimental sepsis, and to identify determinants of RBF.

Method

Using specific search terms we systematically interrogated two electronic reference libraries to identify experimental and human studies of sepsis and septic acute renal failure in which RBF was measured. In the retrieved studies, we assessed the influence of various factors on RBF during sepsis using statistical methods.

Results

We found no human studies in which RBF was measured with suitably accurate direct methods. Where it was measured in humans with sepsis, however, RBF was increased compared with normal. Of the 159 animal studies identified, 99 reported decreased RBF and 60 reported unchanged or increased RBF. The size of animal, technique of measurement, duration of measurement, method of induction of sepsis, and fluid administration had no effect on RBF. In contrast, on univariate analysis, state of consciousness of animals (P = 0.005), recovery after surgery (P < 0.001), haemodynamic pattern (hypodynamic or hyperdynamic state; P < 0.001) and cardiac output (P < 0.001) influenced RBF. However, multivariate analysis showed that only cardiac output remained an independent determinant of RBF (P < 0.001).

Conclusion

The impact of sepsis on RBF in humans is unknown. In experimental sepsis, RBF was reported to be decreased in two-thirds of studies (62 %) and unchanged or increased in one-third (38%). On univariate analysis, several factors not directly related to sepsis appear to influence RBF. However, multivariate analysis suggests that cardiac output has a dominant effect on RBF during sepsis, such that, in the presence of a decreased cardiac output, RBF is typically decreased, whereas in the presence of a preserved or increased cardiac output RBF is typically maintained or increased.

Therapeutic potential for transient inhibition of adenosine deaminase in systemic inflammatory response syndrome

OBJECTIVE

We sought to determine the potential usefulness of 2'-deoxycoformycin (pentostatin), an inhibitor of adenosine deaminase, as a postinsult, or prophylactic treatment for systemic inflammatory response syndrome resulting from fecal peritonitis.

DESIGN

Prospective, randomized, controlled experiment.

SETTING

Small animal basic science laboratory.

SUBJECTS

Male Spague-Dawley rats, weighing 300 to 350 g.

INTERVENTIONS

Rats with fecal peritonitis (intraperitoneal cecal slurry) were treated with 1 mg/kg pentostatin intraperitoneally 24 hrs before, or intravenously when signs of illness presented (2 hrs after induction of peritonitis). Signs of illness included tachycardia, tachypnea, and leukopenia. All rats received 50 mL/kg 0.9% saline resuscitative fluid at 2 hrs.

MEASUREMENTS AND MAIN RESULTS

Survival to day 6 was 100% in nonseptic sham rats, but 33% in untreated septic rats. In rats given pentostatin either 2 hrs after the insult, or 24 hrs before the insult, 6-day survival improved to 81% and 78%, respectively. Histology revealed diffuse peritonitis, and evidence of systemic inflammatory response syndrome, including local and distant site vascular damage and leukocyte activation. These responses to the septic challenge were abrogated by pentostatin treatment. Return of significant amount of tissue adenosine deaminase activity by 24 hrs and later recovery of white blood cell counts argue against any potential for inappropriate immunosuppression by pentostatin.

CONCLUSIONS

These data indicate that the novel use of pentostatin to prevent systemic inflammatory response syndrome secondary to fecal peritonitis shows uncommon promise as a therapeutic tool. All indices of systemic inflammatory response syndrome were abrogated and survival improved when pentostatin was not given until after signs of the illness became manifest. Because protection was afforded with treatment 24 hrs in advance of the inciting insult, pentostatin also has the unique potential for use as a true prophylactic agent.

Adenosine triphosphate-magnesium dichloride during hyperdynamic porcine endotoxemia: effects on hepatosplanchnic oxygen exchange and metabolism

OBJECTIVE

To assess the effects of adenosine triphosphate-magnesium dichloride (ATP-MgCl2) on systemic and hepatosplanchnic hemodynamics, oxygen exchange, and energy metabolism over 24 hrs of hyperdynamic normotensive porcine endotoxemia.

DESIGN

Prospective, randomized, controlled experimental study with repeated measures.

SETTING

Investigational animal laboratory.

SUBJECTS

Seventeen pigs were divided into two groups: eight animals receiving endotoxin served as a control group and nine animals received endotoxin (lipopolysaccharide) and ATP-MgCl2.

INTERVENTIONS

Pigs were anesthetized, mechanically ventilated, and instrumented. Endotoxemia was achieved by continuous intravenous infusion of Escherichia coli lipopolysaccharide. Animals were resuscitated by hetastarch targeted to maintain mean arterial pressure of >75 mm Hg. Twelve hours after the start of the endotoxin infusion, ATP-MgCl2, or its vehicle, were administered for 12 hrs.

MEASUREMENTS AND MAIN RESULTS

Mean arterial pressure was maintained in the control group because of a sustained increase in cardiac output achieved by fluid resuscitation, whereas ATP-MgCl2 significantly decreased mean arterial pressure because of further systemic vasodilatation. ATP-MgCl2 markedly increased portal venous flow. In contrast to the controls, hepatic arterial flow remained unchanged until the end of the experiment, despite the further increase in cardiac output. The ileal mucosal-arterial PCO2 gap (Delta PCO2) progressively increased (p <.05) in control animals, whereas it was restored to prelipopolysaccharide levels during ATP-MgCl2 infusion. Changes in Delta PCO2 correlated with those of portal vein blood flow in these animals (r = -.68, p <.05). Moreover, ATP-MgCl2 blunted the lipopolysaccharide-induced decrease in hepatic lactate balance but did not affect portal venous pH, hepatosplanchnic oxygen exchange, splanchnic lactate/pyruvate ratios, isoprostane, NO2- + NO3-, cytokine concentrations, or tissue nucleotide content.

CONCLUSION

During long-term hyperdynamic porcine endotoxemia, ATP-MgCl2 normalized the otherwise progressive rise of the ileal mucosal-arterial Delta PCO2. Furthermore, it allowed blunting of the continuous decrease in hepatic lactate clearance, thus preserving the metabolic coupling between lactate release from the intestine and lactate utilization by the liver.

Inhibiting adenosine deaminase modulates the systemic inflammatory response syndrome in endotoxemia and sepsis

By pharmacological manipulation of endogenous adenosine, using chemically distinct methods, we tested the hypothesis that endogenous adenosine tempers proinflammatory cytokine responses and oxyradical-mediated tissue damage during endotoxemia and sepsis. Rats were pretreated with varying doses of pentostatin (PNT; adenosine deaminase inhibitor) or 8-sulfophenyltheophylline (8-SPT; adenosine receptor antagonist) and then received either E. coli endotoxin (lipopolysaccharide; 0.01 or 2.0 mg/kg) or a slurry of cecal matter in 5% dextrose in water (200 mg/kg). Resultant levels of tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and IL-10 were measured in serum and in liver and spleen. Untreated, 2 mg/kg lipopolysaccharide elevated serum TNF-alpha, IL-1beta, and IL-10. PNT dose dependently attenuated, without ablating, the elevation in serum TNF-alpha and IL-1beta and raised liver and spleen IL-10. PNT also attenuated elevation of TNF-alpha in serum, liver, and spleen at 4 and 24 h after sepsis induction, and 8-SPT resulted in higher proinflammatory cytokines. Modulating endogenous adenosine was also effective in exacerbated (8-SPT) or diminished (PNT) tissue peroxidation. Survival from sepsis was also improved when PNT was used as a posttreatment. These data indicate that endogenous adenosine is an important modulatory component of systemic inflammatory response syndromes. These data also indicate that inhibition of adenosine deaminase may be a novel and viable therapeutic approach to managing the systemic inflammatory response syndrome without ablating important physiological functions.

Is nitric oxide overproduction the target of choice for the management of septic shock?

Sepsis is a heterogeneous class of syndromes caused by a systemic inflammatory response to infection. Septic shock, a severe form of sepsis, is associated with the development of progressive damage in multiple organs, and is a leading cause of patient mortality in intensive care units. Despite important advances in understanding its pathophysiology, therapy remains largely symptomatic and supportive. A decade ago, the overproduction of nitric oxide (NO) had been discovered as a potentially important event in this condition. As a result, great hopes arose that the pharmacological inhibition of NO synthesis could be developed into an efficient, mechanism-based therapeutic approach. Since then, an extraordinary effort by the scientific community has brought a deeper insight regarding the feasibility of this goal. Here we present in summary form the present state of knowledge of the biological chemistry and physiology of NO. We then proceed to a systematic review of experimental and clinical data, indicating an up-regulation of NO production in septic shock; information on the role of NO in septic shock, as provided by experiments in transgenic mice that lack the ability to up-regulate NO production; effects of pharmacological inhibitors of NO production in various experimental models of septic shock; and relevant clinical experience. The accrued evidence suggests that the contribution of NO to the pathophysiology of septic shock is highly heterogeneous and, therefore, difficult to target therapeutically without appropriate monitoring tools, which do not exist at present.

Interdependence of adenosine and nitric oxide in hepato-splanchnic circulation during sepsis

BACKGROUND

We tested the hypothesis that some of the maintenance of resting, regional hepato-splanchnic perfusion that is mediated by endogenous adenosine (ADO) during sepsis is interdependent with nitric oxide (NO).

MATERIALS AND METHODS

Twenty-four hours after sepsis/sham induction, rats were divided into two groups. Group 1 received a 10-min infusion of the ADO antagonist 8-sulfophenyltheophylline (8-SPT; 0.9 mg/kg x min), followed by 10 min of 8-SPT plus L-NAME (0.5 mg/kg x min). Group 2 received L-NAME first followed by 8-SPT in the presence of L-NAME (all groups: n = 6-10). Hemodynamic and regional hepato-splanchnic blood flow measurements were made prior to infusions, 10 min after initiation of each single agent infusion, and again after double agent infusion.

RESULTS

Twenty-four hours after sepsis hepato-splanchnic blood flow was significantly elevated, compared to nonseptic rats. Both ADO receptor blockade alone and NOS inhibition alone decreased total hepato-splanchnic blood flow to nonseptic values. Decreases in small intestinal and cecal blood flow accounted for the majority of this decrease, but decreased hepatic arterial perfusion contributed as well. No further alterations were seen when 8-SPT was infused in the presence of L-NAME, nor when L-NAME was infused in the presence of 8-SPT.

CONCLUSIONS

These data indicate that there is significant interdependence of endogenous NO and ADO in maintaining resting small bowel, cecal, and hepatic arterial perfusion during sepsis. The lack of responses in other regions of the hepato-splanchnic circulation demonstrate regional specificity of this ADO-NO interdependence.