Variable effects of inhibiting iNOS and closing the vascular ATP-sensitive potassium channel (via its pore-forming and sulfonylurea receptor subunits) in endotoxic shock

Novel relationships between oxidative stress and angiogenesis-related factors in sepsis: New biomarkers and therapies

Sepsis is a systemic uncontrolled inflammatory response in the presence of an infection. It remains a major cause of morbidity and mortality in hospitalized patients. According to its severity, sepsis can progress to three different states: severe sepsis, septic shock, and multiple organ dysfunction syndrome, related to organ dysfunction and/or tissue hypoperfusion. Different processes underlie its pathophysiology; among them are oxidative stress, endothelial and mitochondrial dysfunction, and angiogenesis-related factors. However, no studies have integrated these elements in sepsis. The main difficulty in sepsis is its diagnosis. Currently, the potential of inflammatory biomarkers in septic patients remains weak. In this context, the research into new biomarkers is essential to aid with sepsis diagnosis and prognostication. Furthermore, even though the current management of severe forms of sepsis has been effective, morbimortality remains elevated. Therefore, it is essential to explore alternative approaches to therapy development. The aim of this review is to present an update of evidence supporting the role of oxidative stress and angiogenesis-related factors in the pathophysiology of the different forms of sepsis. It proposes a novel convergence between both elements in their role in the disease, and it will cover their utility as new diagnostic tools, predictors of outcome, and as novel therapeutic targets.

The role of ATP-sensitive potassium channels in cellular function and protection in the cardiovascular system

ATP-sensitive potassium channels (K(ATP)) are widely distributed and present in a number of tissues including muscle, pancreatic beta cells and the brain. Their activity is regulated by adenine nucleotides, characteristically being activated by falling ATP and rising ADP levels. Thus, they link cellular metabolism with membrane excitability. Recent studies using genetically modified mice and genomic studies in patients have implicated K(ATP) channels in a number of physiological and pathological processes. In this review, we focus on their role in cellular function and protection particularly in the cardiovascular system.

Inhibition of vascular adenosine triphosphate-sensitive potassium channels by sympathetic tone during sepsis

OBJECTIVE

Excessive opening of the adenosine triphosphate-sensitive potassium channel in vascular smooth muscle is implicated in the vasodilation and vascular hyporeactivity underlying septic shock. Therapeutic channel inhibition using sulfonylurea agents has proved disappointing, although agents acting on its pore appear more promising. We thus investigated the hemodynamic effects of adenosine triphosphate-sensitive potassium channel pore inhibition in awake, fluid-resuscitated septic rats, and the extent to which these responses are modulated by the high sympathetic tone present in sepsis. Temporal changes in ex-vivo channel activity and subunit gene expression were also investigated.

DESIGN

In vivo and ex vivo animal study.

SETTING

University research laboratory.

SUBJECTS

Male adult Wistar rats.

INTERVENTIONS AND MEASUREMENTS

Fecal peritonitis was induced in conscious, fluid-resuscitated rats. Pressor responses to norepinephrine and PNU-37883A (a vascular adenosine triphosphate-sensitive potassium channel inhibitor acting on the Kir6.1 pore-forming subunit) were measured at 6 or 24 hrs, in the absence or presence of the autonomic ganglion blocker, pentolinium. The aorta and mesenteric artery were examined ex vivo for rubidium efflux as a marker of adenosine triphosphate-sensitive potassium channel activity, and for adenosine triphosphate-sensitive potassium channel subunit gene expression using quantitative reverse transcription-polymerase chain reaction.

MAIN RESULTS

A total of 120 rats (50 sham-operated controls, 70 septic) were included. Septic rats became hypotensive after 12 hrs, with a 24-hr mortality of 51.7% (0% in controls). At 6 hrs, there was an attenuated pressor response to norepinephrine (p < .01) despite blood pressure being elevated (p < .01). PNU-37883A had no pressor effect, except in the presence of pentolinium (p < .01). Kir6.1 subunit mRNA increased significantly in the mesenteric artery while rubidium efflux was increased in both the aorta and mesenteric artery at 24 hrs.

CONCLUSIONS

Despite evidence of increased adenosine triphosphate-sensitive potassium channel activity in sepsis, it appears to be inhibited in vivo by high sympathetic tone. This may explain, at least in part, the reduced efficacy of adenosine triphosphate-sensitive potassium channel blockers in human septic shock.

Early potassium channel blockade improves sepsis-induced organ damage and cardiovascular dysfunction

BACKGROUND AND PURPOSE

There is increasing evidence that potassium channels are involved in the cardiovascular dysfunction of sepsis. This evidence was obtained after the systemic inflammation, cardiovascular dysfunction and organ damage had developed. Here we have studied the consequences of early interference with potassium channels on development of sepsis.

EXPERIMENTAL APPROACH

Sepsis was induced by caecal ligation and puncture (CLP) or sham surgery in Wistar rats. Four hours after surgery, animals received tetraethylammonium (TEA; a non-selective potassium channel blocker) or glibenclamide (a selective ATP-sensitive potassium channel blocker). Twenty-four hours after surgery, inflammatory, biochemical, haemodynamic parameters and survival were evaluated.

KEY RESULTS

Sepsis significantly increased plasma NO(x) levels, expression of inducible nitric oxide synthase (NOS-2) protein in lung and thigh skeletal muscle, lung myeloperoxidase, urea, creatinine and lactate levels, TNF-α and IL-1β, hypotension and hyporesponsiveness to phenylephrine and hyperglycemia followed by hypoglycemia. TEA injected 4 h after surgery attenuated the increased NOS-2 expression, reduced plasma NO(x) , lung myeloperoxidase activity, levels of TNF-α and IL-1β, urea, creatinine and lactate levels, prevented development of hypotension and hyporesponsiveness to phenylephrine, the alterations in plasma glucose and reduced late mortality by 50%. Glibenclamide did not improve any of the measured parameters and increased mortality rate, probably due to worsening the hypoglycemic phase of sepsis.

CONCLUSIONS AND IMPLICATIONS

Early blockade of TEA-sensitive (but not the ATP-sensitive subtype) potassium channels reduced organ damage and mortality in experimental sepsis. This beneficial effect seems to be, at least in part, due to reduction in NOS-2 expression.

An integrated approach to assessing nitroso-redox balance in systemic inflammation

Most studies examining the metabolic fate of NO during systemic inflammation have focused on measuring the quantitatively predominating, stable anions nitrite and nitrate within the circulation. However, these are not necessarily the NO-related products that govern NO metabolism and signaling in tissues. We assessed all major NO derivatives temporally in blood and vital organs during inflammation and explored their relationship to insult severity and redox status. Male rats receiving intraperitoneal endotoxin or vehicle were sacrificed for organ and blood sampling between 0 and 24 h. Endotoxin induced transient and organ-specific changes in a variety of NO metabolites. Nitrite and nitrate increased, peaking at 8 and 12 h, respectively. S- and N-nitrosation and heme-nitrosylation products also peaked at 8 h; these posttranslational protein modifications were associated with decreased myocardial function (echocardiography). Evidence of oxidative stress and systemic inflammation was also obtained. The rise in most NO derivatives was proportional to insult severity. All metabolite levels normalized within 24 h, despite evidence of persisting myocardial dysfunction and clinical unwellness. Our findings point to a complex interplay between NO production, antioxidant defense, and redox status. Although the precise (patho)physiologic roles of specific NO derivatives and their diagnostic/prognostic utility await further investigation, nitroso species in erythrocytes are the most sensitive markers of NO in systemic inflammation, detectable before clinical symptoms manifest.

Differential involvement of potassium channel subtypes in early and late sepsis-induced hyporesponsiveness to vasoconstrictors

This study investigated the involvement of potassium channel subtypes in the hyporesponsiveness to vasoconstrictors of an experimental model of sepsis [cecal ligation and puncture (CLP)], at 2 time points, namely, 6 and 24 hours after sepsis onset. Wistar rats were submitted to CLP or sham surgery, and 6 and 24 hours later, responses to phenylephrine were obtained before and 30 minutes after injection of potassium channel blockers. The potassium channel blockers used were tetraethylammonium (TEA; a nonselective channel blocker), glibenclamide (GLB; an adenosine triphosphate -dependent channel blocker), 4-aminopyridine (4-AP; a voltage-dependent channel blocker), apamin (APA; a small-conductance calcium-dependent channel blocker), and iberiotoxin (IBTX; a large-conductance calcium-dependent channel blocker). It was found that (1) sepsis caused a severe vascular hyporesponsiveness to phenylephrine both 6 and 24 hours after CLP, (2) TEA partially reversed the hyporesponsiveness 6 hours after CLP and completely restored vascular response to phenylephrine 24 hours after CLP, (3) apamin reversed hyporesponsiveness 6 but not 24 hours after CLP, (4) GLB restored phenylephrine response only 24 hours after CLP, and (5) IBTX and 4-AP were ineffective in all periods studied. Our results suggest that potassium channels are important effectors of sepsis-induced vascular dysfunction in vivo and that different subtypes of potassium channels are involved in early (small-conductance calcium-dependent potassium channels) and late (adenosine triphosphate -dependent potassium channels) hyporesponsiveness to vasoconstrictors caused by sepsis.