Ana Gutierrez del Arroyo

Mechanisms of CO2/H+ Sensitivity of Astrocytes

Ventral regions of the medulla oblongata of the brainstem are populated by astrocytes sensitive to physiological changes in PCO2/[H+]. These astrocytes respond to decreases in pH with elevations in intracellular Ca2+ and facilitated exocytosis of ATP-containing vesicles. Released ATP propagates Ca2+ excitation among neighboring astrocytes and activates neurons of the brainstem respiratory network triggering adaptive increases in breathing. The mechanisms linking increases in extracellular and/or intracellular PCO2/[H+] with Ca2+ responses in chemosensitive astrocytes remain unknown. Fluorescent imaging of changes in [Na+]i and/or [Ca2+]i in individual astrocytes was performed in organotypic brainstem slice cultures and acute brainstem slices of adult rats. It was found that astroglial [Ca2+]i responses triggered by decreases in pH are preceded by Na+ entry, markedly reduced by inhibition of Na+/HCO3− cotransport (NBC) or Na+/Ca2+ exchange (NCX), and abolished in Na+-free medium or by combined NBC/NCX blockade. Acidification-induced [Ca2+]i responses were also dramatically reduced in brainstem astrocytes of mice deficient in the electrogenic Na+/HCO3− cotransporter NBCe1. Sensitivity of astrocytes to changes in pH was not affected by inhibition of Na+/H+ exchange or blockade of phospholipase C. These results suggest that in pH-sensitive astrocytes, acidification activates NBCe1, which brings Na+ inside the cell. Raising [Na+]i activates NCX to operate in a reverse mode, leading to Ca2+ entry followed by activation of downstream signaling pathways. Coupled NBC and NCX activities are, therefore, suggested to be responsible for functional CO2/H+ sensitivity of astrocytes that contribute to homeostatic regulation of brain parenchymal pH and control of breathing. SIGNIFICANCE STATEMENT Brainstem astrocytes detect physiological changes in pH, activate neurons of the neighboring respiratory network, and contribute to the development of adaptive respiratory responses to the increases in the level of blood and brain PCO2/[H+]. The mechanisms underlying astroglial pH sensitivity remained unknown and here we show that in brainstem astrocytes acidification activates Na+/HCO3− cotransport, which brings Na+ inside the cell. Raising [Na+]i activates the Na+/Ca2+ exchanger to operate in a reverse mode leading to Ca2+ entry. This identifies a plausible mechanism of functional CO2/H+ sensitivity of brainstem astrocytes, which play an important role in homeostatic regulation of brain pH and control of breathing.

Low-molecular-weight polyethylene glycol improves survival in experimental sepsis.

Objective: For several chronic inflammatory disease states, therapy is enhanced by improving the pharmacokinetic properties of anti-inflammatory drugs through conjugation with polyethylene glycol. We hypothesized that part of the beneficial action of PEGylated drugs may be derived from the anti-inflammatory properties of polyethylene glycol (PEG) itself. Design: Randomized, double-blinded, controlled ex vivo and in vivo laboratory studies. Setting: University research laboratories. Subjects: Human neutrophils and mononuclear cells, macrophage cell line, and adult rats and mice. Interventions: The effect of PEG (either low-molecular-weight [200–400] or high-molecular-weight [>4000]) was assessed on survival after systemic inflammation induced by lipopolysaccharide or zymosan. The effects of PEG on zymosan, lipopolysaccharide, or streptolysin-induced inflammatory and bioenergetic responses of immune cells were also assessed. Measurements and Main Results: Low-molecular-weight PEG reduced inflammatory cytokine expression, pyrexia, and mortality by >50% in both lipopolysaccharide and zymosan models of sepsis. Low-molecular-weight PEG reduced cytokine expression both in vivo and in vitro, and attenuated activation of human neutrophils in response to lipopolysaccharide or zymosan. By contrast, high-molecular-weight PEG conferred less significant survival effects after lipopolysaccharide and zymosan, and it did not exhibit such profound anti-inflammatory effects. Low-molecular-weight PEG attenuated lipopolysaccharide-induced activation of pro-apoptotic pathways (lysophosphatidic acid receptor and caspase-domain signaling) in the livers of endotoxemic rats. Streptolysin-induced necrosis of human neutrophils was reduced by low-molecular-weight PEG, indicating a mechanism that involves coating and/or stabilizing the cellular membrane. Low-molecular-weight PEG preserved human neutrophil responses to septic serum and bioenergetic function in macrophages and neutrophils. Conclusion: PEG is a commonly used, safe, nonimmunogenic molecule possessing hitherto unappreciated anti-inflammatory properties. Low-molecular-weight PEG may potentially play a role in the therapy of systemic inflammation and sepsis.

Baroreflex impairment and morbidity after major surgery.

BACKGROUND Baroreflex dysfunction is a common feature of established cardiometabolic diseases that are most frequently associated with the development of critical illness. Laboratory models show that baroreflex dysfunction impairs cardiac contractility and cardiovascular performance, thereby increasing the risk of morbidity after trauma and sepsis. We hypothesized that baroreflex dysfunction contributes to excess postoperative morbidity after major surgery as a consequence of the inability to achieve adequate perioperative tissue oxygen delivery. METHODS In a randomized controlled trial of goal-directed haemodynamic therapy (GDT) in higher-risk surgical patients, baroreflex function was assessed using the spontaneous baroreflex sensitivity (BRS) method via an arterial line placed before surgery, using a validated sequence method technique (one beat lag). The BRS was calculated during the 6 h postoperative GDT intervention. Analyses of BRS were done by investigators blinded to clinical outcomes. The primary outcome was the association between postoperative baroreflex dysfunction (BRS <6 mm Hg s(-1), a negative prognostic threshold in cardiovascular pathology) and early postoperative morbidity. The relationship between baroreflex dysfunction and postoperative attainment of preoperative indexed oxygen delivery was also assessed. RESULTS Patients with postoperative baroreflex dysfunction were more likely to sustain infectious {relative risk (RR) 1.75 [95% confidence interval (CI): 1.07-2.85], P=0.02} and cardiovascular morbidity [RR 2.39 (95% CI: 1.22-4.71), P=0.008]. Prolonged hospital stay was more likely in patients with baroreflex dysfunction [unadjusted hazard ratio 1.62 (95% CI: 1.14-2.32), log-rank P=0.004]. Postoperative O2 delivery was 36% (95% CI: 7-65) lower in patients with baroreflex dysfunction in those not randomly assigned to GDT (P=0.02). CONCLUSIONS Baroreflex dysfunction is associated with excess morbidity, impaired cardiovascular performance, and delayed hospital discharge, suggesting a mechanistic role for autonomic dysfunction in determining perioperative outcome. CLINICAL TRIAL REGISTRATION ISCRTN76894700.

Autonomic regulation of systemic inflammation in humans: A multi-center, blinded observational cohort study.

OBJECTIVE Experimental animal models demonstrate that autonomic activity regulates systemic inflammation. By contrast, human studies are limited in number and exclusively use heart rate variability (HRV) as an index of cardiac autonomic regulation. HRV measures are primarily dependent on, and need to be corrected for, heart rate. Thus, independent autonomic measures are required to confirm HRV-based findings. Here, the authors sought to replicate the findings of preceding HRV-based studies by using HRV-independent, exercise-evoked sympathetic and parasympathetic measures of cardiac autonomic regulation to examine the relationship between autonomic function and systemic inflammation. METHODS Sympathetic function was assessed by measuring heart rate changes during unloaded pedaling prior to onset of exercise, divided into quartiles; an anticipatory heart rate (AHRR) rise during this period is evoked by mental stress in many individuals. Parasympathetic function was assessed by heart rate recovery (HRR) 60s after finishing cardiopulmonary exercise testing, divided into quartiles. Parasympathetic dysfunction was defined by delayed heart rate recovery (HRR) ≤12.beats.min-1, a threshold value associated with higher cardiovascular morbidity/mortality in the general population. Systemic inflammation was primarily assessed by neutrophil-lymphocyte ratio (NLR), where a ratio >4 is prognostic across several inflammatory diseases and correlates strongly with elevated plasma levels of pro-inflammatory cytokines. High-sensitivity C-reactive protein (hsCRP) was also measured. RESULTS In 1624 subjects (65±14y; 67.9% male), lower HRR (impaired vagal activity) was associated with progressively higher NLR (p=0.004 for trend across quartiles). Delayed HRR, recorded in 646/1624 (39.6%) subjects, was associated with neutrophil-lymphocyte ratio >4 (relative risk: 1.43 (95%CI: 1.18-1.74); P=0.0003). Similar results were found for hsCRP (p=0.045). By contrast, AHRR was not associated with NLR (relative risk: 1.24 (95%CI: 0.94-1.65); P=0.14). CONCLUSIONS Delayed HRR, a robust measure of parasympathetic dysfunction, is independently associated with leukocyte ratios indicative of systemic inflammation. These results further support a role for parasympathetic modulation of systemic inflammation in humans.

Man is the new mouse: Elective surgery as a key translational model for multi-organ dysfunction and sepsis

Translational research in critically ill human patients presents many methodological challenges. Diagnostic uncertainty, coupled with poorly defined comorbidities, make the identification of a suitable control population for case–control investigations an arguably insurmountable challenge. Healthy volunteer experiments using endotoxin infusion as an inflammatory model are methodologically robust, but fail to replicate the onset of, and diverse therapeutic interventions associated with, sepsis/trauma. Animal models are also limited by many of these issues. Major elective surgery addresses many of these shortfalls and offers a key model for exploring the human biology underlying the sepsis syndrome. Surgery triggers highly conserved features of the human inflammatory response that are common to both tissue damage and infection. Surgical patients sustain a predictable and relatively high incidence of sepsis, particularly within the ‘higher risk’ group. The collection of preoperative samples enables each patient to act as their own control. Thus, the surgical model offers unique and elegant experimental design features that provide an important translational bridge between the basic biological understanding afforded by animal laboratory models and the de novo presentation of human sepsis.

Reply: Glucagon-like peptide-1 mediates cardioprotection by remote ischaemic conditioning

We read with interest the Letter by Drs Giblett and Hoole in response to our research article ‘Glucagon-Like Peptide-1 (GLP-1) Mediates Cardioprotection by Remote Ischaemic Conditioning’.1 We thank Drs Giblett and Hoole for their interest in our work and provide our responses to the critical comments raised. First, we believe that it is not entirely appropriate to compare the effects of exogenous application of native GLP-1 or GLP-1 receptor (GLP-1R) agonists with the effects induced by remote ischaemic conditioning (RIc). The efficacy of RIc in protecting against left ventricular dysfunction and myocardial stunning may be compromised by the study design and/or inability of a significant proportion of patients to recruit vagal activity, which appears to be critically important for RIc cardioprotection.2,3 We reported that in rats vagotomy blocks RIc cardioprotection, …

Cardiopulmonary Exercise Capacity and Preoperative Markers of Inflammation

Explanatory mechanisms for the association between poor exercise capacity and infections following surgery are underexplored. We hypothesized that aerobic fitness—assessed by cardiopulmonary exercise testing (CPET)—would be associated with circulating inflammatory markers, as quantified by the neutrophil-lymphocyte ratio (NLR) and monocyte subsets. The association between cardiopulmonary reserve and inflammation was tested by multivariable regression analysis with covariates including anaerobic threshold (AT) and malignancy. In a first cohort of 240 colorectal patients, AT was identified as the sole factor associated with higher NLR (P = 0.03) and absolute and relative lymphopenia (P = 0.01). Preoperative leukocyte subsets and monocyte CD14+ expression (downregulated by endotoxin and indicative of chronic inflammation) were also assessed in two further cohorts of age-matched elective gastrointestinal and orthopaedic surgical patients. Monocyte CD14+ expression was lower in gastrointestinal patients (n = 43) compared to age-matched orthopaedic patients (n = 31). The circulating CD14+CD16− monocyte subset was reduced in patients with low cardiopulmonary reserve. Poor exercise capacity in patients without a diagnosis of heart failure is independently associated with markers of inflammation. These observations suggest that preoperative inflammation associated with impaired cardiorespiratory performance may contribute to the pathophysiology of postoperative outcome.

Arterial pulse pressure and postoperative morbidity in high-risk surgical patients

Background: Systemic arterial pulse pressure (systolic minus diastolic pressure) ≤53 mm Hg in patients with cardiac failure is correlated with reduced stroke volume and is independently associated with accelerated morbidity and mortality. Given that deconditioned surgical and heart failure patients share similar cardiopulmonary physiology, we examined whether lower pulse pressure is associated with excess morbidity after major surgery. Methods: This was a prospective observational cohort study of patients deemed by their preoperative assessors to be at higher risk of postoperative morbidity. Preoperative pulse pressure was calculated before cardiopulmonary exercise testing. The primary outcome was any morbidity (PostOperative Morbidity Survey) occurring within 5 days of surgery, stratified by pulse pressure threshold ≤53 mm Hg. The relationship between pulse pressure, postoperative morbidity, and oxygen pulse (a robust surrogate for left ventricular stroke volume) was examined using logistic regression analysis (accounting for age, sex, BMI, cardiometabolic co‐morbidity, and operation type). Results: The primary outcome occurred in 578/660 (87.6%) patients, but postoperative morbidity was more common in 243/ 660 patients with preoperative pulse pressure ≤53 mm Hg{odds ratio (OR): 2.24 [95% confidence interval (CI): 1.29–3.38]; P<0.001). Pulse pressure ≤53 mm Hg [OR:1.23 (95% CI: 1.03–1.46); P=0.02] and type of surgery were independently associated with all‐cause postoperative morbidity (multivariate analysis). Oxygen pulse <90% of population‐predicted normal values was associated with pulse pressure ≤ 53 mm Hg [OR: 1.93 (95% CI: 1.32–2.84); P=0.007]. Conclusions: In deconditioned surgical patients, lower preoperative systemic arterial pulse pressure is associated with excess morbidity. These data are strikingly similar to meta‐analyses identifying low pulse pressure as an independent risk factor for adverse outcomes in cardiac failure. Low preoperative pulse pressure is a readily available measure, indicating that detailed physiological assessment may be warranted. Clinical trial registration: ISRCT registry, ISRCTN88456378.