Transient ventilatory responses to endotoxin infusion in the cat are mediated by thromboxane A2

Clinical and experimental aspects of breathing modulation by inflammation

Neuroinflammation is produced by local or systemic alterations and mediated mainly by glia, affecting the activity of various neural circuits including those involved in breathing rhythm generation and control. Several pathological conditions, such as sudden infant death syndrome, obstructive sleep apnea and asthma exert an inflammatory influence on breathing-related circuits. Consequently breathing (both resting and ventilatory responses to physiological challenges), is affected; e.g., responses to hypoxia and hypercapnia are compromised. Moreover, inflammation can induce long-lasting changes in breathing and affect adaptive plasticity; e.g., hypoxic acclimatization or long-term facilitation. Mediators of the influences of inflammation on breathing are most likely proinflammatory molecules such as cytokines and prostaglandins. The focus of this review is to summarize the available information concerning the modulation of the breathing function by inflammation and the cellular and molecular aspects of this process. I will consider: 1) some clinical and experimental conditions in which inflammation influences breathing; 2) the variety of experimental approaches used to understand this inflammatory modulation; 3) the likely cellular and molecular mechanisms.

Correlation between transition percentage of minute volume (TMV%) and outcome of patients with acute respiratory failure

PURPOSE

We have previously shown in patients receiving adaptive support ventilation (ASV) that there existed a Transition %MinVol (TMV%) where the patient's work of breathing began to reduce. In this study, we tested the hypothesis that higher TMV% would be associated with poorer outcome in patients with acute respiratory failure.

MATERIALS AND METHODS

In this prospective observational study, we recruited patients with acute respiratory failure on ASV between December 2012 and September 2013 in a mixed ICU. The TMV% was determined by adjusting % MinVol until mandatory respiratory frequency was between 0 and 1breath/min. TMV% was measured on the first two days of mechanical ventilation.

RESULTS

A total of 337 patients (age: 70±16years) were recruited. In patients whose TMV% increased between Day 1 and Day 2, aOR for mortality was 7.0 (95%CI=2.7-18.3, p<0.001) compared to patients whose TMV% decreased. In patients whose TMV% was unchanged between Day 1 and Day2, aOR for mortality was 3.91 (95%CI=1.80-8.22, p<0.01).

CONCLUSIONS

An increase, or lack of decrease, of TMV% from Day 1 to Day 2 was associated with higher risk of in-hospital death.

Lipopolysaccharide-induced carotid body inflammation in cats: functional manifestations, histopathology and involvement of tumour necrosis factor-alpha

In the absence of information on functional manifestations of carotid body (CB) inflammation, we studied an experimental model in which lipopolysaccharide (LPS) administration to pentobarbitone-anaesthetized cats was performed by topical application upon the CB surface or by intravenous infusion (endotoxaemia). The latter caused: (i) disorganization of CB glomoids, increased connective tissue, and rapid recruitment of polymorphonuclear cells into the vascular bed and parenchyma within 4 h; (ii) increased respiratory frequency and diminished ventilatory chemoreflex responses to brief hypoxia (breathing 100% N(2) for 10 s) and diminished ventilatory chemosensory drive (assessed by 100% O(2) tests) during normoxia and hypoxia; (iii) tachycardia, increased haematocrit and systemic hypotension in response to LPS i.v.; and (iv) increased basal frequency of carotid chemosensory discharges during normoxia, but no change in maximal chemoreceptor responses to brief hypoxic exposures. Lipopolysaccharide-induced tachypnoea was prevented by prior bilateral carotid neurotomy. Apoptosis was not observed in CBs from cats subjected to endotoxaemia. Searching for pro-inflammatory mediators, tumour necrosis factor-alpha (TNF-alpha) was localized by immunohistochemistry in glomus and endothelial cells; reverse transcriptase-polymerase chain reaction revealed that the CB expresses the mRNAs for both type-1 (TNF-R1) and type-2 TNF-alpha receptors (TNF-R2); Western blot confirmed a band of the size expected for TNF-R1; and histochemistry showed the presence of TNF-R1 in glomus cells and of TNF-R2 in endothelial cells. Experiments in vitro showed that the frequency of carotid nerve discharges recorded from CBs perfused and superfused under normoxic conditions was not significantly modified by TNF-alpha, but that the enhanced frequency of chemosensory discharges recorded along responses to hypoxic stimulation was transiently diminished in a dose-dependent manner by TNF-alpha injections. The results suggest that the CB may operate as a sensor for immune signals, that the CB exhibits histological features of acute inflammation induced by LPS, that TNF-alpha may participate in LPS-induced changes in chemosensory activity and that some pathophysiological reactions to high levels of LPS in the bloodstream may originate from changes in CB function.

Detection of thromboxane A2 receptor mRNA in rabbit nodose ganglion neurons

Thromboxane A(2) (TXA(2)) is an arachidonic acid metabolite that is released during tissue trauma and elicits platelet aggregation and vascular smooth muscle contraction. Previous research has shown that TXA(2) stimulates pulmonary and cardiac vagal afferent neurons. Therefore, we hypothesized that the presence of the TXA(2) receptor (TP) in vagal neurons would allow for stimulation or modulation of these neurons by TXA(2). To test this hypothesis, single cell RT-PCR was employed using neurons obtained from primary cell cultures of nodose ganglia excised from adult rabbits. Since the sequence for the rabbit TP gene was unknown, a portion of the rabbit TP cDNA was first amplified, cloned, and sequenced. Primer sets for TP were then designed based on this sequence and used in conjunction with a neuronal marker, medium weight neurofilament (NFM), in multiplex RT-PCR reactions. Ninety-three cells were isolated from culture and RT-PCR was carried out on individual cells. Using an aliquot from the initial RT-PCR reaction, a second round of PCR was then employed in which the NFM and TP primer sets were split up into separate reactions. Twenty-three of the 82 cells that were positive for NFM were also positive for TP. Therefore, we conclude that the presence of TP mRNA in a subset of cultured nodose ganglion neurons allows for the possibility that TXA(2) may directly stimulate or modulate vagal afferent neurons.

The pulmonary biology of isoprostanes

Isoprostanes were first recognized as convenient markers of oxidative stress, but their powerful effects on a variety of cell functions are now also being increasingly appreciated. This is particularly true of the lung, which is comprised of a wide variety of different cell types (smooth muscle, innervation, epithelium, lymphatics, etc.), all of which have been shown to respond to exogenously applied isoprostanes. In this review, we summarize these biological responses in the lung, and also consider the roles that isoprostanes might play in a range of pulmonary clinical disorders.

The pulmonary biology of isoprostanes

Isoprostanes are widely recognized as useful markers of membrane lipid peroxidation. It seems to be less well appreciated, however, that they also elicit important biological responses, even though this was first shown at the same time that they were introduced as markers of oxidative stress. The past several years have seen the list of cells/tissues which are sensitive to isoprostanes grow considerably: in fact, as we summarize here, there is now evidence that essentially every cell type in the lung responds in some pathologically relevant way to isoprostanes. In this sense, they might well be considered as not just markers of oxidative stress and inflammation, but also as a novel group of inflammatory mediators. Moreover, in addition to their pathological effects, we summarize here the evidence which has led us to hypothesize that isoprostanes could play an important role in vascular smooth muscle physiology as "endothelium-derived hyperpolarizing factors."

Thromboxane A2-induced inhibition of voltage-gated K+ channels and pulmonary vasoconstriction: role of protein kinase Czeta

Voltage-gated K+ channels (KV) and thromboxane A2 (TXA2) play critical roles in controlling pulmonary arterial tone under physiological and pathological conditions. We hypothesized that TXA2 might inhibit KV channels, thereby establishing a link between these two major pathogenic pathways in pulmonary hypertension. The TXA2 analogue U46619 inhibited IK(V) (Emax=56.1+/-3.9%, EC50=0.054+/-0.019 micromol/L) and depolarized pulmonary artery smooth muscle cells via activation of TP receptors. In isolated pulmonary arteries, U46619 simultaneously increased intracellular Ca2+ concentration and contractile force, and these effects were inhibited by nifedipine or KCl (60 mmol/L). U46619-induced contractions were not altered by the inhibitors of tyrosine kinase genistein or Rho kinase Y-27632 but were prevented by the nonselective protein kinase C (PKC) inhibitors staurosporine and calphostin C. Furthermore, these responses were sensitive to Gö-6983 but insensitive to bisindolylmaleimide I and Gö-6976. Based on the specificity of these drugs, we suggested a role for an atypical PKC in U46619-induced effects. Thus, treatment with a PKCzeta pseudosubstrate inhibitor markedly prevented the vasoconstriction, the inhibition of IK(V), and the depolarization induced by U46619. Western blots showed a transient translocation of PKCzeta from the cytosolic to the particulate fraction on stimulation with U46619. These results indicate that TXA2 inhibits IK(V), leading to depolarization, activation of L-type Ca2+ channels, and vasoconstriction of rat pulmonary arteries. We propose PKCzeta as a link between TP receptor activation and KV channel inhibition.

The effect of Escherichia coli endotoxin infusion on the ventilatory response to hypoxia in unanesthetized newborn piglets

To determine the effects of endotoxemia on the neonatal ventilatory response to hypoxia, 17 chronically instrumented and unanesthetized newborn piglets (</=7 d) were studied before and 30 min after the administration of Escherichia coli O55:B5 endotoxin (n = 8) or normal saline (n = 9). Minute ventilation, oxygen consumption, heart rate, arterial blood pressure, and blood gases were measured during normoxia and 10 min of hypoxia (fraction of inspired oxygen, 0.10). Basal ventilation was not modified by E. coli endotoxin infusion (mean +/- SE, 516 +/- 49 versus 539 +/- 56 mL/min/kg), but the ventilatory response to hypoxia was markedly attenuated at 1 min (955 +/- 57 versus 718 +/- 97 mL/min/kg, p < 0.002, saline versus endotoxin) and at 10 min (788 +/- 51 versus 624 +/- 66 mL/min/kg, p < 0.002). A larger decrease in oxygen consumption was observed during hypoxia and endotoxemia (6.3 +/- 2.8 versus 18.3 +/- 2.7%, p < 0.03, pre- versus post-endotoxin). A significant correlation was demonstrated between the changes in minute ventilation and oxygen consumption with hypoxia during endotoxemia (r = 0.9, p < 0.002). The ventilatory response to hypoxia was not modified by the saline infusion. These data show a significant attenuation in the ventilatory response to hypoxia during E. coli endotoxemia. This decrease in ventilation was associated with a significant decrease in the metabolic rate during hypoxia and endotoxemia.

Activation of lung vagal sensory receptors by circulatory endotoxin in rats

Although endotoxin is known to induce various pulmonary responses that are linked to the function of lung vagal sensory receptors, its effects on these pulmonary receptors are still not clear. This study investigated the effects of circulatory endotoxin on the afferent activity of lung vagal sensory receptors in rats. We recorded afferent activity arising from vagal pulmonary C fibers (CFs), rapidly adapting receptors (RARs), tonic pulmonary stretch receptors (T-PSRs), and phasic pulmonary stretch receptors (P-PSRs) in 64 anesthetized, paralyzed, and artificially ventilated rats. Intravenous injection of endotoxin (50 mg/kg; lipopolysaccharide) stimulated 7 of the 8 CFs, 8 of the 8 RARs, and 4 of the 8 T-PSRs studied, while having no effect on the 8 P-PSRs tested. The stimulation started 3-16 min after endotoxin injection and lasted until the end of the 90-min observation period. The evoked discharge of either CFs or RARs was not in phase with the ventilatory cycle, whereas that of T-PSRs showed a respiratory modulation. Injection of a saline vehicle caused no significant change in the discharge of these receptors. Additionally, endotoxin significantly produced an increase in total lung resistance, and decreases in dynamic lung compliance and arterial blood pressure. Our results demonstrate that a majority of lung vagal sensory receptors are activated following intravenous injection of endotoxin, and support the notion that these pulmonary receptors may function as an important afferent system during endotoxemia.

Isoprostanes: an overview and putative roles in pulmonary pathophysiology

Isoprostanes are produced during peroxidation of membrane lipids by free radicals and reactive oxygen species. Initially, they were recognized as being valuable markers of oxidative stress, and in the past 10 years, dozens of disease states and experimental conditions with diverse etiologies have been shown to be associated with marked increases in urinary, plasma, and tissue levels of isoprostanes. However, they are not just mere markers; they evoke important biological responses on virtually every cell type found within the lung, and these responses exhibit compound-, tissue-, and species-related variations. In fact, the isoprostanes may mediate many of the features of the disease states for which they are used as indicators. In this review, I describe the chemistry, metabolism, and pharmacology of isoprostanes, with a particular emphasis on pulmonary cell types, and the possible roles of isoprostanes in pulmonary pathophysiology.

Isoprostanes: generation, pharmacology, and roles in free-radical-mediated effects in the lung

Isoprostanes are produced during peroxidation of membrane lipids by free radicals and reactive oxygen species, and are currently used as markers of many disease states and experimental conditions in which oxidative stress is a prominent feature. A small number of reports have described the ability of some isoprostanes to evoke important biological effects in smooth muscle and other cell types. However, most of these studies were done using rat tissues, and only two specific isoprostanes - 8-iso-PGE(2)and 8-iso-PGF(2alpha)- were tested. In this review, we describe the generation of isoprostanes during oxidative stress, and their effects on smooth muscle, including our novel findings of their effects on human airway, pulmonary artery and pulmonary vein smooth muscles. Collectively, the data suggest that isoprostanes may not only be markers, but may in fact mediate the effects of free radicals and reactive oxygen species.

The thromboxane A2 mimetic U-46619 inhibits somatomotor activity via a vagal reflex from the lung

Vagal reflexes from the heart and lungs elicit autonomic as well as somatomotor responses. The purpose of the present investigation was to determine whether the inflammatory mediator thromboxane A2 inhibits the knee-jerk reflex via a vagally mediated reflex from either the heart or the lung. The thromboxane A2 mimetic U-46619 (0. 8 +/- 0.08 microg/kg) was injected through a catheter placed near the right atrium (n = 11), near the aortic arch (n = 7), or into the pericardial sac (n = 4) in 11 chloralose-anesthetized cats. The knee-jerk reflex, elicited by striking the patellar tendon with a solenoid-driven hammer, was used to evaluate somatomotor activity. The mean maximum tension produced by the knee-jerk reflex was 306 +/- 21 g (range 154-471 g). Intravenous U-46619 injection inhibited the knee-jerk reflex by 25 +/- 6% and increased peak systolic pressure 53 +/- 7 mmHg on average. Bilateral cervical vagotomy abolished the somatomotor inhibition but did not reduce the pressor response. Intra-arterial U-46619 injection inhibited the knee-jerk reflex in two of seven cats and increased peak systolic pressure by 41 +/- 11 mmHg. Vagotomy abolished the inhibition in one of the two cats but did not reduce the pressor response. Intrapericardial U-46619 injection did not affect the knee-jerk reflex nor blood pressure. The results indicate that U-46619 inhibited the knee-jerk reflex via a vagal reflex from the lung because the inhibition predominated after intravenous injection and was abolished by vagotomy. Speculation is made that the inflammatory mediator thromboxane A2 may contribute via a vagal reflex to the depression of motor activity associated with sickness behavior.

Peripheral neural modulation of endotoxin-induced hyperventilation

OBJECTIVE

To delineate the role of the peripheral neural reflexes involved in modulating hyperventilation during endotoxemia.

DESIGN

A prospective, randomized, controlled, multigroup study.

SETTING

Research animal laboratory.

SUBJECTS

Adult Sprague-Dawley rats (n=43; 354+/-24 g) of either gender.

INTERVENTIONS

Eight rats received a sham operation on their vagus, carotid sinus, and aortic nerves before the administration of a saline vehicle to serve as the time control. In the endotoxin group, 11 rats received a sham operation before endotoxin challenge. The remaining 24 rats received bilateral vagotomy (n=8), perivagal capsaicin treatment (n=8), or denervation of peripheral chemoreceptors (n=8) before endotoxin challenge. After the breathing pattern returned to a steady state, endotoxin (L-4130, serotype 0111, B4 lipopolysaccharide; 50 mg/kg) was injected into the vein. The rat's respiration was then monitored continuously for 5 hrs or until the animal died.

MEASUREMENTS AND MAIN RESULTS

The respiratory rate and tidal volume did not change over the 5-hr observation period in the time control group. In the endotoxin group, the respiratory rate increased significantly from baseline (135.4%) 2 hrs after endotoxin challenge and increased persistently until the rats died. The tidal volume increased gradually to < or =132.8% of baseline 4 hrs after endotoxin challenge. Bilateral cervical vagotomy and perineural capsaicin treatment of the vagus nerves eliminated the tachypnea response to endotoxin injection. Denervation of the peripheral chemoreceptor accentuated the hyperventilation response to endotoxin, and resulted in the shortest survival time.

CONCLUSIONS

Both lung vagal C-fiber afferents and peripheral chemoreceptors are involved in modulating the hyperventilation response after endotoxin challenge in rat models. Stimulation of vagal C-fiber afferents increased the respiratory rate. Conversely, the role of peripheral chemoreceptors was to restrain the hyperventilatory response and these receptors may play a protective role during endotoxemia.

TxA2-induced pulmonary artery contraction requires extracellular calcium

In order to examine the role of extracellular calcium in the pulmonary arterial vasoconstriction that is elicited by thromboxane A2 (TxA2), rabbits were sacrificed and the main trunk of the pulmonary artery removed. Contractile responses of the isolated vessel to the TxA2 mimetic, U46 619, were measured in a temperature controlled (37 degrees C) organ bath. Compared with control responses, U46 619 microM) contractions were nearly eliminated when 1 mM EGTA was added to the buffer. In the presence of normal extracellular calcium concentrations, antagonists of voltage sensitive calcium channels (e.g. verapamil and nifedipine) attenuated the U46 619-induced contractions. These voltage sensitive calcium channel blockers were more effective in eliminating contractile responses to high KCl concentrations (6) or 120 mM KCl). The inability of these calcium channel antagonists to completely eliminate U46 619 responses was confirmed in the anesthetized rabbit where both nifedipine and verapamil failed to block the increase in pulmonary arterial blood pressure resulting from intravenous U46 619 infusion. These results indicate that extracellular calcium is essential for U46 619-induced pulmonary vascular contraction, and that mechanisms in addition to voltage operated calcium channels participate in the movement of extracellular calcium through the plasma membrane.

Effects of levodropropizine on vagal afferent C-fibres in the cat

1. Levodropropizine (LVDP) is an effective antitussive drug. Its effects on single-unit discharge of vagal afferent C-fibres were tested in anaesthetized cats to assess whether an inhibition of vagal C-fibres is involved in its antitussive properties. Vagal C-fibres, identified by their response to phenylbiguanide (PBG), were recorded via suction electrodes from the distal part of the cut vagus. Based on their response to lung inflation, C-fibres were classified as pulmonary (19 fibres) or non-pulmonary (6 fibres). 2. PBG increased the discharge rate of both C-fibre types and activated a respiratory reflex causing apnoea. This reflex was abolished when the second vagus nerve was cut as well, while PBG-mediated stimulation of the C-fibres was not affected by vagotomy. 3. LVDP was administered intravenously and the C-fibre response to PBG was compared with that before administration of the drug. LVDP reduced both the duration of apnoea and the response of the C-fibre to PBG. 4. Comparison of the C-fibre responses to PBG and to a mixture of PBG and LVDP revealed that the period of apnoea was shortened and the discharge rate of the C-fibre reduced when LVDP was present. 5. The LVDP-induced inhibition of the C-fibre response to PBG was on average 50% in pulmonary and 25% in non-pulmonary fibres. 6. These results suggest that LVDP significantly reduces the response of vagal C-fibres to chemical stimuli. It is, thus, likely that the antitussive effect of LVDP is mediated through its inhibitory action on C-fibres.