Pulmonary Receptors Research Articles

Demonstration of pulmonary β2-adrenergic receptor binding in vivo with [18F]fluoroethyl-fenoterol in a guinea pig model

The new beta2 radioligand (R,R)(S,S) 5-(2-(2-[4-(2-[18F]fluoroethoxy)phenyl]-1-methylethylamino)-1-hydroxyethyl)-benzene-1,3-diol ([18F]FE-fenoterol; [18F]FEFE), a fluoroethylated derivative of racemic fenoterol, was evaluated in vivo and ex vivo using a guinea pig model. Dynamic PET studies over 60 min with [(18)F]FEFE were performed in nine Hartley guinea pigs in which a baseline (group 1, n=3), a predose (group 2, n=3; 2 mg/kg fenoterol 5 min prior to injection of [18F]FEFE) or a displacement study (group 3, n=3; 2 mg/kg fenoterol 5 min post injection of [18F]FEFE) was conducted. In all animal groups, the lungs could be visualised and semi-quantified separately by calculating uptake ratios to non-specific binding in the neck area. Premedication with non-radioactive fenoterol and displacement tests showed significant reduction of lung uptake, by 94% and 76%, respectively. These data demonstrate specific binding of the new radioligand to the pulmonary beta2-receptors in accordance with ex vivo measurements. Therefore, [18F]FEFE seems to be suitable for the in vivo visualisation and quantification of the pulmonary beta2-receptor binding in this animal model.

Cholangiocyte Endothelin 1 and Transforming Growth Factor β1 Production in Rat Experimental Hepatopulmonary Syndrome

Cholangiocyte Endothelin 1 and Transforming Growth Factor β1 Production in Rat Experimental Hepatopulmonary Syndrome

High Expression of Pulmonary Proteinase-activated Receptor 2 in Acute and Chronic Lung Injury in Preterm Infants

Proteinase-activated receptor 2 (PAR(2)), a G-protein-coupled receptor activated by serine proteinases such as trypsin, has been suggested to play an important role in inflammatory and fibroproliferative processes. In preterm infants, the development of bronchopulmonary dysplasia (BPD) is characterized by early pulmonary inflammation and subsequent interstitial fibrosis. High pulmonary trypsin-2 has been shown to be associated with the development of BPD. We studied the expression and distribution of PAR(2) and trypsin-2 by immunohistochemistry in autopsy lung specimens of fetuses (n = 10), of preterm infants who died of acute or prolonged respiratory distress syndrome (RDS) (n = 8 and n = 7, respectively) or BPD (n = 6), and of newborn infants without lung disease (n = 5) who served as controls. In prolonged RDS and BPD, PAR(2) immunoreactivity was significantly higher in bronchial epithelium when compared with infants without pulmonary pathology (p < 0.05 and p < 0.005, respectively). In alveolar epithelium, expression of PAR(2) was elevated in prolonged RDS when compared with newborn infants without pulmonary pathology (p < 0.05). Moreover, strong expression of PAR(2) was detected in myofibroblasts of thickened and fibrotic alveolar walls in prolonged RDS or BPD. Trypsin-2 was co-localized with PAR(2) in bronchoalveolar epithelium. These findings suggest that PAR(2), possibly activated by trypsin-2, may participate in inflammation and fibroproliferation associated with progression of RDS toward BPD in preterm infants.

Hyperthermia increases sensitivity of pulmonary C‐fibre afferents in rats

This study was carried out to investigate whether an increase in tissue temperature alters the excitability of vagal pulmonary C-fibres. Single-unit afferent activities of 88 C-fibres were recorded in anaesthetized and artificially ventilated rats when the intrathoracic temperature (T(it)) was maintained at three different levels by isolated perfusion of the thoracic chamber with saline: control (C: approximately 36 degrees C), medium (M: approximately 38.5 degrees C) and high (H: approximately 41 degrees C), each for 3 min with 30 min recovery. Our results showed: (1) The baseline fibre activity (FA) of pulmonary C-fibres did not change significantly at M, but increased drastically (>5-fold) at H. (2) The C-fibre response to right-atrial injection of capsaicin (0.5 microg kg(-1)) was markedly elevated at H (deltaFA = 5.94 +/- 1.65 impulses s(-1) at C and 13.13 +/- 2.98 impulses s(-1) at H; P < 0.05), but not at M. Similar increases in the C-fibre responses to other chemical stimulants (e.g. adenosine, etc.) were found at H; all the enhanced responses returned to control in 30 min. (3) The C-fibre response to lung inflation was also significantly potentiated at H. In sharp contrast, there was no detectable change in either the baseline activity or the responses to lung inflation and deflation in 10 rapidly adapting pulmonary receptors and 10 slowly adapting pulmonary receptors at either M or H. (4) The enhanced C-fibre sensitivity was not altered by pretreatment with indomethacin or capsazepine, a selective antagonist of the transient receptor potential vanilloid type 1 (TRPV1) receptor, but was significantly attenuated by ruthenium red that is known to be an effective blocker of all TRPV channels. (5) The response of pulmonary C-fibres to a progressive increase in T(it) in a ramp pattern further showed that baseline FA started to increase when T(it) exceeded 39.2 degrees C. In conclusion, a pronounced increase in the baseline activity and excitability of pulmonary C-fibres is induced by intrathoracic hyperthermia, and this enhanced sensitivity probably involves activation of temperature-sensitive ion channel(s), presumably one or more of the TRPV receptors, expressed on the C-fibre endings.

A role for platelet-derived growth factor β-receptor in a newborn rat model of endothelin-mediated pulmonary vascular remodeling

Newborn rats exposed to 60% O2 for 14 days develop endothelin (ET)-1-dependent pulmonary hypertension with vascular remodeling, characterized by increased smooth muscle cell (SMC) proliferation and medial thickening of pulmonary resistance arteries. Using immunohistochemistry and Western blot analyses, we examined the effect of exposure to 60% O2 on expression in the lung of receptors for the platelet-derived growth factors (PDGF), which are implicated in the pathogenesis of arterial smooth muscle hyperplasia. We observed a marked O2-induced upregulation of PDGF-alpha and -beta receptors (PDGF-alphaR and -betaR) on arterial smooth muscle. This led us to examine pulmonary vascular PDGF receptor expression in 60% O2-exposed rats given SB-217242, a combined ET receptor antagonist, which we found prevented the O2-induced upregulation of PDGF-betaR, but not PDGF-alphaR, on arterial smooth muscle. PDGF-BB, a major PDGF-betaR ligand, was found to be a potent in vitro inducer of hyperplasia and DNA synthesis in cultured pulmonary artery SMC from infant rats. A critical role for PDGF-betaR ligands in arterial SMC proliferation was confirmed in vivo using a truncated soluble PDGF-betaR intervention, which attenuated SMC proliferation induced by exposure to 60% O2. Collectively, these data are consistent with a major role for PDGF-betaR-mediated SMC proliferation, acting downstream of increased ET-1 in a newborn rat model of 60% O2-induced pulmonary hypertension.

Responses of Pulmonary Platelet-Derived Growth Factor Peptides and Receptors to Hyperoxia and Nitric Oxide in Piglet Lungs

The peptides platelet-derived growth factor-A (PDGF-A) and especially -B have important roles in lung development. The effect of hyperoxic exposure with and without inhaled nitric oxide (iNO) on lung expression of PDGF and its receptors is unknown. We hypothesized that hyperoxia exposure would suppress mRNA expression and protein production of these ligands and their receptors. The addition of iNO to hyperoxia may further aggravate the effects of hyperoxia. Thirteen-day-old piglets were randomized to breathe 1) room air (RA); 2) 0.96 fraction of inspired oxygen (O2), or 3) 0.96 fraction of inspired oxygen plus 50 ppm of NO (O2+NO), for 5 d. Lungs were preserved for mRNA, Western immunoblot, and immunohistochemical analyses for PDGF-A and -B and their receptors PDGFR-alpha and -beta. PDGF-B mRNA expression was greater than that of PDGF-A or PDGFR-alpha and -beta in RA piglet lungs (p<0.05). Hyperoxia with or without iNO reduced lung PDGF-B mRNA and protein expression relative to the RA group lungs (p<0.01). PDGF-B immunostain intensity was significantly increased in the alveolar macrophages, which were present in greater numbers in the hyperoxia-exposed piglet lungs, with or without NO (p<0.01). PDGFR-beta immunostaining was significantly increased in airway epithelial cells in O2- and O2+NO-exposed piglets. PDGF-A and PDGFR-alpha immunostain intensity and distribution pattern were unchanged relative to the RA group. Sublethal hyperoxia decreases PDGF-B mRNA and protein expression but not PDGF-A or their receptors in piglet lungs. iNO neither aggravates nor ameliorates this effect.

Cardiovascular Responses in Unrestrained WKY Rats to Inhaled Ultrafine Carbon Particles

Based on epidemiologic observations, the issue of adverse health effects of inhaled ultrafine particles (UFP) is currently under intensive discussion. We therefore examined cardiovascular effects of UFP in a controlled animal exposure on young, healthy WKY rats. Short-term exposure (24 h) to carbon UFPs (38 nm, 180 μg m−3), generated by spark discharging, induced a mild but consistent increase in heart rate (18 bpm, 4.8%), which was associated with a significant decrease in heart-rate variability during particle inhalation. The timing and the transient character of these responses point to a particle induced alteration of cardiac autonomic balance, mediated by a pulmonary receptor activation. After 24 h of inhalation exposure, bronchoalveolar lavage revealed significant but low-grade pulmonary inflammation (clean air 1.9% vs. UFPs 6.9% polymorphonuclear cells) and on histopathology sporadic accumulation of particle-laden macrophages was found in the alveolar region. There was no evidence of an inflammation-mediated increase in blood coagulability, as UFP inhalation did not induce any significant changes in plasma fibrinogen or factor VIIa levels and there were no prothrombotic changes in the lung or the heart at both the protein and mRNA level. Histological analysis revealed no signs of cardiac inflammation or cardiomyopathy. This study therefore provides toxicological evidence for UFP-associated pulmonary and cardiac effects in healthy rats. Our findings suggest that the observed changes are mediated by an altered sympatho-vagal balance in response to UFP inhalation, but do not support the concept of an inflammation-mediated prothrombotic state by UFP.

Pulmonary–Hepatic vascular Disorders (PHD)

⇓“The tantalizing problem of the connective link in cirrhotic patients between oxygen unsaturation and possible arteriovenous shunting in the lungs remains unsolved, and any relation between arterial unsaturation and pulmonary vasodilation remains obscure.” Fig. 1.— Working model of molecular alterations in the pulmonary microcirculation in experimental hepatopulmonary syndrome (HPS). a) In the normal microvasculature, a balance of vasoconstrictive and vasodilatory factors, including paracrine endothelin (ET)-1-mediated vasoconstriction through the ETA receptor (▪) on smooth muscle cells (SMCs) and ET-1-mediated vasodilatation mediated through the ETB receptor (□) linked to endothelial nitric oxide synthase (eNOS) in endothelial cells (ECs), maintain tone. b) During the development of HPS, a number of alterations, both directly and indirectly related to hepatic injury and portal hypertension, result in the production or release of mediators into the venous circulation, where they influence the pulmonary microcirculation. Increased expression of pulmonary endothelial ETB receptors and increased hepatic production and release of ET-1 contribute to an increase in eNOS expression and enhanced nitric oxide (NO) production in the microvascular endothelium during the initiation of HPS. Tumour necrosis factor (TNF)-α-mediated accumulation of intravascular macrophage-like cells also occurs after chronic common bile duct ligation. Haem oxygenase (HO)-1 and inducible nitric oxide synthase (iNOS) expression increase in these cells and contribute to the progression of HPS. CO: carbon monoxide. Fig. 2.— Algorithm for screening and therapeutic decisions in hepatopulmonary syndrome (HPS). OLT: orthotopic liver transplantation; P a,O2: arterial oxygen tension; P A–a,O2: alveolar–arterial oxygen tension difference; CEE: contrast-enhanced echocardiography; PFT: pulmonary function test; MAA: macroaggregated albumin. #: high-resolution thoracic computed tomographic scanning is highly recommended in order to exclude chronic respiratory comorbid conditions; ¶: high risk for post-operative OLT mortality. 1 mmHg=0.133 kPa. Fig. 3.— Plot demonstrating the relationship between cardiac output ( Q ') and transpulmonary pressure gradient (TPG; mean pulmonary arterial …

Evolutionary trends in airway CO2/H+ chemoreception

In many species of air-breathing vertebrates CO2-sensitive airway receptors play an important role in ventilatory control. In ectotherms, olfactory receptors often inhibit breathing and prolong breath holding when environmental CO2 levels are high. CO2/H+ sensitive pulmonary receptors (intra pulmonary chemoreceptors (IPC) and pulmonary stretch receptors (PSR)) regulate breathing patterns in all vertebrates in a manner that reduces dead space ventilation and enhances the efficiency of CO2 excretion under conditions of environmental hypercarbia, and/or reduces CO2 loss from hyperventilation. The greater CO2 sensitivity of IPC may allow them to also serve as a venous CO2 receptor (at least transiently when levels of metabolically produced CO2 begin to rise), prevent alkalosis during hyperpnea/polypnea, and may have contributed to the evolution of the extremely thin air/blood barrier and increased diffusion capacity associated with the rigid avian lung. The presence of all three receptor groups with different degrees of CO2 sensitivity in most reptiles, however, gives rise to what appear to be anomalous responses to environmental CO2.

Augmented pulmonary IL-4 and IL-13 receptor subunit expression in idiopathic interstitial pneumonia

Background: Some idiopathic interstitial pneumonias (IIPs) are characterised by fibroproliferation and deposition of extracellular matrix. Because efficacious treatment options are limited, research has been directed towards understanding the cytokine networks...

Effect of dietary clenbuterol and cimaterol on muscle composition, beta-adrenergic and androgen receptor concentrations in broiler chickens.

Illegal dietary supplementation with beta(2)-agonists has been shown to increase protein deposition and decrease fat accretion in domestic animals. In poultry the metabolic and endocrine responses to beta(2)-agonists are not fully elucidated. In this trial the effects of dietary clenbuterol (1 p.p.m.) and cimaterol (1 p.p.m.) on muscle composition and endocrine response of male broiler chickens were studied. Dietary clenbuterol induced a slight, but in general not significant, improvement of zootechnical performances and carcass yields. Chemical composition of muscle was not influenced by dietary treatments, even if a slight improvement of protein content was observed in treated groups. No effects on fatty acid composition of meat were detected. Both clenbuterol and cimaterol treatments caused a downregulation in testicular androgen receptors and in pulmonary, cardiac and central nervous system beta-adrenergic receptors.

Experimental pulmonary hypertension markedly induces pulmonary artery transient receptor potential protein expression

Increased pulmonary vascular resistance and smooth muscle hypertrophy characterize pulmonary hypertension (PAH). Only a few downstream mediators can induce such changes among these being altered Ca++ signaling. A family of ion channels, the transient receptor potential proteins (TrpCs), has been identified that activate Ca++ flux and affect cell function. Thus we tested the hypothesis that PAH induces specific TrpCs.

Tiotropium (Spiriva) - a long-acting inhaled anticholinergic for the treatment of chronic obstructive pulmonary disease (COPD)

Anticholinergics are agents of first choice for the symptomatic treatment of patients with COPD. Tiotropium (Ba 679 BR, Spiriva) is a long-acting inhaled anticholinergic designed for once-daily bronchodilator treatment of COPD. Tiotropium is a selective antagonist of pulmonary M1 and M3 muscarinic receptor subtypes, that produces a long-lasting (24 hours), dose-dependent bronchodilation and bronchoprotection against constrictive stimuli, e. g. methacholine, following inhalation of single doses. Clinical trials with tiotropium in COPD patients over a maximum treatment duration of one year have confirmed a persisting bronchodilator effect of tiotropium compared with placebo and ipratropium, as well as meaningful clinical improvements in lung function, hyperinflation, exercise tolerance, symptom control and quality of life. Moreover, recent trials indicate that treatment with tiotropium also reduces the frequency of COPD exacerbations and hospitalizations. Comparative trials further suggest that the bronchodilator potency of tiotropium may be superior to those of available COPD treatments. Besides a higher incidence of dry mouth, the side effect profile was comparable to ipratropium bromide. In conclusion, present clinical data suggest that tiotropium has the potential of a first-line treatment for patients with COPD.

Endothelin B receptor deficiency predisposes to pulmonary edema formation via increased lung vascular endothelial cell growth factor expression.

Endothelin (ET) may contribute to pulmonary edema formation, particularly under hypoxic conditions, and decreases in ET-B receptor expression can lead to reduced ET clearance. ET increases vascular endothelial cell growth factor (VEGF) production in vitro, and VEGF overexpression in the lung causes pulmonary edema in vivo. We hypothesized that pulmonary vascular ET-B receptor deficiency leads to increased lung ET, that excess ET increases lung VEGF levels, promoting pulmonary edema formation, and that hypoxia exaggerates these effects. We studied these hypotheses in ET-B receptor-deficient rats. In normoxia, homozygous ET-B-deficient animals had significantly more lung vascular leak than heterozygous or control animals. Hypoxia increased vascular leak regardless of genotype, and hypoxic ET-B-deficient animals leaked more than hypoxic control animals. ET-B-deficient animals had higher lung ET levels in both normoxia and hypoxia. Lung HIF-1alpha and VEGF content was greater in the ET-B-deficient animals in both normoxia and hypoxia, and both HIF-1alpha and VEGF levels were reduced by ET-A receptor antagonism. Both ET-A receptor blockade and VEGF antagonism reduced vascular leak in hypoxic ET-B-deficient animals. We conclude that ET-B receptor-deficient animals display an exaggerated lung vascular protein leak in normoxia, that hypoxia exacerbates that leak, and that this effect is in part attributable to an ET-mediated increase in lung VEGF content.

Hypersensitivity of pulmonary C fibers induced by adenosine in anesthetized rats.

Compelling clinical evidence implicates the potential role of adenosine in development of airway hyperresponsiveness and suggests involvement of pulmonary sensory receptors. This study was carried out to determine the effect of a low dose of adenosine infusion on sensitivity of pulmonary C-fiber afferents in anesthetized open-chest rats. Infusion of adenosine (40 microg x kg-1x min-1 i.v. for 90 s) mildly elevated baseline activity of pulmonary C fibers. However, during adenosine infusion, pulmonary C-fiber responses to chemical stimulants and lung inflation (30 cmH2O tracheal pressure) were markedly potentiated; e.g., the response to right atrial injection of capsaicin (0.25 or 0.5 microg/kg) was increased by more than fivefold (change in fiber activity = 2.64 +/- 0.67 and 16.27 +/- 3.11 impulses/s at control and during adenosine infusion, n = 13, P < 0.05), and this enhanced response returned to control in approximately 10 min. The potentiating effect of adenosine infusion was completely blocked by pretreatment with 8-cyclopentyl-1,3-dipropylxanthine (100 microg/kg), a selective antagonist of the adenosine A1 receptor, but was not affected by 3,7-dimethyl-1-propargylxanthine (1 mg/kg), an A2-receptor antagonist, or 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (2 mg/kg), an A3-receptor antagonist. This potentiating effect was also mimicked by N6-cyclopentyladenosine (0.25 microg x kg-1 x min-1 for 90 s), a selective agonist of the adenosine A1 receptor. In conclusion, our results showed that infusion of adenosine significantly elevated the sensitivity of pulmonary C-fiber afferents in rat lungs and that this potentiating effect is likely mediated through activation of the adenosine A1 receptor.

Structure of slowly adapting pulmonary stretch receptors in the lung periphery.

Pulmonary sensory receptors are the initiating sites for lung reflexes; however, little is known about their structure, especially the relationship between the structure and function of these receptors. Using a novel approach (combining electrophysiological and morphological techniques), we examined the structures of the typical slowly adapting pulmonary stretch receptors (SARs) located in the lung periphery. We recorded SAR activities in the cervical vagus nerve, identified the receptive field, dissected the SARs in blocks, fixed and processed these blocks for immunohistochemical staining using anti-Na+/K+-ATPase, and examined the blocks under a confocal microscope. These SAR structures have multiple endings that have terminal knobs. Some structures that are located in the airway walls have terminal knobs buried in smooth muscle. Others are in the most peripheral part of the lung, and their terminal knobs have no obvious relation to smooth muscle, suggesting that muscle contraction may not be a direct factor for SAR activation.

Endothelium Derived factors and Pulmonary Hypertension

Extracted from text ... REGISTRAR PRIZE 45 Endothelium derived factors and pulmonary hypertension level. Several genetic factors clearly play a role in this disease. It has recently been discovered that primary pulmonary hypertensives lack a receptor for Transforming Growth Factor-??which may allow clonal expansion of endothelial cells.4 Nitric Oxide Previously known as Endothelin Derived Relaxant Factor, Nitric Oxide (NO) is a potent vasodilator of pulmonary and systemic blood vessels. Pulmonary selectivity relies on inhaled NO being delivered directly to ventilated pulmonary vasculature with rapid plasma inactivation before reaching the systemic circulation. Inhaled NO has been used for persistent pulmonary hypertension of the newborn, ..

Functional morphology of pulmonary neuroepithelial bodies: extremely complex airway receptors.

Innervated groups of neuroendocrine cells, called neuroepithelial bodies (NEBs), are diffusely spread in the epithelium of intrapulmonary airways in many species. Our present understanding of the morphology of NEBs in mammalian lungs is comprehensive, but none of the proposed functional hypotheses have been proven conclusively. In recent reviews on airway innervation, NEBs have been added to the list of presumed physiological lung receptors. Microscopic data on the innervation of NEBs, however, have given rise to conflicting interpretations. Using neuronal tracing, denervation, and immunostaining, we recently demonstrated that the innervation of NEBs is much more complex than the almost unique vagal nodose sensory innervation suggested by other authors. The aim of the present work is to summarize our present understanding about the origin and chemical coding of the profuse nerve terminals that selectively contact pulmonary NEBs. A thorough knowledge of the complex interactions between the neuroendocrine cells and at least five different nerve fiber populations is essential for defining the position(s) of NEBs among the many pulmonary receptors characterized by lung physiologists.

Introduction to the Special Section: “Morphology, cell biology, physiology, and pathology of airway receptors”

At the Experimental Biology 2002 meeting, held in New Orleans, the second Wiley Symposium was focused on morphological and functional aspects of distinct types of airway receptors. Sensory nerves in the lower airways are involved in the regulation of breathing patterns, modulation of airway autonomic neural tone, intrapulmonary mucosal vasodilation, the initiation of the cough reflex, and many other actions. Main players in this group of sensory nerves are the vagal afferents, to be subdivided into pulmonary C-fibers and two subgroups of myelinated afferents, i.e., the rapidly and slowly adapting pulmonary receptors. This list of presumed physiological intrapulmonary receptors has been recently extended to include complexly innervated neuroendocrine structures, the so-called neuroepithelial bodies. Not surprisingly, normal lung functioning will be drastically affected by abnormalities in some or more of these specialized sensors, which as such may directly underlie multiple airway pathologies. The continuous increase in air pollution, amongst others leading to a higher exposure to chemical irritants in the environment, has an ever increasing negative impact on this highly delicate and complex “sensing” system of the lung and accounts for the strong current interest in this field of investigation. The contributions of the various research laboratories to this special issue aim to offer an up-to-date status, both from a morphological and a physiopharmacological perspective, of the fundamental scientific research on airway receptors carried out in the different subareas.

Saturation of adrenomedullin receptors plays an important role in reducing pulmonary clearance of adrenomedullin during the late stage of sepsis

Adrenomedullin (AM) is a potent vasodilator that plays a major role in the cardiovascular response during the progression of sepsis. Although pulmonary clearance of AM (i.e., the primary site of AM clearance) is reduced during the late, hypodynamic stage of sepsis, the role of AM receptors under such conditions remains unclear. This study was carried out to test the hypothesis that saturation of AM receptors is responsible for the decreased clearance of AM in the lungs during sepsis. Polymicrobial sepsis was induced in male adult rats by cecal ligation and puncture (CLP). At 20 h after CLP (i.e., the late phase), 125I-labeled rat AM was administered through the jugular vein, both with (+) and without (−) pre-injection of the human AM fragment AM 22–52 (an AM receptor antagonist). Pulmonary tissue samples were harvested after 30 min and the radioactivity was determined. In addition, lung levels of AM were determined at 5 and 20 h after CLP by radioimmunoassay. Alterations in gene expression of the recently identified AM receptor subunits calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein-2 and -3 (RAMP-2 and -3) were assessed in the lungs by reverse transcription–polymerase chain reaction (RT–PCR) at 5 and 20 h after CLP. The results indicate that there was a significant decrease in pulmonary [ 125I]AM clearance at 20 h in −AM 22–52 CLP animals. Lung clearance in +AM 22–52 sham animals was significantly lower than in −AM 22–52 sham animals and was not statistically different from the −AM 22–52 CLP group. There was no statistical difference between +AM 22–52 and −AM 22–52 CLP groups. However, there was a significant increase in lung AM levels at 20 but not 5 h after CLP. In addition, RAMP-3 expression was significantly upregulated at 5 but not 20 h after CLP. There were no alterations in the expression of CRLR or RAMP-2 at either time point. These results suggest that pulmonary AM receptors become saturated as more AM enters the bloodstream, thereby reducing the ability of the lungs to clear this peptide during late sepsis. Early upregulation of RAMP-3 may be a compensatory mechanism to help clear the upregulated AM from the bloodstream. The lack of upregulation of RAMP-3 during late sepsis could also contribute to the decreased clearance observed during this phase.