Enkephalinase: selective peptide inhibitors

Curcumin induces peripheral antinociception by opioidergic and cannabinoidergic mechanism: Pharmacological evidence

BACKGROUND

Curcumin is one of the compounds present in plants of the genus Curcuma sp., being very used not only as condiment but also with medicinal purposes. As an analgesic, papers highlight the efficacy of curcumin in the treatment of various types of pain.

AIMS

In this study we evaluated the peripheral antinociceptive effect of curcumin and by which mechanisms this effect is induced.

MAIN METHODS

The mice paw pressure test was used on animals which had increased pain sensitivity by intraplantar injection of carrageenan. All the drugs were administered in the right hind paw.

KEY FINDINGS

Curcumin was administered to the right hind paw animals induced antinociceptive effect. Non -selective antagonist of opioid receptors naloxone reverted the antinociceptive effect induced by curcumin. Selective antagonists for μ, δ and κ opioid receptors clocinnamox, naltrindole and nor- binaltorphimine, respectively, reverted the antinociceptive effect induced by curcumin. Bestatin, enkephalinases inhibitor that degrade peptides opioids, did not change the nociceptive response. Selective antagonists for CB₁ and CB₂ cannabinoid receptors, AM251 and AM630, respectively, reversed the antinociceptive effect induced by curcumin. The MAFP inhibitor of the enzyme FAAH which breaks down anandamide, JZL184, enzyme inhibitor MAGL which breaks down the 2-AG, as well as the VDM11 anandamide reuptake inhibitor potentiated the antinociceptive effect of curcumin.

SIGNIFICANCE

These results suggest that curcumin possibly peripheral antinociception induced by opioid and cannabinoid systems activation and possibly for endocannabinoids and opioids release.

[Characteristics and Functional Roles of Opioids Originally Present in Vivo]

The characteristics and functional roles of opioids originally present in vivo (endogenous opioids) in guinea-pig ileum were investigated. The release of endogenous opioids was determined by the inhibitory twitch response evoked by 0.1 Hz stimulation after 10 Hz stimulation (post-tetanic twitch inhibition). The effects of peptidase inhibitors increased the post-tetanic twitch inhibition, prevented by β-funaltrexamine and nor-binaltorphimine, which are selective μ- and κ-opioid receptor subtype antagonists, respectively. Dopamine receptor antagonists (haloperidol, sultopride and domperidone) increased the post-tetanic twitch inhibition. These results suggest that dopamine receptors are involved in modulation of the ileal opioid system, so as to diminish endogenous opioid release by tetanic stimulation, and dopamine antagonists increase the opioid action, that might depend more on the increased release of endogenous opioids. The post-tetanic twitch inhibition was inhibited by adrenalectomy, and showed the supersensitivity of the opioid receptors, resulting from a decrease of endogenous opioids by adrenalectomy. These findings suggest that the increase in morphine-analgesia by adrenalectomy was due to this process. In the presence of naloxone, an opioid antagonist, an increase in basal tension after tetanic stimulation (10 Hz stimulation) (post-tetanic contraction) was observed, and was blocked by spantide, a substance P antagonist, and indomethacin, a prostaglandins-biosynthesis inhibitor. This contraction increased with morphine or peptidase inhibitor exposure, depending on the length of time the ileum was exposed to the morphine or peptidase inhibitor. Post-tetanic contraction might be a useful indicator of the formation of physical dependence to morphine or endogenous opioids in the ileum.

Identification of protein fold topology shared between different folds inhibited by natural products

Natural products have withstood the test of time as therapeutics, but new lead-generation strategies have focussed away from natural products. A new approach that uses natural-product recognition to drive an understanding of biological space might provide an impetus for renewed focus on natural-product starting points. Protein fold topology (PFT) has been shown to be an underlying factor for natural-product recognition. An investigation of natural product inhibitors of the Zincin-like fold has demonstrated their capacity also to inhibit targets of different fold types. Analysis of crystal structure complexes for natural products cocrystallised within different fold types has shown similarity at the PFT level. Two new PFT(T) (where subscript T denotes PFT shared between therapeutic targets) relationships have been established: the Zincin-like- metallohydrolase/oxidoreductase PFT(T) and the Zincin-like-phosphorylase/hydrolase PFT(T). The PFT relationship between a natural product's biosynthetic enzyme and therapeutic target, and now between different fold targets of the same natural product, suggests that PFT is the simplest descriptor of biological space. This fundamental factor for recognition could facilitate a rational approach to drug development guided by natural products.

In vitro cytocidal effects on Trypanosoma brucei and inhibition of Leishmania major GP63 by peptidomimetic metalloprotease inhibitors

Peptidomimetic inhibitors of mammalian zinc metalloproteases have been tested as potential agents for intervention in disease caused by kinetoplastid protozoa. Certain metalloprotease inhibitors were able to inhibit the release of variant surface glycoprotein from cultured transgenic procyclic Trypanosoma brucei, confirming our previous identification of a cell surface zinc metalloprotease activity in this stage of the trypanosome lifecycle [Bangs, JD et al. Expression of bloodstream variant surface glycoproteins in procyclic stage Trypanosoma brucei: role of GPI anchors in secretion, EMBO J. 1997;16:4285]. Selected peptidomimetics were also found to be toxic for cultured bloodstream trypanosomes with IC50 values in the low micromolar range. The paradigm for zinc metalloproteases in kinetoplastids are the GP63 surface enzymes of Leishmania. Peptidomimetics at low micromolar concentrations were able to inhibit in vitro cleavage of a synthetic peptide substrate by purified GP63 from L. major. Our results suggest that zinc metalloproteases perform essential functions in different stages of the trypanosome lifecycle and we hypothesize that these activities may be affected by the recently discovered trypanosomal homologues of GP63 [El-Sayed, NMA and Donelson, JE. African trypanosomes have differentially expressed genes encoding homologues of Leishmania GP63 surface protease, J. Biol. Chem. 1997;272:26742]. Development of higher affinity metalloprotease inhibitors may provide a novel avenue for treatment of parasitic diseases.

Role of brain dynorphin in nitrous oxide antinociception in mice

Earlier studies indicate that nitrous oxide antinociception is mediated by opioid receptors, and we have hypothesized that nitrous oxide stimulates a neuronal release of an endogenous opioid peptide (EOP) that stimulates opioid receptors. To further test this hypothesis, male NIH Swiss mice were pretreated intracerebroventricularly with rabbit antisera to opioid peptides or with various inhibitors of peptidases involved in the degradation of EOPs. Mice were subsequently exposed to three different concentrations of nitrous oxide in oxygen, and their antinociceptive responsiveness was measured using the acetic acid abdominal constriction test. Nitrous oxide antinociception was significantly attenuated by 24-h pretreatment with antisera to various fragments of dynorphin (DYN) but not by antisera against methionine-enkephalin (ME) or beta-endorphin (beta-EP). In other experiments, nitrous oxide antinociception was significantly enhanced by 30-min pretreatment with phosphoramidon, an inhibitor of endopeptidase 24.11, which has been implicated in DYN degradation, but not bestatin or captopril, which inhibit aminopeptidase and angiotensin-converting enzyme, respectively. The latter enzymes have been implicated in degradation of certain EOPs albeit not DYN. These findings support the hypothesis that nitrous oxide antinociception in the mouse abdominal constriction test is mediated by endogenous DYN acting in the central nervous system.

Administration of SIN-1 induces guinea pig airway hyperresponsiveness through inactivation of airway neutral endopeptidase

BACKGROUND

Peroxynitrite plays an important role in the pathogenesis of airway inflammation. We have already found that peroxynitrite may contribute to decreased beta(2)-adrenoceptor responses in airway smooth muscle. However, it is not known whether peroxynitrite can alter neutral endopeptidase (EC 3.4.24.11; NEP) activity in the airways. This study was designed to determine whether peroxynitrite induces airway hyperresponsiveness to substance P (SP) and endothelin-1 (ET-1) through the inactivation of airway NEP.

METHODS

We examined whether the administration of S-morpholinosydnonimine (SIN-1), a compound that releases peroxynitrite, increased bronchoconstrictor responses to SP and ET-1 in anesthetized guinea pigs. In addition, we assayed NEP activity in the airways of SIN-1-exposed guinea pigs.

RESULTS

Though SIN-1 (10(-7) M) alone had no effect on pulmonary resistance, pretreatment with SIN-1 significantly enhanced SP- and ET-1-induced bronchoconstriction. Pretreatment with phosphoramidon, an NEP inhibitor, also enhanced SP- and ET-1-induced bronchoconstriction. However, simultaneous administration of phosphoramidon and SIN-1 had no additive effect on SP- and ET-1-induced bronchoconstriction. Peroxynitrite formation by SIN-1 was completely inhibited by N-acetylcysteine (NAC) and glutathione (GSH) in vitro, and pretreatment with NAC and GSH significantly reversed the potentiation by SIN-1 of SP-induced bronchoconstriction. In addition, the NEP activity of the trachea after SIN-1 exposure was significantly reduced compared to the level in control guinea pigs (solvent for SIN-1: 30.0+/-4.2 fmol.min(-1).mg tissue(-1); 10(-7) M SIN-1; 15.5+/-4.5 fmol.min(-1).mg tissue(-1), p<0.05).

CONCLUSIONS

These findings suggest that peroxynitrite induces airway hyperresponsiveness to SP and ET-1 through the inactivation of airway NEP, and that peroxynitrite is an important mediator of the alterations in airway functions.

Respiratory syncytial virus upregulates expression of the substance P receptor in rat lungs

Respiratory syncytial virus (RSV) is a major respiratory pathogen in infants. The first goal of this study was to determine whether the infection following endotracheal inoculation of RSV in Fischer 344 rats results in increased inflammatory responses to substance P (SP) either released by capsaicin from sensory nerves or injected into the circulation. Five days after inoculation, the extravasation of Evans blue-labeled albumin after capsaicin or SP was significantly greater in RSV-infected airways than in pathogen-free controls. The peptide-degrading activity of the regulatory enzyme neutral endopeptidase was unaffected by RSV. However, SP(NK(1)) receptor mRNA levels increased fivefold in RSV-infected lungs, and the density of SP binding sites in the bronchial mucosa increased threefold. These data suggest that RSV makes the airways abnormally susceptible to the proinflammatory effects of SP by upregulating SP(NK(1)) receptor gene expression, thereby increasing the density of these receptors on target cells. This effect may contribute to the inflammatory reaction to the virus and could be a target for the therapy of RSV disease and its sequelae.

Breast cancer cell-associated endopeptidase EC 24.11 modulates proliferative response to bombesin

We have investigated the production, growth and inactivation of gastrin-releasing peptide (GRP)-like peptides in human breast cancer cell lines. Radioimmunoassay detected GRP-like immunoreactivity (GRP-LI) in T47D breast cancer cells but not in the conditioned medium, indicating rapid clearance. No GRP-LI was found in the ZR-75-1 or MDA-MB-436 cells or their conditioned medium. High-performance liquid chromatography (HPLC) analysis of the GRP-LI in the T47D cells revealed a major peak, which co-eluted with GRP(18-27), and a minor more hydrophilic peak. In vitro stimulation of T47D cell growth by bombesin (BN) was enhanced to 138% of control levels (bombesin alone) by the addition of the selective endopeptidase EC 3.4.24.11 inhibitor phosphoramidon (0.1 ng ml(-1)). Fluorogenic analysis using whole cells confirmed low levels of this phosphoramidon-sensitive enzyme on the T47D cells. This enzyme, previously unreported in human breast cancer cells, significantly modulates both T47D growth and its response to BN-induced growth.

Airway hyper-responsiveness to neurokinin A and bradykinin following Mycoplasma pneumoniae infection associated with reduced epithelial neutral endopeptidase

To determine whether mycoplasma infection produces airway hyper-responsiveness to tachykinins and bradykinin and, if so, to elucidate the role of neutral endopeptidase (NEP), isolated hamster tracheal segments were studied under isometric conditions in vitro. Nasal inoculation with Mycoplasma pneumoniae potentiated contractile responses to neurokinin A and bradykinin, causing a leftward shift of the dose-response curves to a lower concentration by 1 log unit for each agonist, whereas there was no response with acetylcholine. Pretreatment of tissues with the NEP inhibitor phosphoramidon augmented neurokinin A- and bradykinin-induced contractions in saline-treated control tissues, but did not further potentiate the responsiveness in M. pneumoniae-infected tissues. NEP activity in the tracheal epithelium, but not in epithelium-denuded tissues, was decreased in infected animals. These results suggest that M. pneumoniae infection causes airway bronchoconstrictor hyper-responsiveness to neurokinin A and bradykinin and that this effect may be associated with an inhibition of epithelial NEP activity.

Tachykinin NK2 receptors predominantly mediate tachykinin-induced contractions in ovine trachea

In vitro studies were conducted to characterize the contractile effects of tachykinins in normal ovine trachea with a view in the future to compare tachykinin contractile responses in allergic tissue. Tracheal smooth muscle strips were prepared for in vitro studies of isometric contraction in response to cumulative addition of carbachol, acetylcholine, histamine, neuropeptide gamma, substance P, neurokinin A, neurokinin B, [Sar9, Met(O2)11]substance P, [Nle10]neurokinin A-(4-10), and [Succinyl-Asp6, Me-Phe8]substance P-(6-11) (senktide). The rank order of potency was neuropeptide gamma > carbachol > neurokinin A > or = [Nle10]neurokinin A-(4-10) > acetylcholine > or = histamine. Phosphoramidon enhanced the contractile response to neurokinin A and substance P, but not to neuropeptide gamma, [Sar9, Met(O2)11]substance P or senktide. Repeated cumulative concentration responses for acetylcholine, substance P, neurokinin A, [Sar9, Met(O2)11]substance P and histamine were also conducted to test for tachyphylaxis. No tachyphylaxis to acetylcholine, substance P, or neurokinin A was observed, however, [Sar9, Met(O2)11]substance P and histamine did exhibit tachyphylaxis. Atropine had no effect on tracheal contractions to neurokinin A and substance P, while [Sar9, Met(O2)11]substance P contractions were atropine sensitive. Pyrilamine did not affect substance P-induced tracheal smooth muscle contractions, indicating that the response to substance P was not mediated by histamine release. These results show that, in vitro, natural tachykinins induce tracheal smooth muscle contraction predominantly by a direct effect mediated by tachykinin NK2 receptors, and a small tachykinin NK1 receptor mediated cholinergic mechanism.

Tachykinin receptors mediating non-cholinergic contraction of the guinea-pig isolated main bronchus in response to field stimulation

1. The primary aim of the present study was to classify the receptors activated by tachykinins released by field stimulation of intramural nerve terminals of the guinea-pig isolated main bronchus by using the novel non-peptide NK1 and NK2 receptor-selective antagonists SR 140333 and SR 48968, respectively. 2. Log concentration-response curves to substance P (SP), the NK1 receptor-selective agonist [Sar9, Met(O2)11]-SP and the NK2 receptor-selective agonist [Nle10]-neurokinin (NK) A(4-10) were constructed in the presence of indomethacin (2 mumol/L) and phosphoramidon (5 mumol/L). Substance P was the least potent of these agonists. 3. In left and right main bronchi, SR 140333 (100 nmol/L) antagonized concentration-related contractions evoked by SP yielding pKB values of 8.02 and 7.68, respectively. SR 140333 (10 nmol/L) antagonized the effects of [Sar9, Met(O2)11]-SP on the left bronchus with a pKB value of 8.04. 4. SR 48968 (100 nmol/L) antagonized the effects of SP yielding pKB estimates of 7.88 (left bronchus) and 7.31 (right bronchus). 5. [Nle10]-NKA(4-10) was more potent in the left than in the right main bronchus. SR 48968 (0.1-10 nmol/L) antagonized the effects of [Nle10]-NKA(4-10) on the left bronchus with pKB estimates of 8.26-10.25. 6. In the presence of indomethacin (2 mumol/L), phosphoramidon (5 mumol/L) and atropine (1 mumol/L), electrical field stimulation (EFS; 30 V, 1 ms, 15 s at 1, 3, 10 and 30 Hz) produced prolonged contractions. SR 48968 (0.1 mumol/L) markedly reduced responses to stimulation, whereas SR 140333 (0.3 mumol/L) caused a small but significant rightward displacement of the log frequency-response curve. In combination, these concentrations of SR 48968 and SR 140333 produced complete inhibition to field stimulation at 10 Hz. 7. These results indicate that: (i) both NK1 and NK2 receptor subtypes are present in the guinea-pig main bronchi with the left bronchus being more sensitive to an NK2 receptor agonist; and (ii) EFS of the main bronchus leads to frequency dependent contractions due to the release of tachykinin(s) that predominantly activate an NK2 receptor.

Neutral endopeptidase 24.11 in neutrophils modulates protective effects of natriuretic peptides against neutrophils-induced endothelial cytotoxity

This study was performed to determine effects of atrial and brain natriuretic peptides (ANP, BNP) on neutrophils-induced endothelial injury which is known to play a role in the pathophysiology of ischemia/reperfusion myocardial injury and to examine whether the effects of ANP and BNP on neutrophils are modulated by neutral endopeptidase 24.11 (NEP) in neutrophils themselves. The incubation of human neutrophils with ANP and BNP inhibited the neutrophils-induced detachment of cultured human endothelial cells (HEC). The inhibitory effect of ANP and BNP was associated with the suppressions of the neutrophils adhesiveness to HEC, CD18 expression on the neutrophils and elastase release from the neutrophils. Coincubation with UK73967 or phosphoramidon, inhibitors of NEP, potentiated all of the effects of ANP and BNP on the neutrophil functions, and the NEP inhibitors protected degradation of ANP and BNP by the neutrophils. NEP enzymatic activity in the particulate fractions and immunoreactive NEP expression were found to increase in the neutrophils from patients with early phase of acute myocardial infarction (AMI) by 5.2- and by 4.2-fold of the neutrophils from patients with late phase of AMI, respectively. In an in vivo canine model of myocardial ischemia/reperfusion, the intravenous administration of UK73967 suppressed the neutrophil adherence to endothelium and the neutrophil accumulation in the ischemic/reperfused myocardium. The results indicate that ANP and BNP, which are known to increase in AMI, modulate the neutrophil functions and exert protective effects against the neutrophils-induced endothelial cytotoxity. But the effects are suppressed due to their degradation by the neutrophil own NEP. Thus, neutrophil NEP, which also increases in AMI, may play a role in the pathophysiology of neutrophils-mediated ischemia/reperfusion endothelial and myocardial injury.

Endothelin-1 does not modulate O2.release and [Ca(2+)]i variations in resting or differentiated HL-60 cells

Endothelin-1 (ET-1) by itself was not an effective stimulus for inducing superoxide (O2.) generation in human resting or DMSO-differentiated neutrophil-like HL-60 cells. ET-1 (0.01-100 nM) was not able to modulate O2. generation stimulated by the chemotactic peptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP, EC50 = 4.24 +/- 1.63 nM in the absence and 3.16 +/- 1.95 nM in the presence of ET-1). Neither did ET-1 (0.01-100 nM) promote the mobilization of intracellular calcium ions or modulate fMLP-induced [Ca(2+)]i increase in this model of human neutrophils. Phosphoramidon, a neutral endopeptidase inhibitor, was not able to reveal any biological (O2.) or biochemical ([Ca(2+)]i response to ET-1 in the absence or in the presence of fMLP in these cells. These results indicate that DMSO-differentiated neutrophil-like HL-60 cells are not sensitive to ET-1 in terms of O2. generation or [Ca(2+)]i variations.

Tachykinins stimulate nitric oxide generation by canine cultured tracheal epithelium

We studied the effects of tachykinins on the generation of nitric oxide (NO) from canine cultured tracheal epithelial cells using a specific amperometric sensor for this molecule. Immersion of the NO-selective electrode in the medium bathing the cells detected the baseline current of 30.5-61.7 pA, which corresponded to NO concentration ([NO]) at 44.0 +/- 7.6 nM (mean +/- S.E.M.). Substance P (SP, 10(-6) M) increased the current from 51.3 +/- 9.8 to 73.6 +/- 11.4 pA (P < 0.001), an effect that was not affected by NG-nitro-D-arginine methylester, but inhibited by NG-nitro-L-arginine methylester by 83 +/- 9% (P < 0.001), and this inhibition was restored by the subsequent addition of L-arginine, but not by D-arginine. SP and neurokinin A (NKA) increased [NO] in a dose-dependent manner, the maximal increases from the baseline level being 71.0 +/- 14.9 and 33.4 +/- 8.5 nM, respectively (P < 0.001 for each), whereas neurokinin B (NKB) had no effect. In the presence of phosphoramidon, the response of each tachykinin was augmented, but the rank order of potency was still NKA > SP >> NKB. These results suggest that NO is spontaneously released from airway epithelium and that tachykinins stimulate epithelial NO generation via NK2 receptors.

A role for endothelin in bicuculline-induced neurogenic pulmonary oedema in rats

1. The possible contribution of endogenous endothelin (ET) to the pathogenesis of seizure-associated pulmonary oedema was examined in mechanically ventilated rats after intravenous bolus injection of the gamma-aminobutyric acid (GABA) antagonist, bicuculline (1.2 mg kg-1). 2. Recurrent seizure activity elicited by bicuculline injection led to rapidly developing pulmonary oedema. Within 4 min after bicuculline application (1.2 mg kg-1), arterial O2 partial pressure (PaO2) significantly dropped from 17.49 +/- 1.20 kPa to 7.51 +/- 2.21 kPa (P < 0.01) and arterial CO2 partial pressure (PaCO2) significantly increased from 4.64 +/- 0.56 kPa to 8.15 +/- 0.99 kPa (P < 0.01). Gradually a progressive acidosis developed. Moreover, mean arterial blood pressure (MABP) and end-inspiratory airway pressure (Paw) rapidly increased. 3. Concomitantly there was a time-dependent increase of big ET-1 and ET-1 levels in bronchoalveolar lavage (BAL) as determined by combined reverse phase high performance liquid chromatography (h.p.l.c.) and radioimmunoassay. BAL levels of both peptides increased up to 8 min after bicuculline injection and slowly decreased subsequently. In contrast, BAL from animals injected with vehicle did not contain detectable amounts of ET. 4. Pretreatment with the endothelin-converting enzyme inhibitor, phosphoramidon (5.4 mg kg-1, i.v.) for 5 min significantly (P < 0.001) reduced peak ET-1 levels in BAL fluid by 65.4 +/- 9.9% at 8 min after bicuculline injection. Simultaneously it afforded protection from hypoxia. PaCO2 did not increase and PaO2 decreased only slightly from 14.63 +/- 1.00 kPa to 12.97 +/- 0.61 kPa (P > 0.05) after phosphoramidon pretreatment. In contrast, vehicle-treated animals that received bicuculline showed both significant hypercapnia as well as profound hypoxia. Phosphoramidon significantly diminished the maximum increase in Paw by 76.7 +/- 12.4% (P <0.005), but only slightly affected the MABP. Phosphoramidon pretreatment had no effect on the acidosis.5. Pretreatment with the ETA receptor antagonist, BQ-123 (1 mg kg-1, i.v.), for 5 min did not affect the levels of ET-1 in the BAL fluid at 8 min after bicuculline injection but did ameliorate the development of hypoxia. No hypercapnia developed and Pa02 decreased only moderately from 16.65 +/-0.25 kPa to 14.19 +/-2.15 kPa (P>0.05) in BQ-123-treated animals. In contrast, vehicle-treated animals that received bicuculline exhibited significant hypercapnia as well as profound hypoxia. BQ-123 significantly reduced the increase in Paw by 51.3 +/- 12.8% (P < 0.01). It affected MABP only slightly and had no effect on the acidosis.6. These results suggest that ET peptides play a significant role in this model of neurogenic pulmonary oedema and may act as mediators of respiratory distress. The deleterious effects of endogenous ET in this model are primarily mediated via the ETA receptor, for they were inhibited by the ETA receptor antagonist, BQ-123. ETA receptor antagonists may therefore be of potential therapeutic value in respiratory distress.

Effect of an inhaled neutral endopeptidase inhibitor, phosphoramidon, on baseline airway calibre and bronchial responsiveness to bradykinin in asthma

BACKGROUND

Bradykinin is a potent vasoactive peptide which has been proposed as an important inflammatory mediator in asthma since it provokes potent bronchoconstriction in asthmatic subjects. Little is known at present about the potential role of lung peptidases in modulating bradykinin-induced airway dysfunction in vivo in man. The change in bronchial reactivity to bradykinin was therefore investigated after treatment with inhaled phosphoramidon, a potent neutral endopeptidase (NEP) inhibitor, in a double blind, placebo controlled, randomised study of 10 asthmatic subjects.

METHODS

Subjects attended on six separate occasions at the same time of day during which concentration-response studies with inhaled bradykinin and histamine were carried out, without treatment and after each test drug. Subjects received nebulised phosphoramidon sodium salt (10(-5) M, 3 ml) or matched placebo for 5-7 minutes using an Inspiron Mini-neb nebuliser 5 minutes before the bronchoprovocation test with bradykinin or histamine. Agonists were administered in increasing concentrations as an aerosol generated from a starting volume of 3 ml in a nebuliser driven by compressed air at 8 1/min. Changes in airway calibre were measured as forced expiratory volume in one second (FEV1) and responsiveness as the provocative concentration causing a 20% fall in FEV1 (PC20).

RESULTS

Phosphoramidon administration caused a transient fall in FEV1 from baseline, FEV1 values decreasing 6.3% and 5.3% on the bradykinin and histamine study days, respectively. When compared with placebo, phosphoramidon elicited a small enhancement of the airways response to bradykinin, the geometric mean PC20 value (range) decreasing from 0.281 (0.015-5.575) to 0.136 (0.006-2.061) mg/ml. In contrast, NEP blockade failed to alter the airways response to a subsequent inhalation with histamine, the geometric mean (range) PC20 histamine value of 1.65 (0.17-10.52) mg/ml after placebo being no different from that of 1.58 (0.09-15.21) mg/ml obtained after phosphoramidon.

CONCLUSIONS

The small increase in bronchial reactivity to bradykinin after phosphoramidon exposure suggests that endogenous airway NEP may play a modulatory role in the airways response to inflammatory peptides in human asthma.

Hydrogen peroxide inhibits lung neutral endopeptidase

Oxidants have been implicated in the pathogenesis of many inflammatory airway diseases. Neutral endopeptidase (also called enkephalinase, EC 3.4.24.11) is a peptidase that is involved in the degradation of several proinflammatory peptides, such as tachykinins and kinins. Indirect evidence suggests that airway neutral endopeptidase is inactivated by oxidants. To determine whether hydrogen peroxide inactivates neutral endopeptidase, we studied the activity of this peptidase in washed crude preparations of membranes from guinea pig lungs. Washed crude membrane preparations were exposed to increasing concentrations of hydrogen peroxide (1.25-25 mM) in the presence or absence of two different concentrations of catalase (300 and 700 U/mL). Neutral endopeptidase activity was inhibited by hydrogen peroxide in a concentration-dependent fashion (p = .0001). Addition of catalase prevented, in a concentration-dependent fashion, the inhibition of neutral endopeptidase induced by hydrogen peroxide (p = .0001). Mannitol (40 mM) and L-methionine (20 mM) did not prevent inhibition of neutral endopeptidase induced by hydrogen peroxide (2.5 mM). It can be concluded that neutral endopeptidase is inactivated by hydrogen peroxide, an effect that is prevented by catalase. Hydrogen peroxide-induced inactivation of neutral endopeptidase is not mediated by spontaneous generation of either hydroxyl radical or hypochlorous acid in the membrane preparation. Our results suggest that neutral endopeptidase inactivation may occur in airway diseases associated with exposure to or production of oxidants.

Neutral endopeptidase in the heart. Neutral endopeptidase inhibition prevents isoproterenol-induced myocardial hypoperfusion in rats by reducing bradykinin degradation

Peptide mediators may play a role in the control of myocardial perfusion. We found immunohistochemical evidence of the peptide-degrading enzyme neutral endopeptidase (NEP) in cultured rat myocytes. Therefore, we examined the effect of an NEP inhibitor, phosphoramidon, on myocardial perfusion in rats after (1) stimulating sensory nerves with capsaicin and (2) inducing myocardial hypoperfusion with isoproterenol, with or without pretreatment with selective antagonists of the substance P (NK1) and bradykinin (B2) receptors. Three to five sequential determinations of myocardial blood flow were made in anesthetized rats by injecting 100,000 radionuclide-labeled microspheres suspended in 70% dextrose into the left ventricle. Phosphoramidon doubled coronary blood flow in response to a dose of capsaicin that was ineffective in the absence of the inhibitor. Isoproterenol (50 mg/kg IP) caused an immediate fall in blood pressure and coronary blood flow; after 20 minutes, flow had returned to normal but pressure was still subnormal. Administration of phosphoramidon reduced the recovery of blood pressure but greatly increased coronary blood flow. These changes were not altered by a substance P NK1 receptor blocker but were completely abolished by a selective bradykinin B2 receptor blocker. Our data indicate that (1) NEP is present in the rat myocardium, (2) sensory nerve-induced coronary vasodilation is markedly potentiated by NEP inhibition, (3) isoproterenol-induced myocardial hypoperfusion is prevented by NEP inhibition, and (4) this effect of NEP inhibition is due to reduced degradation of bradykinin.

Epithelium modulates the kinetics of the response to substance P and its intrinsic activity in the guinea-pig trachea

The contractile response of guinea-pig tracheal preparations with or without epithelium to substance P has been studied in the presence or absence of thiorphan, an endopeptidase 24.11 inhibitor, paying special attention to the kinetics of the response. Without thiorphan, the response to substance P was greater in tracheal preparations without epithelium than in tracheal preparations with epithelium. The concentration-response curve was shifted to the left in the absence of the epithelium. In the presence of 10 microM thiorphan, the maximal contractile response induced by single doses of substance P (0.1 to 10 microM) was lower in tracheal preparations without epithelium. The maximal responses required 10 min in tracheal preparations with epithelium and 2 min in tracheal preparations without epithelium. These epithelium-dependent differences of reactivity remained in the presence of lipoxygenase or cyclooxygenase inhibitors and of selective antagonists of muscarinic, serotoninergic and histaminergic receptors, after the pre-treatment of tissues with capsaicin or compound 48/80 and in the presence of tetrodotoxin. The profile of the cumulative concentration-response curves for substance P was largely dependent on the time between two successive doses. When this time was short (2-4 min), curves established with or without the epithelium were parallel and both reached similar maximal values (2696 +/- 214 mg and 2780 +/- 62 mg, respectively). The curve in tracheal preparations without epithelium was slightly shifted to the left (EC50s: 24 +/- 10 nM and 78 +/- 19 nM). When this time was longer (10 min, ie corresponding to the time required for a full response to a single dose in intact trachea) the potency of substance P was not modified (EC50s: 13 +/- 3 nM and 52 +/- 11 nM), but a lower maximal response was observed with tracheal preparations without epithelium (1440 +/- 182 mg and 2832 +/- 209 mg). Similar results were observed with neurokinin A and neurokinin B. Thus, the removal of the epithelium led to a more rapid contraction and to a decrease of the maximal response to neurokinins, ie a decreased intrinsic activity, a property known to be drug- and tissue-dependent. These data suggest that the intrinsic activity of drugs depends on the cellular environment of the target cells in a tissue and is partly related to the diffusion and metabolism of drugs and to drug-induced hyporeactivity of the target cells.

Inhibition of neutral endopeptidase potentiates bronchoconstriction induced by neurokinin A in asthmatic patients

The endogenous tachykinins exhibit a range of properties which may be relevant in the pathophysiology of asthma. Their effects on the airways seem to be modulated by a variety of lung peptidases, including neutral endopeptidase (NEP). In order to evaluate the potential role of endogenous NEP activity in modulating tachykinins-induced bronchoconstriction in man in vivo, six atopic asthmatic patients, with a mean FEV1 value of 3.38 +/- 0.76 l, and a histamine PD20 mean value of 0.024 mg, were studied. The influence of inhaled phosphoramidon (a potent NEP inhibitor) was examined against the NKA-induced bronchospasm in a double-blind, placebo-controlled randomized study. Changes in airway calibre were followed as FEV1 and agonists responsiveness expressed as PD20 and PD15 for histamine and NKA respectively. Patients received nebulized phosphoramidon sodium salt (10(-5) M) or a control solution 10 min prior to the bronchoprovocation test with NKA. No significant difference was noticed between any of the study days and after inhaled phosphoramidon on baseline FEV1 values (3.29 +/- 0.90 l) in comparison with the control solution (3.31 +/- 0.79 l). Inhaled NKA produced a dose-dependent fall in FEV1 values in all the subjects studied with a mean PD15 value of 20.91 x 10(-9) mol. Phosphoramidon administered by inhalation elicited a significant (P < 0.01 vs baseline and control solution) potentiation in the airway responsiveness to inhaled NKA, the NKA PD15 value decreasing to 9.45 x 10(-9) mol. The present study confirms that inhaled NKA induces a dose-related bronchoconstriction in asthmatic patients and demonstrates that inhaled phosphoramidon potentiates NKA-induced bronchoconstriction.

Relative contributions of direct and indirect mechanisms mediating endothelin-induced contraction of guinea-pig trachea

1. The present study was undertaken to determine the mechanism of action of endothelin-1 (ET-1)-induced contraction of the guinea-pig isolated trachea. 2. ET-1 (1 nM-0.3 microM) produces a concentration-dependent contraction of guinea-pig trachea with an EC50 of approximately 25 nM. The combination of the peptidoleukotriene receptor antagonist, SK&F 104353 (10 microM) and the H1-histamine receptor antagonist, mepyramine (10 microM), which abolishes antigen-induced contraction in guinea-pig trachea, was without effect on ET-1 concentration-response curves. Furthermore, the platelet-activating factor (PAF) receptor antagonist, WEB 2086, (1 or 10 microM) did not inhibit ET-induced contraction. 3. ET-1 (0.3 microM) did not stimulate histamine or immunoreactive peptidoleukotriene release from guinea-pig isolated trachea. 4. The release of various prostanoids from guinea-pig trachea was increased significantly by ET-1 (0.3 microM); the profile of release was prostaglandin D2 (PGD2) = PGE2 = 6-keto PGF1 alpha (PGI2 metabolite) > thromboxane B2 = PGF2 alpha >> 9 alpha, 11 beta PGF2 (PGD2 metabolite). ET-1-induced release of prostaglandins, which was about 30% of that elicited by antigen in sensitized tissues, was not affected by epithelium removal and was observed in tissues from which the smooth muscle had been removed. Previous studies in our laboratory indicated that indomethacin potentiated contraction produced by high concentrations of ET-1, whereas a thromboxane receptor antagonist was without appreciable effect on ET-1 concentration-response curves. 5. Pretreatment of tissues for 1 h with capsaicin (10 microM), which depletes different sensory neurones, produced a small, but significant, inhibitory effect on ET-1 concentration-response curves in the presence but not the absence of the epithelium. The combination of the NK1 tachykinin receptor antagonist,CP-96,345 (0.1 microM), and the NK2 tachykinin receptor antagonist, SR 48968 (0.1 microM), was without effect on ET-l concentration-response curves but substantially antagonized capsaicin-induced contraction.6. The present data suggest that in guinea-pig isolated trachea, ET- 1 produces contraction predominantly via a direct mechanism: there is no significant contribution of the release of histamine,leukotrienes, PAF, or tachykinins (acting on NK1 or NK2 receptors). Although ET-1 evokes the release of an array of prostanoids from the trachea they do not appear to have a major influence on the contractile response.

Bronchopulmonary responses to endothelin-1 in sensitized and challenged guinea-pigs: role of cyclooxygenase metabolites and platelet-activating factor

The effect of phosphoramidon on the increase in pulmonary inflation pressure (PIP) induced by endothelin-1 (ET-1) administered by aerosol in ovalbumin (OA)-sensitized and challenged guinea-pigs was investigated after pretreatment or not of the animals with the neutral endopeptidase inhibitor, phosphoramidon, the cyclooxygenase inhibitor, indomethacin or the platelet activating factor (PAF) antagonist, BN 50730. When guinea-pigs were pretreated by phosphoramidon (0.1 mM, aerosol for 15 min), a significant enhancement of PIP was observed after administration of ET-1 (1 or 3 micrograms ml-1, aerosol for 2 min), whereas these doses of the peptide were only slightly active in control animals. In sensitized and unchallenged guinea-pigs, ET-1 (1 or 3 micrograms.ml-1, aerosol for 2 min) induced, as in controls, a moderate increase in PIP. In contrast, aerosol exposure of OA in sensitized guinea-pigs developed an increased PIP following ET-1 (1 and 3 micrograms.ml-1, aerosol for 2 min) administration, that was non significantly affected by pretreatment of the animals with phosphoramidon after the dose of 3 micrograms ml-1 ET-1. Guinea-pigs exposed to phorphoramidon and treated with indomethacin (10 mg kg-1, i.v.) or BN 50730 (25 mg kg-1, per os) significantly reduced the increase in PIP upon administration of ET-1 (3 micrograms.ml-1, aerosol for 2 min). No inhibitory effect of indomethacin was noted when ET-1 (3 micrograms.ml-1, aerosol for 2 min) was administered to sensitized and OA-exposed guinea-pigs, pretreated or not with phosphoramidon. In contrast, BN 50730 significantly reduced the increase in PIP induced by ET-1 observed in sensitized and OA-exposed guinea-pigs. Moreover, this drug was moderately active in reducing the increase in PIP induced by ET-1, when the animals were pretreated by phosphoramidon. These results suggest that a phosphoramidon-sensitive endopeptidase-like enzyme, present in the airway tissue modulates the effect of ET-1. Furthermore, the increase in PIP to ET-1 observed in aerosol-sensitized and antigen-exposed guinea-pigs appears to be mediated by PAF rather than cyclooxygenase metabolites, even though the participation of other mediators in this process is open.

Intracisternal neutral endopeptidase-24.11 inhibitors produce inhibition in gastric acid output: independence from opiate, bombesin, or neurotensin-mediated mechanisms

Intracisternal (ic) injection of the neutral endopeptidase-24.11 inhibitor phosphoramidon (1-100 nmol) produced a dose-dependent inhibition of gastric acid secretion in 2-h pylorus-ligated rats. The response resulted from a reduction in acid concentration and volume. Likewise, ic injection of another neutral endopeptidase-24.11 inhibitor Zincov (200 nmol) produced a 63% inhibition in gastric acid output. In contrast, neither intravenous injection of phosphoramidon (100 nmol) nor ic injection of the aminopeptidase inhibitor amastatin (100 nmol) produced any change in gastric acid secretion. The inhibitory effect of ic phosphoramidon (10 nmol) was not reversed by a dose of naloxone sufficient to antagonize the acid inhibitory effects of ic [D-Ala2-D-met5]enkephalinamide (8.5 nmol). Moreover, phosphoramidon-induced inhibition of acid was not reduced by the centrally effective bombesin antagonist N-acetyl-GRP(20-26)-O-CH3 or by reserpine pretreatment at a dose effective to antagonize ic neurotensin-induced inhibition in acid secretion. These results suggest that an endogenous neutral endopeptidase-24.11 sensitive substrate may act in the brain to inhibit gastric acid output by mechanisms independent of CNS opiate, bombesin or neurotensin activity.

Inhibitors of enkephalin-degrading enzymes as potential therapeutic agents

A limited number of enzymes such as membrane metalloendopeptidase (enkephalinase) and angiotensin converting enzyme appear to be involved in deactivation and modulation of circulatory regulatory peptides. Peptides such as the enkephalins are also involved in a large number of physiological processes. This multiplicity of physiological roles has made it difficult to establish the therapeutic role of enkephalin-degrading enzyme inhibitors. Other factors such as difficulty in quantification and thus measurement of processes involved in pain and mental illness have also hindered the process of establishing any therapeutic role of enkephalin-degrading enzyme inhibitors in these conditions. However, they have proved to be useful pharmacological 'tools'. The most likely therapeutic role at present appears to be in the treatment of cardiovascular disorders. As a 'profile' of pharmacological actions of enkephalin-degrading enzymes emerges, it is becoming apparent that bioavailability rather than a high degree of specificity or inhibitory potency may be the most important factor. This may be used to an advantage in future developments by the use of less specific or combined inhibitors in the form of prodrugs, designed to be active at specific sites such as the central nervous system.

Phosphoramidon potentiates the bronchoconstriction induced by inhaled bombesin in guinea pigs

The effect of the neutral endopeptidase inhibitor, phosphoramidon, on the bronchoconstriction induced by aerosolized bombesin in the guinea pig was investigated. Administered by aerosol for 1 min, bombesin (0.01 or 0.1 mg/ml) induced a dose-dependent increase in pulmonary inflation pressure. Pretreatment of the guinea-pigs with phosphoramidon (0.1 mM), administered by aerosol for 15 min, 15 min prior to challenge, markedly potentiated the increase in pulmonary inflation pressure induced by bombesin (0.01 mg/ml) and substance P (0.1 mg/ml). This result suggests a local hydrolysis of bombesin by airway neutral endopeptidase reducing the activity of this peptide on smooth muscle.

Enkephalins, brain and immunity: modulation of immune responses by methionine-enkephalin injected into the cerebral cavity

There is a large number of interactions at molecular and cellular levels between the nervous system and the immune system. It has been demonstrated that the opioid neuropentapeptide methionine-enkephalin (Met-Enk) is involved in humoral and cell-mediated immune reactions. Met-Enk injected peripherally produces a dual and dose-dependent immunomodulatory effect: high doses suppress, whereas low doses potentiate the immune reactivity. The present mini-review concerns the immunological activity of Met-Enk after its administration into the lateral ventricles of the rat brain, and describes the extraordinary capacity of centrally applied Met-Enk to regulate/modulate the immune function. This survey is composed of sections dealing with (a) the role of opioid peptides in the central nervous system (CNS); (b) the activity of opioid peptides in the immune system; (c) the application of Met-Enk into the cerebral cavity; (d) the influence of centrally administered Met-Enk on nonspecific local inflammatory reaction; (e) the effect of Met-Enk injected intracerebroventricularly (i.c.v.) on specific delayed hypersensitivity skin reaction, experimental allergic encephalomyelitis, anaphylactic shock, plaque-forming cell response, and hemagglutinin production; (f) the central antagonizing action of quaternary naltrexone, an opioid antagonist that does not cross the brain-blood barrier, on Met-Enk-induced immunomodulation; (g) the alteration of immune responsiveness by i.c.v. injection of enkephalinase-degrading enzymes; (h) the participation of the brain-blood/blood-brain barrier in the CNS-immune system interaction; and (i) the role of opioid receptors in immunological activity of Met-Enk. A hypothesis has been advanced for the reaction of Met-Enk and opioid receptor sitting on the cell membrane. This concept suggests that the constellation of chemical residues of enkephalin and receptor in the microenvironment determines the binding between the opioid partners. The plurality of conformational structures of enkephalins and receptors makes possible their involvement in a variety of processes which occur in different physiological systems, including the nervous system and the immune system, and intercommunications between the two systems.

Cultured human synovial fibroblasts rapidly metabolize kinins and neuropeptides

Kinins and substance P have been implicated in the pathogenesis of inflammatory arthritis by virtue of their abilities to induce vasodilation, edema, and pain. The relative biological potencies of these peptides in vivo would depend at least in part upon their rates of catabolism in the joint. We hypothesized that human synovial lining cells may regulate intraarticular levels of kinins and neuropeptides via degradation by cell surface-associated peptidases. We exposed intact human synovial fibroblasts to kinins and substance P, in the presence or absence of specific peptidase inhibitors, and measured the amount of intact substrate remaining and degradation product(s) generated over time. Aminopeptidase M (AmM; EC 3.4.11.2), neutral endopeptidase-24.11 (NEP-24.11; EC 3.4.24.11), and dipeptidyl(amino)peptidase IV (DAP IV; EC 3.4.14.5) were identified on the cell surface of synovial cells. Bradykinin degradation was due entirely to NEP-24.11 (1.39 +/- 0.29 nmol/min per well). Lysylbradykinin was also degraded by NEP-24.11 (0.80 +/- 0.19 nmol/min per well); however, in the presence of phosphoramidon, AmM-mediated conversion to bradykinin (3.74 +/- 0.46 nmol/min per well) could be demonstrated. The combined actions of NEP-24.11 (0.93 +/- 0.15 nmol/min per well) and DAP IV (0.84 +/- 0.18 nmol/min per well) were responsible for the degradation of substance P. AmM (2.44 +/- 0.33 nmol/min per well) and NEP-24.11 (1.30 +/- 0.45 nmol/min per well) were responsible for the degradation of the opioid peptide, [Leu5]enkephalin. The identity of each of the three peptidases was confirmed via synthetic substrate hydrolysis, inhibition profile, and immunological identification. The profiles of peptidase enzymes identified in cells derived from rheumatoid and osteoarthritic joints were identical. These data demonstrate the human synovial fibroblast to be a rich source of three specific peptidases and suggest that it may play a prominent role in regulating peptide levels in the joint.

Angiotensin II potentiates neurally mediated contraction of rabbit airway smooth muscle

The effect of angiotensin II (AT II) on cholinergic neurotransmission in rabbit tracheal segments was studied under isometric conditions in vitro. AT II concentration-dependently potentiated the contractile response to electrical field stimulation (EFS), and caused a leftward shift of the frequency-response curves for EFS, so that the stimulus frequency required to produce a half-maximal effect (ES50), decreased from 7.0 +/- 0.1 to 3.0 +/- 0.1 Hz (P less than 0.01). In contrast, the contractile response to acetylcholine was not affected. Non-peptide AT II receptor antagonist CV-2961 attenuated the effect of AT II on the EFS-induced contraction. Pretreatment of tissues with thiorphan or phosphoramidon did not alter the action of AT II. Thus, AT II may prejunctionally potentiate the neurally-mediated contraction of airway smooth muscle through activation of AT II receptors on the cholinergic nerve terminals, and this effect may not be modulated by endogenous neutral endopeptidase.

Ketomethylene analogues of phosphoryl dipeptides related to phosphoramidon: synthesis and inhibition of proteases

Non-rhamnose-containing phosphoramidon analogues, in which the amide bond was replaced by the isosteric ketomethylene group, have been synthesized in order to stabilize these compounds to peptidase degradation. The key step in this synthesis was suitable alkylation of a 4-ketodiester, prepared from Z-Leu chloromethyl ketone and dimethyl malonate. The ketomethylene dipeptide derivatives P-Leu psi (COCH2)(RS)Xaa-OMe (Xaa = Trp, Phe) are good inhibitors of thermolysin, ACE and specially enkephalinase.

Effect of epithelium removal and of enkephalin inhibition on the bronchoconstrictor response to three endothelins of the human isolated bronchus

The three endothelins ET-1, ET-2 and ET-3 induced a potent contractile response in the human isolated bronchus with an intact epithelium, which proceeded on two different stages. The first stage was observed at low concentrations (high potency) (10(-12) to 10(-9) M) but corresponded to a low intrinsic activity (Emax maximal effect induced by ACh 10(-3) M), the second stage appeared at higher concentrations and corresponded to higher intrinsic activity. The rank order of potency for the two stages of contractile activity was ET-1 greater than ET-2 = ET-3. Removal of the epithelium significantly enhanced the two stages of the contractile responses to the three endothelins and abolished the differences in potency efficacy that were observed between ET-1, ET-2 and ET-3 when the epithelium was present. Phosphoramidon (10(-5) M), an enkephalinase inhibitor, was as potent as epithelium removal in enhancing the contractile responses to these agonists at low concentrations (first stage of contraction, 10(-16) to 10(-9) M). However, with high concentrations of endothelins (greater than 10(-9) M, second stage of contraction), phosphoramidon was less potent than epithelium removal in enhancing the contractile responses. In epithelium-denuded strips, preincubation with phosphoramidon did not further increase the maximal contractions induced by/or the potencies of ET-1, ET-2 or ET-3. After epithelium removal, responses to low doses of endothelins were attenuated by nicardipine (10(-6) M) whereas responses to high doses of the endothelins were not affected, as was also observed when the epithelium was present. (ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged antinociceptive activity of pseudodipeptide analogues of Lys-Trp(Nps) and Trp(Nps)-Lys

Peptide bond substitution in the molecules of Lys-Trp(Nps) (LTN) and Trp(Nps)-Lys (TNL) by an aminomethylene and ketomethylene bond, respectively, afforded pseudodipeptides with analgesic activity. The new compounds Lys psi(CH2NH)-Trp(Nps)-OMe (LTNAM) and Trp(Nps)psi(COCH2)(R,S)-Lys (TNLKM) induced a dose-dependent and naloxone-reversible analgesia following intracerebroventricular (ICV) administration to mice. The antinociceptive effects were longer lasting compared to those induced by the parent compounds. The pseudodipeptides protected Met-enkephalin degradation by rat striatal slices and, combined with an ineffective dose of the opioid peptide, induced analgesia. LTNAM and TNLKM were as potent as LTN to inhibit brain aminopeptidase in vitro and ex vivo. An increased resistance to proteolysis of the pseudodipeptides may explain their prolonged analgesic activity.

Novel inhibitors of enkephalin-degrading enzymes. IV: Structure-activity relationships within the penicillins as enkephalinase inhibitors

A range of penicillins have been examined as competitive reversible inhibitors of enkephalinase (neutral endopeptidase; EC 3.4.24.11). Carfecillin (Ki = 0.18 microM) was the most potent inhibitor in the series, whereas cloxacillin (27.5 microM), ampicillin (41.0 microM), nafcillin (58.7 microM) and carbenicillin (158 microM) had moderate potency and benzyl penicillin (885 microM), mezlocillin (473 microM) and azlocillin (556 microM) were weak inhibitors. Structure-activity relationships within the series have been rationalised from a consideration of molecular graphics analysis of the match between receptor binding groups with thiorphan as well as log P values.

Naloxone blocks conditioned place preference induced by substance P and [pGlu6]-SP(6-11)

The effect of prior treatment with the opioid receptor (opioceptor) antagonist naloxone on conditioned place preference produced by the neurotachykinin substance P (SP) and its C-terminal hexapeptide analog [pGlu6]-SP(6-11) (SPC) was investigated in rats. Place conditioning was assessed using a circular open field partitioned into four quadrants that were equally preferred by the rats prior to drug treatment. On three successive days, rats received an intraperitoneal (i.p.) injection of naloxone-HCl (1 mg/kg) or vehicle 15 min before an i.p. injection of either 37 nmol/kg SP, equimolar dosed SPC or corresponding diluent vehicle. After injection the rats were placed into their assigned treatment corral for 15 min. During the test for conditioned corral preference (CCP), when provided a choice between the four quadrants, rats injected with SP or SPC spent more time in the treatment corral compared to vehicle controls, indicative of a positive reinforcing action of these peptides. The pre-treatment with naloxone blocked the positive reinforcing effects of both SP and SPC; when injected alone, naloxone did not influence the preference behavior. Gross locomotor activity was affected by neither treatment. Thus, the positive reinforcing effects of SP and SPC may be mediated via interactions with the endogenous opioid system(s).

Metabolism of bradykinin agonists and antagonists by plasma aminopeptidase P

In addition to angiotensin I converting enzyme (ACE; EC 3.4.15.1) and carboxypeptidase N (CPN; EC 3.4.17.3), other peptidases contribute to bradykinin (BK) degradation in plasma. Rat plasma degraded BK by hydrolysis of the N-terminal Arg1-Pro2 bond, and the characteristics of hydrolysis are consistent with identification of aminopeptidase P (APP; EC 3.4.11.9) as the responsible enzyme. BK and BK[1-5] N-terminal hydrolysis was optimal at neutral pH, was inhibited by 2-mercaptoethanol, dithiothreitol, o-phenanthroline and EDTA, but was unaffected by the aminopeptidase inhibitors amastatin, puromycin and diprotin A, the endopeptidase-24.11 inhibitors phosphoramidon and ZINCOV, and the ACE and CPN inhibitors captopril and D,L-mercapto-methyl-3-guanidinoethylthiopropanoic acid (MERGETPA), respectively. Although kallidin (Lys-BK) was not metabolized directly by APP, conversion to BK by plasma aminopeptidase M (EC 3.4.11.2) resulted in subsequent degradation by APP. BK analogs containing N-terminal Arg1-Pro2 bonds, including [Tyr8-(OMe)] BK and [Phe8 psi(CH2NH)Arg9]BK (B2 agonists), des-Arg9-BK and [D-Phe8]des-Arg9-BK (B1 agonists), and [Leu8]des-Arg9-BK (B1 antagonist), were degraded by APP with Km and Vmax values comparable to those found for BK (Km = 19.7 +/- 2.6 microM; Vmax = 12.1 +/- 1.2 nmol/min/mL). In contrast, B2 antagonists containing D-Arg0 N-termini, including D-Arg[Hyp3,Thi5.8,D-Phe7]BK and D-Arg[Hyp3,D-Phe7,Phe8 psi(CH2NH)Arg9]BK, were resistant to APP-mediated hydrolysis. These data support a role for plasma aminopeptidase P in the degradation of circulating kinins, and a variety of B2 and B1 kinin agonists and antagonists. However, APP does not participate in the degradation of D-Arg0-containing antagonists.

Enkephalinase-inhibitors modulate immune responses

Exogenous opioid peptides, and enkephalins in particular, modulate a variety of immune performances in vivo and in vitro. In this study, the immunomodulatory role of endogenous opioids was investigated by means of central and peripheral administration of four peptidase inhibitors in the rat. Animals sensitized with sheep red blood cells (SRBC) were daily treated intraperitoneally (i.p.) with 0.2 mg/kg or 1 mg/kg of each inhibitor, or intracerebroventricularly (i.c.v.) with 0.02 mg/kg and 0.2 mg/kg of bestatin, des-tyrosine-methionine-enkephalin, (Des-Tyr1)Met-Enk; and actinonin, and 0.005 and 0.5 mg/kg of N-Carboxymethyl-phenilalanine-leucine,(N-C)Phe-Leu. Controls were injected i.p. and i.c.v. with saline. The results revealed that in animals treated i.p. with 0.2 mg/kg of bestatin and (N-C)Phe-Leu potentiated the plaque-forming cell (PFC) response and hemagglutinin production. In contrast, these immune responses were suppressed by 1 mg/kg. On the other hand, i.p. doses of 0.2 and 1 mg/kg of actinonin and (Des-Tyr1)Met-Enk potentiated humoral immune responses. When given i.c.v., all of the inhibitors used exerted clear dose-dependent immunomodulatory effects, i.e. increase in the PFC response and hemagglutinin production when given at lower doses (0.005-0.02 mg/kg), and decrease when injected with higher doses (0.2-0.5 mg/kg). These effects of enkephalin-related peptidase inhibitors, applied i.p. and i.c.v., suggest the involvement of endogenous enkephalins in immune mechanisms.

Neutral endopeptidase and kininase II mediate glucocorticoid inhibition of neurogenic inflammation in the rat trachea

Glucocorticoids inhibit plasma extravasation induced in the rat tracheal mucosa by substance P and other tachykinins released from sensory nerves. This study was performed to determine whether this antiinflammatory effect of glucocorticoids is mediated by the tachykinin-degrading enzymes neutral endopeptidase (NEP) and kininase II (angiotensin converting enzyme, ACE). In addition, we studied the effect of dexamethasone on a nonpeptide inflammatory mediator, platelet-activating factor (PAF), which is not degraded by NEP or ACE. Adult male pathogen-free F344 rats were treated for 2 d with dexamethasone (0.5 mg/kg per d i.p.), or with the vehicle used to dissolve the steroid. The magnitude of plasma extravasation produced by an intravenous injection of substance P (5 micrograms/kg) or PAF (10 micrograms/kg) was then assessed by using Monastral blue pigment as an intravascular tracer. The role of NEP and ACE activities in the changes produced by dexamethasone was investigated by examining the effect of the selective inhibitors of these enzymes, phosphoramidon and captopril. Dexamethasone reduced the substance P-induced extravasation by 57% but did not affect the PAF-induced extravasation. The suppressive effect of dexamethasone on substance P-induced extravasation was completely reversed by simultaneously inhibiting NEP and ACE activities, but the inhibition of these enzymes had no effect on PAF-induced extravasation, regardless of whether the rats were pretreated with dexamethasone or not. These results suggest that NEP and ACE mediate a selective inhibitory effect of glucocorticoids on neurogenic plasma extravasation.

Vasoactive intestinal peptide stimulates ciliary motility in rabbit tracheal epithelium: modulation by neutral endopeptidase

We studied the effect of vasoactive intestinal peptide (VIP) on ciliary activity in rabbit cultured tracheal epithelium by a photoelectric method in vitro. Administration of VIP (10(-7) M) elicited an increase in ciliary beat frequency (CBF) from the baseline values of 970 +/- 52 to 1139 +/- 75 beats/min (mean +/- S.E., P less than 0.01). This ciliostimulatory effect was dose-dependent, with the maximal increase and EC50 value being 17.4 +/- 1.0% (P less than 0.05) and 6.10(-11) M, respectively. The VIP-induced increase in CBF was abolished by pretreatment of cells with [4-Cl-D-Phe6, Leu17]-VIP, a VIP receptor antagonist. The neutral endopeptidase inhibitor phosphoramidon (10(-5) M) potentiated the effect of VIP, so that the CBF dose-response curve for VIP was shifted to lower concentrations by 0.5 log U. The administration of VIP increased cyclic AMP levels in epithelial cells, an effect that was also potentiated by phosphoramidon. These results suggest that VIP may interact with its specific receptors and stimulate airway ciliary activity probably through the activation of adenylate cyclase, and that neutral endopeptidase may play a role in modulating this effect of VIP.

Novel inhibitors of enkephalin-degrading enzymes. III: 4-Carboxymethylamino-4-oxo-3 (phenylamino) butanoic acids as enkephalinase inhibitors

4-Carboxymethylamino-4-oxo-3-(4'-aminophenylamino) butanoic acid (25), its ethyl ester (26) and the corresponding unsubstituted-aryl analogues (17) and (16) are fairly potent inhibitors of enkephalinase (neutral endopeptidase; EC 3.4.24.11), Ki = 0.14-0.39 microM, with weak inhibitory potency, Ki = 15-75 microM, towards aminopeptidase MII. In the mouse abdominal constriction test, the esters (26) and (16) showed systemic inhibitory (antinociceptive) activity with ED50 values 62 +/- 3.05 and 81 +/- 1.74 mg/kg respectively. In the mouse tail immersion test, both (26) and (16) exhibited antinociceptive activity when administered intracerebroventricularly and (26) also exhibited a systemic effect which was only partially reversed by naltrexone. The antinociceptive effect seen with (26) reflects its ranking in vitro as an inhibitor of enkephalinase (Ki = 0.14 microM) but it is possible that this effect is not totally opioid-mediated. Compounds (26) and (16) represent the first combined inhibitors of enkephalinase and aminopeptidase MII.

Angiotensin converting enzyme in the human basal forebrain and midbrain visualized by in vitro autoradiography

angiotensin converting enzyme converts angiotensin I to angiotensin II, a peptide that plays an important role in the central regulation of blood pressure and fluid and electrolyte homeostasis. However, the distribution of this enzyme in the human brain has not been well described. In this study, angiotensin converting enzyme was mapped in the human basal forebrain and midbrain by using quantitative in vitro autoradiography employing a derivative of a potent converting enzyme inhibitor, 125I-351A, as radioligand. This radioligand binds specifically and with high affinity to angiotensin converting enzyme and also exhibited these properties in binding to slide-mounted sections of human basal ganglia. In the basal ganglia, high levels of binding of 125I-351A are found in the caudate nucleus, putamen, nucleus accumbens, both divisions of the globus pallidus, and substantia nigra pars reticulata. High densities of labelling also occur in the ventral pallidum. In the hypothalamus, a moderate level occurs in the paraventricular and supraoptic nuclei, and a diffuse, low level of binding is found throughout the periventricular region. The organum vasculosum of the lamina terminalis, one of the circumventricular organs, displays the highest concentration of binding. The choroid plexus contains only moderate density of labelling in contrast to other mammalian species previously studied. Major fibre tracts are devoid of activity except for the posterior limb of the internal capsule, which contains fascicles of intense activity. In the midbrain, a moderate density of binding is detected in the periaqueductal gray. The dorsal, central linear, and, more caudally, the centralis superior medialis raphe nuclei also contain moderate densities of labelling. Angiotensin converting enzyme is heterogeneously distributed in the caudate nucleus and putamen, with distinct patches of high concentration surrounded by a matrix of diffuse, lower levels. In the caudate nucleus, these patches of high binding corresponded to striosomes since they register with acetylcholinesterase-poor zones. The high concentration of angiotensin converting enzyme found in the basal ganglia suggests that the enzyme may be involved in processing neuropeptides that occur in high concentrations in these structures. Possible substrates for converting enzyme include not only angiotensin I but also substance P and enkephalins, which are also concentrated in striosomes.

Peptidase modulation of the pulmonary effects of tachykinins in tracheal superfused guinea pig lungs

The effects of the angiotensin converting enzyme (ACE) inhibitor captopril and the neutral endopeptidase (NEP) inhibitors thiorphan and SCH 32615 on the changes in airway opening pressure (PaO) and the recovery of offered peptide were studied after intratracheal administration of substance P (SP) and neurokinin A (NKA) in isolated guinea pig lungs superfused through the trachea. Pao changes and the recovery of offered peptide were significantly greater in NEP inhibitor-treated lungs than in control lungs. Captopril did not cause a significant change in the physiological effects or the recovery of SP and NKA. HPLC analysis of [3H]Pro2,4-SP and 125I-Histidyl1-NKA perfused through the airways showed major cleavage products consistent with NEP action. We conclude that there is significant degradation of both SP and NKA after tracheal infusion of peptides by NEP-like but not by ACE activity; this effect significantly influences the physiological effects of these peptides.

Receptors mediating the effects of substance P and neurokinin A on mucus secretion and smooth muscle tone of the ferret trachea: potentiation by an enkephalinase inhibitor

1. The effects of substance P (SP) and neurokinin A (NKA) were examined on tracheal smooth muscle tone, mucus volume output, lysozyme output and albumin transport across the ferret in vitro whole trachea in the presence and absence of the enkephalinase inhibitor, thiorphan. 2. SP (0.001-3 microM) and NKA (0.01-10 microM) contracted the tracheal smooth muscle and increased mucus volume, lysozyme and albumin outputs into the tracheal lumen. The EC50 values for SP and NKA for all of the variables measured were significantly reduced, and all of the maximum responses were significantly enhanced by thiorphan (10 microM). 3. In the presence of thiorphan, SP (1 microM) and NKA (10 microM) produced albumin concentrations in the secreted mucus (8.9 and 7.2 micrograms microliters-1) which were greater than those in the submucosal buffer (4.2 micrograms microliters-1). 4. In the presence of thiorphan, NKA was approximately 5 times more potent than SP at contracting the tracheal smooth muscle. Conversely SP was 23, 15 and 22 times more potent than NKA at stimulating mucus volume, lysozyme and albumin outputs respectively. 5. Thus, there is neutral endopeptidase in the ferret trachea in vitro which cleaves exogenously applied SP and NKA, thereby reducing the magnitude and potency of their actions. SP and NKA contract the ferret tracheal muscle probably by an action at NK2 (or NK3)-receptors but stimulate mucus volume output, lysozyme output and albumin transport across the tracheal wall probably by an action on NK1 receptors.

Guinea-pig tracheal epithelium and endothelin

Removal of the epithelium increased the responsiveness of isolated guinea-pig trachea to the contractile effects of endothelin. This phenomenon was observed in the presence of indomethacin (5 microM), captopril (10 microM), bacitracin (20 micrograms/ml) or leupeptin (50 microM), but was inhibited by phosphoramidon (10 microM). Bacitrain enhanced contraction produced by endothelin. The increased responsiveness of denuded guinea-pig trachea to endothelin may be due to removal of an epithelium-derived phosphoramidon-sensitive peptidase.

Cigarette smoke induces bronchoconstrictor hyperresponsiveness to substance P and inactivates airway neutral endopeptidase in the guinea pig. Possible role of free radicals

We examined the effects of acute exposure to cigarette smoke on the airway responses to substance P in anesthetized guinea pigs and on the activity of airway neutral endopeptidase (NEP). After exposure to air or to cigarette smoke we measured the change in total pulmonary resistance (RL) induced by increasing concentrations of aerosolized substance P in the absence or presence of the NEP inhibitor phosphoramidon. In the absence of phosphramidon the bronchoconstrictor responses to substance P were greater in cigarette smoke-exposed guinea pigs than in air-exposed animals. Phosphoramidon did not further potentiate the responses to substance P in smoke-exposed guinea pigs, whereas it did so in air-exposed animals. In the presence of phosphoramidon, bronchoconstrictor responses to substance P in animals exposed to air or to cigarette smoke were not different. Aerosols of SOD delivered before cigarette smoke exposures dramatically reduced smoke-induced hyperresponsiveness to substance P, whereas heat-inactivated SOD had no effect on smoke-induced hyper-responsiveness to substance P. Cigarette smoke solution inhibited NEP activity from tracheal homogenate in a concentration-dependent fashion, an inhibitory effect that was mostly due to the gas phase of the smoke, but not to nicotine. The mild chemical oxidant N-chlorosuccinimide mimicked the concentration-dependent inhibitory effect of smoke solution on airway NEP activity. We conclude that cigarette smoke causes enhanced airway responsiveness to substance P in vivo by inactivating airway NEP. We suggest that cigarette smoke-induced inhibition of airway NEP is due to effects of free radicals.

Enkephalinase inhibition increases plasma atrial natriuretic peptide levels, glomerular filtration rate, and urinary sodium excretion in rats with reduced renal mass

To investigate the in vivo effects of inhibition of endopeptidase 24.11, an enkephalinase enzyme shown to be involved in atrial natriuretic peptide (ANP) breakdown in vitro, we infused phosphoramidon, a specific inhibitor of endopeptidase 24.11, into rats with reduced renal mass (and chronic extracellular volume expansion) and into normal rats. Relative to baseline values in rats with remnant kidneys, phosphoramidon led to elevations of plasma ANP levels and concomitant increases in urinary sodium excretion, fractional excretion of sodium, glomerular filtration rate, filtration fraction, and urinary cyclic GMP excretion. Similar changes in renal function and urinary cyclic GMP excretion were obtained with thiorphan, another endopeptidase 24.11 inhibitor. These enhanced ANP levels and renal actions were not observed with phosphoramidon in normal rats. These results show that plasma ANP levels can be modulated in rats with reduced renal mass by inhibition of endopeptidase 24.11.