Comparison of muscarinic and beta adrenergic receptors between bovine peripheral lung and tracheal smooth muscles: a striking difference in the receptor concentration

In vitro dry powder inhaler formulation performance considerations

It has long been desired to match airflow conditions during formulation evaluation to those of relevance to lung deposition. In this context several strategies have been adopted involving sampling at different: flow rate (without consideration of flow conditions, e.g. shear, Reynolds number, work function); pressure drop (with and without consideration of flow conditions) and; flow rate and pressure drop. Performance testing has focused on the influence of these sampling conditions on delivered dose uniformity and aerodynamic particle size distribution. However, in order to be physiologically relevant it is also important to know when the drug was delivered with respect to initiation of airflow as variation in this parameter would influence lung deposition. A light obscuration method of detecting the dose delivered from a dry powder inhaler while sampling for aerodynamic particle size distributions (APSD) by inertial impaction has been developed. Four formulations of albuterol sulfate and budesonide in sieved and milled lactose, respectively, were dispersed and their rate of delivery monitored. The differences observed have the potential to impact the site of delivery in the lungs. The rate of delivery of drug is clearly an important companion measurement to delivered dose and APSD if the intent is to predict the similarity of in vivo performance of dry powder inhaler products.

Segment-dependent expression of muscarinic acetylcholine receptors and G-protein coupling in the equine respiratory tract

Muscarinic receptors are considered to be of comparable clinical importance in chronic obstructive pulmonary disease (COPD) in equines and in humans. At present, data are scarce on the expression and distribution of probable subtypes of these receptors and their signalling pathways in airway segments, including lung parenchyma and bronchial and tracheal epithelium with the underlying smooth muscle in horses. Specific [N-methyl-3H]scopolamine chloride ([3H]NMS) binding to all three tissues was saturable and of high affinity, with KD values ranging between 1.6+/-0.7 and 1.9+/-0.3 nmol/L. [3H]NMS binding identified a higher density of total muscarinic receptors (fmol/mg protein) in the trachea (720+/-59 nmol/L) than in bronchi (438+/-48 nmol/L) or lung (22 +/- 3 nmol/L). Competitive binding studies using [3H]NMS and the unlabelled subtype-selective antagonists pirenzepine and telenzepine (M1), methoctramine and himbacine (M2), 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) (M3), tropicamide (M4) and mamba toxin (MT-3) (M4) indicated the presence of at least three muscarinic receptor subtypes in peripheral lung tissue (50:40:24-28%: M2>M3>M1), whereas in bronchus and trachea M2 subtypes (87-90%) predominated over M3 (14-22%), and M1 subtypes were lacking. No differences were found between tissues in high-affinity binding sites for carbachol in the absence (31-36%) or presence of guanosine 5'-triphosphate (GTP) (approximately 100%). Western blotting for G-protein alpha-subunits showed a much more robust expression of G(alphai1/2) in the trachea (with highest receptor density) than in the lung or bronchi, whereas G(alphas)-protein was dominantly expressed in bronchus. Concomitantly, carbachol inhibited isoproterenol- and GTP-stimulated adenylyl cyclase activity with increasing muscarinic receptor expression (trachea > bronchi > lung). We conclude that the expression and signalling pathways of muscarinic receptors in the equine respiratory tract are segment-dependent. These receptors might contribute to the pathogenesis of COPD in the horse and could provide potential drug targets for the therapeutic use of anticholinergics in this species.

Manufacture, characterization, and pharmacodynamic evaluation of engineered ipratropium bromide particles

PURPOSE

The intent of this research was to generate and characterize respirable particles of ipratropium bromide (IPB), a short-acting anticholinergic bronchodilator, to achieve demonstrable sustained-release properties. The value of a long-acting anticholinergic agent is evident in the use of tiotropium for the treatment of chronic obstructive pulmonary disease.

METHODS

Hollow, spherical particles of ipratropium bromide suitable for inhalation were generated using a spray-drying process and characterized by laser diffraction particle size analysis, scanning electron microscopy, dynamic vapor sorption, X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, and dissolution testing. Experimental design techniques were used to identify critical process parameters and optimize the spray drying process. Pharmacodynamic studies were conducted to determine duration of effect.

RESULTS

Crystalline, stable, respirable particles with a range of dissolution profiles were manufactured by application of polylactic acid (PLA) coatings of 1, 5, 10, 15, 30, and 50% w/w. A novel, robust, modified Type IV dissolution method discriminated between formulations and guided their development. Preliminary studies in guinea pigs indicated an increased duration of bronchodilatory effect for 30% PLA-coated particles (56.3 min) particles compared with IPB powders alone (11.0 min).

CONCLUSIONS

Sustained-release respirable particles of ipratropium bromide were developed using a PLA spray coating approach and a trend for increased duration of effect was demonstrated in guinea pigs.

Contractility and Ca2+ signaling of smooth muscle cells in different generations of mouse airways

The control and mechanisms of airway smooth muscle cell (SMC) contraction were investigated with a sequential series of lung slices from different generations of the same airway from the cardiac lobe of the mouse lung. Airway contraction was measured by monitoring the changes in airway lumen area with phase-contrast microscopy. Changes in intracellular calcium concentration of the SMCs were studied with a custom-built confocal or two-photon microscope. The distribution of the airway SMCs and the muscarinic M(3) or 5-HT(2A) receptors was determined with immunofluorescence. Methacholine and 5-HT induced a concentration-dependent airway contraction and Ca(2+) oscillations within the SMCs of each airway generation. The airway contraction in response to the same agonist concentration was greater in the middle generation compared with the distal or proximal generations of the same airway. Similarly, the Ca(2+) oscillations varied in different generations of the same airway, with a slower frequency in the SMCs of the distal zone as compared with the middle or proximal zones of airways. By contrast, high KCl induced minimal contraction and very slow Ca(2+) oscillations throughout the whole intrapulmonary airway. The slower agonist-induced Ca(2+) oscillations in the distal zone correlated with a reduced expression of agonist receptors. The layer of SMCs increased in thickness in the middle and proximal zones. These results indicate that the contractility of airway SMCs varies at different positions along the same airway and that this response partially results from different Ca(2+) signaling and the total amount of the SMCs.

Agonist-independent alteration in beta-adrenoceptor-G-protein-adenylate cyclase system in an equine model of recurrent airway obstruction

We examined the inhibitory sympathetic beta-adrenergic mechanisms in peripheral lung, bronchi and trachea of an equine model of recurrent airway obstruction (RAO), to support the hypothesis that the beta-adrenergic receptor dysfunction is not only restricted to cell surface receptor density but rather encompasses a mechanistic defect apart from the receptor, to the intracellular signaling components. The non-asthmatic lung possessed 3.2-fold more beta-adrenergic receptors than bronchi (496 +/- 19.4 vs. 155.1+/- 19.6 fmol/mg protein; P < 0.01) and 6.2-fold higher than in the trachea (79.8 +/- 12.6 fmol/mg protein; P < 0.001) (assessed by radioligand binding assays using (-)-[(125)I]-iodocyanopindolol, ICYP) and in all tissues a greater proportion of the beta(2)- than the beta(1)-subtype (75-80%). The receptor density (B(max)) in lung parenchyma and bronchial membranes was 33 and 42%, respectively, lower (P < 0.001) in RAO than in control animals, attributable to a decrease in the beta(2)-subtype. This receptor down-regulation was accompanied with an attenuated coupling efficiency of the receptor to the stimulatory G(S)-protein (P < 0.05 vs. control). Concomitantly, activation of adenylate cyclase evoked by isoproterenol was significantly reduced in lung and bronchial membranes of animals with RAO, whereas effects of 10 microM GTP, 10mM NaF, 10 microM forskolin and 10 mM Mn(2+) were not altered. There was no difference in beta-adrenergic receptor density, G(S)-protein or adenylate cyclase coupling in the trachea between asthmatic and control animals. In conclusion, in stable asthma the pulmonary beta-adrenergic receptor-G(S)-protein-adenylate cyclase system is impaired, thus the pathologic process involves all signaling components, and due to its close similarity, this animal model seems to serve as a suitable model, at least partly, of chronic asthmatic patients.

A Randomized Controlled Trial To Assess the Optimal Dose and Effect of Nebulized Albuterol in Acute Exacerbations of COPD

STUDY OBJECTIVES

Despite the widespread use of short-acting, inhaled beta(2)-agonists in acute exacerbations of COPD (AECOPDs), little is known about their optimal dose. The aims of this study are to compare the bronchodilator response to incremental doses of inhaled albuterol during and after recovery from an AECOPD, and to compare the effects of regular nebulized albuterol, 2.5 mg and 5 mg, on the speed of recovery.

METHODS

Eighty-six patients admitted with an AECOPD were recruited. Each patient was administered incremental doses of inhaled albuterol on hospital admission and following recovery. Dose-response curves were constructed based on FEV(1) and peak expiratory flow rate (PEFR) recorded after each incremental dose. Patients were then randomized in a double-blind fashion to receive 2.5 mg or 5 mg of nebulized albuterol q4h until recovery. Twice-daily PEFR, the number of extra doses of bronchodilators, and side effects reported were recorded.

RESULTS

Maximal bronchodilation (Emax) FEV(1) (maximal bronchodilatory response to albuterol) increased from 0.64 +/- 0.27 L/min during the exacerbation to 0.94 +/- 0.38 L/min during recovery (p < 0.001). The Emax PEFR increased from 147.53 +/- 62.46 L/min to 222.94 +/- 73.82 L/min after recovery (p = < 0.001). There was no significant difference in rate of recovery of PEFR (p = 0.684), duration of hospital stay (p = 0.084), or side effects (p = 0.506) between the groups receiving 2.5 mg or 5 mg of nebulized albuterol.

CONCLUSIONS

There was significant improvement in Emax to inhaled albuterol as the COPD exacerbation resolved. There was no significant difference in outcomes including length of hospital stay or recovery of lung function between patients treated with regular 2.5 mg vs 5 mg of nebulized albuterol during an AECOPD.

Effects of inhalation exposures to an M1-receptor agonist on ventilation in rhesus monkeys

Information was needed on effects of possible occupational inhalation exposure to an M1-receptor agonist (xanomeline) such as might occur during the manufacturing process. Both acute and repeated inhalation exposures to xanomeline were carried out in six male rhesus monkeys using a head-dome exposure system. Exposure concentrations ranged from 0.3 to 10 mg/m3. The exposure durations were up to 2 weeks. Decreases in tidal volume and increases in respiratory frequency were both time and concentration related during acute exposures. These effects were blocked with atropine pre-treatment. Correlation with pulmonary resistance measurements in two monkeys suggested that these were bronchoconstrictive changes that increased with severity with time at a given concentration and with concentration when measured after a constant exposure time. The dose-response was relatively steep with 10 mg/m3 becoming intolerable to the monkeys after approximately 15 minutes, but no measurable effects were observed at 0.3 mg/m3 after up to 4 hours of exposure. To investigate the effects of repeated exposures, monkeys were exposed for 4 hr/day, 5 days/wk for 2 weeks to 0.0 (air only), 0.3, and 1.2 mg xanomeline/m3 of air. When compared to the air-only exposure, 0.3 mg/m3 caused no significant changes in tidal volume. In contrast, 1.2 mg/m3 caused a rapid and significant decrease in tidal volume that was sustained throughout the 4-hr exposure. A slower rise in breathing frequency also occurred. Repeated exposures did not alter the effects seen after a single exposure. It is concluded that xanomeline, a M1-receptor agonist, can acutely alter normal ventilation in non-human primates at airborne concentrations > or = 0.6 mg/m3 and should be carefully controlled in a manufacturing environment. The no-observed-effect concentration was 0.3 mg/m3.

Muscarinic receptors and control of airway smooth muscle

Contraction of airway smooth muscle is mediated by M3 muscarinic receptors on the airway smooth muscle. However, there is no evidence suggesting that hyperresponsiveness results from any alterations in function of these M3 muscarinic receptors. In contrast, there is clearly increased release of the neurotransmitter acetylcholine in animal models of hyperactivity and in asthma. Release of acetylcholine is controlled by inhibitory M2 muscarinic receptors, and it appears that it is these M2 receptors that are dysfunctional in animal models of hyperresponsiveness. Allergen-induced M2 receptor dysfunction is absolutely dependent upon an influx of eosinophils into the airways. Activated eosinophils release major basic protein, which binds to M2 receptors and prevents binding of acetylcholine. Thus, the normal negative feedback control of acetylcholine release is lost, and acetylcholine release is increased. In conclusion, loss of function of inhibitory M2 muscarinic receptors on the airway parasympathetic nerves causes vagally mediated bronchoconstriction and hyperresponsiveness following antigen challenge.

The influence of ageing on muscarinic receptors, beta-adrenoceptors and adenylate cyclase activity in the bovine lung

Muscarinic and beta-adrenoceptors were identified in airway epithelium, smooth muscle and lung parenchyma from Holstein-Friesian calves and cows and were characterized with [3H]quinuclidinyl benzilate and [3H]dihydroalprenolol, respectively. The muscarinic receptor density in the smooth muscle of cows (Bmax = 4803 +/- 245 fmol/mg protein) was 33% greater (p < 0.01) than in calves. Low receptor numbers were detected in the epithelium and parenchyma. In both calves and cows, the density of epithelial beta-adrenoceptors was twice as high as in smooth muscle and parenchyma. The quantity of beta-adrenoceptors in the tracheal epithelium (Bmax = 994 +/- 83 fmol/mg protein) and smooth muscle (Bmax = 492 +/- 41 fmol/mg protein) in cows was respectively 37% (p < 0.001) and 35% (p < 0.01) lower than in calves. Adenylate cyclase (AC) assays indicated that the basal and the (-)-isopropylnoradrenaline- (ISO-) stimulated cAMP production were not significantly different between the calves and cows. After stimulation with NaF, significantly higher cAMP production was found in all tissues from cows. Significant correlations were found between absolute AC responses to NaF and beta-adrenoceptor density in epithelium (r = -0.75, p < 0.001) and smooth muscle (r = -0.63, p < 0.01). It seems that, in older animals, the production of cAMP is independent of the number of receptors, indicating the presence of fully active compensatory mechanisms.

Effect of clenbuterol on beta-adrenoceptors and adenylate cyclase activity in smooth muscle and epithelium of the trachea of calves

A potent beta-agonist (clenbuterol) was administered perorally to young calves for 50 days. After this period the animals were slaughtered and beta-adrenoceptor density, ligand affinity, and basal and stimulated adenylate cyclase activities were studied in smooth muscle and epithelium of the trachea. Although the density of lung beta-adrenoceptors was down regulated by clenbuterol, cAMP production remained constant (epithelium) or even increased (smooth muscle). Therefore desensitization of beta-adrenoceptors in the trachea was not observed. This might be a reason for the effectiveness of long-term treatment with beta-agonists.

Alpha 1-adrenergic and muscarinic receptors in adult and neonatal rat type II pneumocytes

Binding characteristics of the alpha 1-adrenergic radioloigand [3H]prazosin, and the muscarinic cholinergic radioligand, [3H]quinuclidinyl benzilate, were determined both in intact cell preparations of rat alveolar type II pneumocytes (TIIPs) and in membrane preparations of rat lung tissue. Binding in adult and neonatal (< 24 h postnatal age) rats was also compared. Binding affinities for both receptor classes on TIIPs and whole lung membrane preparations alike did not vary significantly with age. In lung membrane preparations, the concentrations of both receptor classes were higher in neonates than adults. In TIIPs, the alpha 1-adrenergic receptor concentration was higher in neonates, but muscarinic receptor concentration was higher in adults. To begin investigation of the functional significance of these receptors, the effects of alpha 1-adrenergic and muscarinic agonists on intracellular calcium ion concentration ([Ca2+]i) were also measured. Both agonists induced consistent increases in [Ca2+]i, which were blocked by respective antagonists. These data indicate the presence of receptors on TIIPs for alpha 1-adrenergic and muscarinic agonists that may influence cellular function via modulation of [Ca2+]i.

Muscarinic receptor subtypes, beta-adrenoceptors and cAMP in the tracheal smooth muscle of conventional and double-muscled calves

The total muscarinic (M1 + M2 + M3) and beta-adrenergic receptors in the tracheal smooth muscle of conventional and double-muscled calves were identified and characterized with the non-specific antagonists [3H]quinuclidinyl benzilate ([3H]QNB) and [3H]dihydroalprenolol ([3H]DHA) respectively. Although the quantity of beta-adrenoceptors in double-muscled calves was 25% lower (p < 0.05) than in conventional calves (Bmax = 327 +/- 89 fmol/mg protein), adenylate cyclase assays indicated that the basal adenylate cyclase activity and the (-)-isopropylnoradrenaline (ISO)- and sodium fluoride (NaF)-stimulated values were not significantly different between these calves. However, the density of muscarinic receptors in double-muscled calves was 40% higher (p < 0.01) than in conventional calves (Bmax = 2955 +/- 625 fmol/mg protein). Subtypes of muscarinic receptors were studied with [3H]telenzepine (M1-receptors), [3H]AF-DX 384 (M2-receptors) and [3H]4DAMP (M1 and M3-receptors). It was found that in both double-muscled and conventional calves about 40% of the receptors were of the M3-subtype, the remaining 60% being M2-receptors. From these results, it is suggested that inflammation of the respiratory tract in double-muscled calves may be complicated by an imbalance between the cholinergic bronchoconstrictor and the beta-adrenergic bronchodilator components of the autonomic nervous system.

Response to nebulized ipratropium bromide and terbutaline in acute severe asthma

Outpatient studies on asthmatics have shown that inhaled anti-cholinergic agents decrease in efficacy as FEV1 falls. To determine whether there are changes in response to inhaled anti-cholinergics during acute bronchoconstriction we have examined the effects of nebulized ipratropium and terbutaline in nine hospitalized patients recovering from acute severe asthma. At 6 a.m. each day throughout the admission, baseline PEFR was recorded. Ipratropium bromide, 1 mg, was nebulized and PEFR measured again 1 h later. Following this, terbutaline, 5 mg, was nebulized with further measurement of PEFR 15 min after nebulization. Results were analysed by paired t-tests. Mean baseline PEFR rose from 157 l m-1 on patients worst day to 300 l m-1 on their best day (P less than 0.01). Ipratropium improved mean PEFR by 55 l m-1 and 42 l m-1 on patients worst and best days respectively (P less than 0.01). Subsequent terbutaline improved mean PEFR on patients worst day by 23 l m-1 (P less than 0.01) but only by a non-significant 4 l m-1 on their best day (P = 0.09). Hence, ipratropium produced 96% of total bronchodilatation when baseline was highest, but achieved only 71% of total response when baseline was lowest, a highly significant change in response (P less than 0.01). We conclude that in acute severe asthma as baseline PEFR rises response to inhaled ipratropium improves, compared with total response to combined ipratropium followed by terbutaline.

A minority of muscarinic receptors mediate rabbit tracheal smooth muscle contraction

To enhance our understanding of cholinergic mechanisms and muscarinic receptors in bronchoconstriction, we have characterized the muscarinic receptor subtypes in rabbit tracheal smooth muscle using radioligand binding and functional assays. The Kd for [3H]quinuclidinyl benzilate ([3H](-)QNB) binding determined from saturation isotherms was 12.6 x/divided by 1.1 pM (geometric mean x/divided by SEM), and the Bmax was 269 +/- 7 fmol/mg protein (arithmetic mean +/- SEM). Competitive inhibition studies with the muscarinic antagonists pirenzepine (PZ), 11[[2-[(diethylamino)-methyl]1-piperidinyl]acetyl]-5,11-dihydro-6H- pyrido[2,3-b][1,4]benzodiazepine-6-one (AF-DX116), 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP), and hexahydrosiladifenidol (HHSiD) demonstrated heterogeneity of muscarinic receptor subtypes in rabbit tracheal smooth muscle. PZ bound with low affinity to a single receptor site, indicative of an absence of M1 receptors. AF-DX116 (M2 selective) bound with high affinity to approximately 83% of muscarinic binding sites, and 4-DAMP and HHSiD (M3 antagonists) bound with high affinity to approximately 24 and 28% of muscarinic binding sites, respectively. Additionally, direct binding studies with [3H]4-DAMP demonstrated high-affinity binding with 23% of muscarinic binding sites. Thus, the majority of muscarinic receptors in rabbit tracheal smooth muscle bound with high affinity to an M2-selective antagonist, and the remaining receptor sites bound with high affinity to M3 antagonists. The inhibitory effects of atropine, PZ, AF-DX116, and 4-DAMP on methacholine-induced contraction of rabbit tracheal rings were compared. 4-DAMP was a potent inhibitor of methacholine-induced contraction, but PZ and AF-DX116 demonstrated low potency.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenoceptors in human airway tissue: relationship between functional responsiveness and receptor number

Functional organ bath experiments and radiolabelled ligand binding studies were used to investigate the relationship between beta-adrenoceptor-mediated relaxation and the total number of beta-adrenoceptors in human lung parenchymal tissue and bronchial tissue. Sensitivity to the beta-adrenoceptor agonist isoprenaline (pD2) varied almost 10-fold (pD2 values 6.00 to 6.85) for lung parenchymal preparations and 35-fold for bronchial preparations (pD2 values 6.16 to 7.67) between patients. The total number of [3H] DHA labelled beta-adrenoceptors (Bmax) varied almost 6-fold for lung parenchymal membrane preparations (Bmax 164 to 936 fmol/mg protein) and less than 2-fold for bronchial tissue membrane preparations (Bmax 188 to 342 fmol/mg protein) between patients. Comparison of sensitivity to isoprenaline and beta-adrenoceptor number for lung parenchymal tissue from the same patient demonstrated a negative correlation (r = -0.80 [95% confidence intervals: -0.13, -0.96], 6 d.f., P less than 0.05), suggesting that beta-adrenoceptor-mediated sensitivity of lung parenchymal tissue is inversely related to the number of beta-adrenoceptors. However, there was an absence of correlation between sensitivity to isoprenaline and beta-adrenoceptor number in bronchial tissue from the same patient. Thus, the findings of the present study do not support the possibility of a direct relationship between the beta-adrenoceptor-mediated responsiveness and the beta-adrenoceptor number of human airway preparations.

Muscarinic receptor subtypes in human and guinea pig lung

Muscarinic receptor subtypes in human and guinea pig lung membranes were characterised using selective muscarinic antagonists. Competition experiments were carried out against [3H](-)-quinuclidinyl benzilate binding at 25 degrees C in Tris-HCl buffer; non-specific binding was determined in the presence of 1 microM atropine. Of all the antagonists examined, only the M1-selective antagonist pirenzepine exhibited a heterogeneous binding profile (nH less than 1.0), best described by two-binding sites of high and low affinity. Binding of [3H]pirenzepine confirmed the presence of a high proportion of high affinity (M1) receptors (60% of total) in human peripheral lung. The high potency of M3-selective antagonists 4-diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP) and hexahydrosiladifenidol suggested the presence of M3 receptors, but the low potency of AF-DX 116 and methoctramine indicated that there was no significant population of M2 receptors present. The existence of muscarinic receptor subtypes in lung may have important clinical implication but their cellular localisation remains to be determined.

Isolation of bovine lymphocytes for beta 2-adrenoceptor determination by means of the radioligand technique: some practical implications

The present investigation was undertaken to demonstrate saturable beta 2-adrenoceptor binding sites on cell membranes of bovine lymphocytes by means of the non-selective antagonist (3H)dihydroalprenolol. Although the classical density gradient centrifugation technique was used as described for the isolation of these cells from human blood, striking differences in binding characteristics were observed. It is suggested that differences in physical properties of blood might be responsible for these phenomena. Since receptor density and affinity of beta 2-adrenoceptors in lymphocytes are used in human medicine (and will be in the future in veterinary medicine) for correlation with several physiological and pathophysiological conditions, attention is drawn to the effect of dilution of blood during the preparation of these cells; in lactating cows, the best results were obtained with the dilution 1:0.5.

Characterization of cholinergic muscarinic receptors in cow tracheal muscle membranes. Effect of maturation

The parasympathetic nervous system is important in the control of basal airway muscle tone and caliber. We characterized muscarinic cholinergic receptors in isolated tracheal membranes from cows of three age groups (immature, less than 2 weeks; transition, 3-5 months; and mature, greater than 5 years) using l-[3H]quinuclidinyl benzilate (l-[3H]QNB) as the radioligand. There were significant decreases in the densities of l-[3H]QNB binding sites with maturation (Bmax: 2344 +/- 169 vs 1381 +/- 85 vs 1116 +/- 80 fmol/mg protein for tissues from immature, transition and mature cows respectively). No change in the dissociation constant was observed with maturation (Kd: 0.38 +/- 0.09 vs 0.55 +/- 0.06 vs 0.50 +/- 0.07 nM for tissues from immature, transition and mature animals respectively). The association and dissociation rate constants did not vary between tissues from immature and mature animals. The specific activity of the enzyme, acetylcholinesterase, was correlated with the density of l-[3H]QNB binding sites present in the tracheal homogenates; that is, with maturation, there were significant decreases in acetylcholinesterase activity [0.28 +/- 0.01 vs 0.16 +/- 0.02 vs 0.08 +/- 0.01 mol X l-1 X min-1 X (mg protein)-1 for tissues from immature, transition and mature animals respectively]. All competition binding studies using muscarinic antagonists exhibited single site binding and did not show any differences in drug affinities between the age groups. In contrast, multiple binding sites were observed with carbachol, methacholine and muscarine, and there were significant decreases in receptor affinities for the muscarinic agonists. No changes in the proportion of high and low affinity sites were found. These results indicate that with maturation there are alterations in the properties of muscarinic receptors in tracheal smooth muscle.

Innervation of airway smooth muscle. Efferent mechanisms

The classical view, with one excitatory (cholinergic) and one inhibitory (noradrenergic) component, of the innervation of airway smooth muscle is incomplete and at least two other, possibly peptidergic, types of innervation must be included when the innervation of airways is considered. A summary of these neuronal components is given in Fig. 1 and their possible origin is outlined. Besides the inhibitory noradrenergic innervation of the airways observed in some species, an inhibitory NANC (i-NANC) innervation has been demonstrated. The polypeptide, VIP, seems to be the most likely candidate for the neurotransmitter in the i-NANC innervation of the airways. The excitatory cholinergic innervation is present in the airways from the trachea down to the peripheral bronchi. In the guinea-pig bronchi an excitatory NANC (e-NANC) innervation has been demonstrated as well. The e-NANC nerves may correspond to chemosensitive primary afferent nerves with substance P or a related tachykinin as transmitter. When the innervation of airway smooth muscle of different mammalian species is compared it is evident that all nerve components except the cholinergic, show a considerable variability among species. The cholinergic innervation seems to be present in all mammalian species whereas the other components may be completely absent from some species. Distinct regional variations in the innervation of the airways may occur, which is exemplified by the distribution of the autonomic innervation in the guinea-pig tracheo-bronchial tree. Cholinergic neurotransmission in for example the guinea-pig and human airways can be modulated by NA via prejunctional inhibitory alpha 2-adrenoceptors. Furthermore, the e-NANC neurotransmission in the guinea-pig airways may be modulated by NA or by selective alpha 2-adrenoceptor agonists, acting via prejunctional inhibitory alpha 2-adrenoceptors. The clinical importance of the NANC innervation in relation to asthma is discussed. The i-NANC nerves may exert a modulating effect on bronchoconstriction, and a functional defect would presumably lead to an exaggerated response to constrictor stimuli. The e-NANC nerves in the airways may also be clinically relevant since the transmitter (tachykinins) from these nerves can produce bronchoconstriction and promote inflammation of the airway epithelium, either by direct mechanisms or indirectly by activation of mast cells, and thus contribute to the features of asthma.(ABSTRACT TRUNCATED AT 400 WORDS)

Biochemical characteristics of muscarinic cholinoreceptors in swine tracheal smooth muscle

The tritiated muscarinic cholinoreceptor antagonist quinuclidinyl benzilate, [3H]QNB, was used to characterize the muscarinic receptors associated with homogenized membrane of the smooth muscle from swine trachea. Based on receptor binding assays, the homogenate had specific, saturable, high-affinity receptors for [3H]QNB. Specific binding was time- and temperature-dependent. The association of [3H]QNB with the muscarinic receptor reached equilibrium much sooner at 37 degrees C than 25 degrees C at a [3H]QNB concentration of 180 pM (30 min and 2 h, respectively). Equilibrium at both temperatures was attained within 5 min at a [3H]QNB concentration of 1800 pM. All remaining experiments were performed at 37 degrees C. Binding was saturable with respect to [3H]QNB and tissue concentrations. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 51 +/- 20 pM and a maximum receptor density (Bmax) of 2.17 +/- 0.27 pmole/mg protein. The Hill coefficient for [3H]QNB binding was 1.07 +/- 0.16. The association (K1) and dissociation (K-1) rate constants were determined to be (5.51 +/- 0.16) X 10(8) M-1 min-1 and (1.41 +/- 0.18) X 10(-2) min-1, respectively. KD calculated from the ratio of K1 and K-1 was 26.3 +/- 3.8 pM; this value is close to the value of KD calculated from Scatchard plots of binding isotherms. The density of muscarinic receptor binding sites was 10-fold greater in tracheal smooth muscle than in tracheal epithelium (0.20 +/- 0.03 pmole/mg protein). There is no difference between weanling and young adult swine in the density of muscarinic receptors in tracheal smooth muscle. The nonselective muscarinic antagonists atropine, scopolamine and quinuclidinyl benzilate (QNB) competitively inhibited [3H]QNB binding to the homogenate with Hill coefficients of 0.9-1.0 and inhibition constants (Ki) of nanomolar range. Competition with selective muscarinic antagonists pirenzepine and 3-quinuclidinyl xanthene-9-carboxylate (QNX) gave Ki values, 0.26 M and 0.78 nM, respectively, and Hill coefficients of approximately 1. There was a single population of [3H]QNB binding sites of the M2 subtype for all tested muscarinic antagonists. Competition with selective muscarinic agonists pilocarpine and carbachol yielded Ki values of micromolar range, Hill coefficients of less than 1, and revealed the existence of two binding sites (P less than 0.01).

Evidence for a similar receptor site for binding of [3H] leukotriene E4 and [3H] leukotriene D4 to the guinea-pig crude lung membrane

To determine whether the action of leukotriene E4 is mediated by its cross reaction with leukotriene D4 receptor sites, we compared [3H] leukotriene E4 and [3H]leukotriene D4 binding activities in selected tissues as well as their competition results in the guinea-pig crude lung membrane. We demonstrated good correlation of [3H]leukotriene E4 and [3H]leukotriene D4 binding activities among the tissues studied. A significant correlation was demonstrated between the ability of leukotriene C4, D4 and E4, FPL-55712 and arachidonic acid to inhibit lung [3H]leukotriene E4 and [3H]leukotriene D4 binding. These correlations suggest that leukotriene E4 binds to a site which is similar to or close to the leukotriene D4 receptor.

Simultaneous assay of muscarinic and beta-adrenergic receptors using a double isotope technique

Muscarinic receptor and beta-adrenergic receptor binding were measured simultaneously in a membrane fraction of bovine tracheal smooth muscle using [3H]-L-quinuclidinyl benzilate and [125I]-(-) iodocyanopindolol. The binding characteristics, affinity and receptor density, obtained in the double receptor assay and in the control experiments were the same within experimental error. Moreover, there appears to be neither a significant influence of an excess of d,1-propranolol on [3H]-L-quinuclidinyl benzilate binding nor a significant influence of an excess of 1-quinuclidinyl benzilate on [125I]-(-)iodocyanopindolol binding. The method is advantageous where both receptors have to be assayed and where limited amounts of biological material, like in biopsy specimen, are available.

Identification of calcium antagonist receptor binding sites using (3H)nitrendipine in bovine tracheal smooth muscle membranes

(3H)Nitrendipine binding to the bovine tracheal muscle membrane at 25 degrees C was rapid, saturable (Bmax = 14.8 +/- 3.9 fmol/mg protein) and of high affinity (Kd = 0.15 +/- 0.04 nM). The rank order of Ca2+ antagonists competing for airway (3H)nitrendipine binding was nitrendipine not equal to nisoldipine not equal to nifedipine much greater than verapamil. Cromolyn, however, neither inhibited nor increased the binding.

Effect of the serine-borate complex on the relative ability of leukotriene C4, D4 and E4 to inhibit lung and brain [3H] leukotriene D4 and [3H] leukotriene C4 binding: demonstration of the agonists' potency order for leukotriene D4 and leukotriene C4 receptors

To define the potency order of the leukotrienes for inhibition of [3H] leukotriene D4 and [3H] leukotriene C4 binding, we investigated leukotriene C4, D4 and E4 competition with and without the serine-borate complex in guinea pig lung and brain homogenates. Without it, the rank order of their potency for inhibition of lung [3H] leukotriene D4 or [3H] leukotriene C4 binding was leukotriene C4 = leukotriene D4 greater than leukotriene E4. Presence of the complex altered the potency order for both competition studies: for the [3H] leukotriene D4 competition it was leukotriene D4 greater than leukotriene E4 = leukotriene C4 and for the [3H] leukotriene C4 competition it was leukotriene C4 much greater than leukotriene D4 greater than or equal to leukotriene E4.

Identification of leukotriene D4 receptor binding sites in guinea pig lung homogenates using [3H]leukotriene D4

We have characterized [3H]leukotriene D4 binding to guinea pig lung homogenates. Both biphasic dissociation kinetics and curvilinear Scatchard plots indicated the presence of [3H]leukotriene high and low affinity states of the binding sites. The rank order of potency for the competition study was leukotriene C4 = leukotriene D4 greater than leukotriene E4 much greater than arachidonic acid, and for their contractile effect on lung strips was leukotriene C4 = leukotriene D4 = leukotriene E4 much greater than arachidonic acid. FPL-55712 was the only other agent tested that inhibited binding. These results suggest that binding of [3H]leukotriene D4 to the homogenate is consistent with its binding to specific leukotriene D4 receptor sites.

Operational models of pharmacological agonism

The traditional receptor-stimulus model of agonism began with a description of drug action based on the law of mass action and has developed by a series of modifications, each accounting for new experimental evidence. By contrast, in this paper an approach to modelling agonism is taken that begins with the observation that experimental agonist-concentration effect, E/[A], curves are commonly hyperbolic and develops using the deduction that the relation between occupancy and effect must be hyperbolic if the law of mass action applies at the agonist-receptor level. The result is a general model that explicitly describes agonism by three parameters: an agonist-receptor dissociation constant, KA; the total receptor concentration, [R0]; and a parameter, KE, defining the transduction of agonist-receptor complex, AR, into pharmacological effect. The ratio, [R0]/KE, described here as the 'transducer ratio', tau, is a logical definition for the efficacy of an agonist in a system. The model may be extended to account for non-hyperbolic E/[A] curves with no loss of meaning. Analysis shows that an explicit formulation of the traditional receptor-stimulus model is one particular form of the general model but that it is not the simplest. An alternative model is proposed, representing the cognitive and transducer functions of a receptor, that describes agonist action with one fewer parameter than the traditional model. In addition, this model provides a chemical definition of intrinsic efficacy making this parameter experimentally accessible in principle. The alternative models are compared and contrasted with regard to their practical and conceptual utilities in experimental pharmacology.

Mapping of adrenergic receptors in the trachea by autoradiography

We investigated the distribution of adrenergic receptors in ferret trachea using autoradiography. [3H]Dihydroalprenolol, used to identify beta-adrenoceptors, revealed a high density of specific binding sites over surface epithelium and submucosal glands, with less labelling of smooth muscle. [3H]prazosin labelling showed that alpha 1-receptors were numerous in glands and epithelium, but sparse in smooth muscle. Comparison of adrenergic receptor densities in tracheal sections from the same animals showed a rank order for submucosal glands of alpha 1 greater than beta, for epithelium beta greater than alpha 1 and for smooth muscle beta greater than alpha 1. Within the submucosal glands, alpha 1- and beta-adrenergic receptors were differentially distributed, with alpha 1-receptors being significantly more numerous over serous than mucous cells and beta receptors being significantly more numerous over mucous than serous cells. This technique provides insight into adrenergic regulation of airway function and should be useful in investigations of how relative receptor densities may be altered in disease.

GTP increases airway muscarinic antagonist binding sites: an effect regulated by Mg2+

Using [3H]QNB, we demonstrated that GTP and Gpp(NH)p increased muscarinic receptor high affinity sites in bovine tracheal muscle preparations; however, neither the dissociation constant of [3H]QNB binding sites nor pulmonary beta-adrenergic receptor sites was altered. The GTP effect on increasing the receptor sites was relatively small (16%), sensitive (ED50 0.48 microM) and specific (Gpp(NH)p greater than GTP = GDP greater than GMP much greater than ATP). Mg2+ potentiated this increase by up to 94.8%, whereas Na+, K+ and Ca2+ had no such effect.

Muscarinic receptors in lung and trachea: autoradiographic localization using [3H]quinuclidinyl benzilate

[3H]Quinuclidinyl benzilate binding to slide-mounted frozen sections of ferret lung was of high affinity (KD 63 +/- 14 pM, mean +/- S.E., n = 4), and characteristic of interaction with muscarinic cholinergic receptors. Light microscopic autoradiography showed muscarinic receptors to be localized predominantly to smooth muscle of trachea and intrapulmonary cartilaginous airways, and to submucosal glands. There was much less labelling of bronchiolar smooth muscle, airway epithelium and vascular smooth muscle and no labelling of alveoli. This distribution of receptors parallels that of cholinergic innervation.