Measuring airway dimensions during bronchoscopy using anatomical optical coherence tomography.

Airway dimensions are difficult to quantify bronchoscopically because of optical distortion and a limited ability to gauge depth. Anatomical optical coherence tomography (aOCT), a novel imaging technique, may overcome these limitations. This study evaluated the accuracy of aOCT against existing techniques in phantom, excised pig and in vivo human airways. Three comparative studies were performed: 1) micrometer-derived area measurements in 10 plastic tubes were compared with aOCT-derived area; 2) aOCT-derived airway compliance curves from excised pig airways were compared with curves derived using an endoscopic technique; and 3) airway dimensions from the trachea to subsegmental bronchi were measured using aOCT in four anaesthetised patients during bronchoscopy and compared with computed tomography (CT) measurements. Measurements in plastic tubes revealed aOCT to be accurate and reliable. In pig airways, aOCT-derived compliance measurements compared closely with endoscopic data. In human airways, dimensions measured with aOCT and CT correlated closely. Bland-Altman plots showed that aOCT diameter and area measurements were higher than CT measurements by 7.6% and 15.1%, respectively. Airway measurements using aOCT are accurate, reliable and compare favourably with existing imaging techniques. Using aOCT with conventional bronchoscopy allows real-time measurement of airway dimensions and could be useful clinically in settings where knowledge of airway calibre is required.

Potent bronchodilation and reduced stiffness by relaxant stimuli under dynamic conditions.

Airway relaxation in response to isoprenaline, sodium nitroprusside (SNP) and electrical field stimulation (EFS) was compared under static and dynamic conditions. The capacity of relaxants to reduce airway stiffness and, thus, potentially contribute to bronchodilation was also investigated. Relaxation responses were recorded in fluid filled bronchial segments from pigs under static conditions and during volume oscillations simulating tidal and twice tidal manoeuvres. Bronchodilation was assessed from the reduction in carbachol-induced lumen pressure, at isovolume points in pressure cycles produced by volume oscillation, and stiffness was assessed from cycle amplitudes. Under static conditions, all three inhibitory stimuli produced partial relaxation of the carbachol-induced contraction. Volume oscillation alone also reduced the contraction in an amplitude-dependent manner. However, maximum relaxation was observed when isoprenaline or SNP were combined with volume oscillation, virtually abolishing contraction at the highest drug concentrations. The proportional effects of isoprenaline and EFS were not different under static or oscillating conditions, whereas relaxation to SNP was slightly greater in oscillating airways. All three inhibitory stimuli also strongly reduced carbachol-induced airway stiffening. The current authors conclude that bronchoconstriction is strongly suppressed by combining the inhibitory stimulation of airway smooth muscle with cyclical mechanical strains. The capacity of airway smooth muscle relaxants to also reduce stiffness may further contribute to bronchodilation.

Airway smooth muscle contraction - perspectives on past, present and future.

Past and contemporary views of airway smooth muscle (ASM) have led to a high level of understanding of the control and intracellular regulation of force or shortening of ASM and of its possible role in airway disease. As well as the multitude of cellular mechanisms that regulate ASM contraction, a number of structural and mechanical factors, which are only present at the airway and lung level, provide overriding control over ASM. With new knowledge about the cellular physiology and biology of ASM, there is increasing need to understand how ASM contraction is regulated and expressed at these airway and system levels.

Airway mechanics and methods used to visualize smooth muscle dynamics in vitro.

Contraction of airway smooth muscle (ASM) is regulated by the physiological, structural and mechanical environment in the lung. We review two in vitro techniques, lung slices and airway segment preparations, that enable in situ ASM contraction and airway narrowing to be visualized. Lung slices and airway segment approaches bridge a gap between cell culture and isolated ASM, and whole animal studies. Imaging techniques enable key upstream events involved in airway narrowing, such as ASM cell signalling and structural and mechanical events impinging on ASM, to be investigated.

The relevance of pharmacological dose--response curves to airway narrowing.

A defect in the smooth muscle of airways has been discarded as a possible cause of asthma in recent years because of the lack of correlation between airflow obstruction in patients and the contractile responsiveness of the isolated airway smooth muscle. Howard Mitchell and Malcolm Sparrow question the relevance of comparing parameters obtained from pharmacological dose-response curves (e.g. EC50) of strips of airways in vitro with those describing airways narrowing in vivo (e.g. resistance). They point out that in small airways the upper half of the dose-response curves seen in strips of airway wall is not represented in perfused tubular airway segments because they are fully constricted at or near the EC50 of the strip.

Elastic properties of the bronchial mucosa: epithelial unfolding and stretch in response to airway inflation.

The bronchial mucosa contributes to elastic properties of the airway wall and may influence the degree of airway expansion during lung inflation. In the deflated lung, folds in the epithelium and associated basement membrane progressively unfold on inflation. Whether the epithelium and basement membrane also distend on lung inflation at physiological pressures is uncertain. We assessed mucosal distensibility from strain-stress curves in mucosal strips and related this to epithelial length and folding. Mucosal strips were prepared from pig bronchi and cycled stepwise from a strain of 0 (their in situ length at 0 transmural pressure) to a strain of 0.5 (50% increase in length). Mucosal stress and epithelial length in situ were calculated from morphometric data in bronchial segments fixed at 5 and 25 cmH(2)O luminal pressure. Mucosal strips showed nonlinear strain-stress properties, but regions at high and low stress were close to linear. Stresses calculated in bronchial segments at 5 and 25 cmH(2)O fell in the low-stress region of the strain-stress curve. The epithelium of mucosal strips was deeply folded at low strains (0-0.15), which in bronchial segments equated to < or =10 cmH(2)O transmural pressure. Morphometric measurements in mucosal strips at greater strains (0.3-0.4) indicated that epithelial length increased by approximately 10%. Measurements in bronchial segments indicated that epithelial length increased approximately 25% between 5 and 25 cmH(2)O. Our findings suggest that, at airway pressures <10 cmH(2)O, airway expansion is due primarily to epithelial unfolding but at higher pressures the epithelium also distends.

Inhibitory effect of sodium cromoglycate on pulmonary responses to histamine administered after indomethacin in anaesthetized guinea-pigs.

1. Histamine (2-4 micrograms kg-1 i.v.) increased airways resistance (Raw) and decreased dynamic lung compliance (Cdyn) in urethane-anaesthetized guinea-pigs. The effects on Raw were almost abolished by atropine (0.1 mg kg-1 i.v.) and reduced by vagal cooling (11-16 degrees C). 2. Histamine-induced changes in Raw and Cdyn were significantly (P less than 0.05) enhanced by indomethacin (1 mg kg-1 i.v.). 3. In animals not treated with indomethacin, exposure to an aerosol containing sodium cromoglycate (0.01-2% for 30 s) failed to affect subsequent (3 min) histamine-induced bronchoconstriction. 4. Administration of an aerosol containing low (0.01-0.2%) concentrations of sodium cromoglycate had no effect on the enhanced responses (i.e. hyperreactivity) seen after indomethacin. However, more concentrated sodium cromoglycate aerosols (greater than 0.2%) reduced or abolished the hyperreactivity to histamine seen after indomethacin. 5. It was concluded that sodium cromoglycate can prevent the development of hyperreactivity to histamine, possibly by interacting with some mechanism utilized by both histamine and indomethacin in this model.

Indomethacin enhances the effect of histamine on airways resistance in the anaesthetized guinea-pig.

In anaesthetized guinea-pigs intravenous histamine caused an increase in airways resistance (RA) and a fall in dynamic compliance (CDyn). Indomethacin (1 mg kg-1, i.v.) significantly enhanced the effect of histamine on RA. Indomethacin also increased the basal RA and the RA response to a histamine infusion. The effect of indomethacin on CDyn was less consistent but here also there was a trend for an increased response to histamine. Sodium carbonate (the vehicle for indomethacin, 0.05 ml 100 mM solution) had no effect on RA or CDyn in control experiments. Propranolol (0.1 mg kg-1, i.v.) enhanced the effect of histamine on RA in animals pretreated with either indomethacin or Na2CO3 vehicle, but the effect was more consistent in indomethacin pretreated animals. Indomethacin also tended to enhance the effect of histamine on RA in animals pretreated with reserpine or BW755c but it had little effect on the CDyn response to histamine. The results show that indomethacin augments the responsiveness of the airways to histamine in the anaesthetized guinea-pig. The results with propranolol and reserpine suggest that an operational beta-adrenergic system is not required for the effect of indomethacin on RA. No confirmation for lipoxygenase involvement was obtained with the lipoxygenase inhibitor, BW755.

The effect of inhibitors of arachidonic acid metabolism on drug-induced contractions in isolated tracheal smooth muscle of the pig.

1 The regulation of drug-induced tone in airways smooth muscle was examined in an isolated preparation of swine tracheal smooth muscle. 2 The trachea contracted (isometric) to histamine, 2-pyridylethylamine (2-PEA), acetylcholine and K+ but no response to histamine H2-receptor agonists were observed. 3 Histamine-induced contractions (100 microM) were potentiated by 213.3% by indomethacin (1 microM) and by 126.9% by sodium salicylate (250 microM). These inhibitors had only slight or no effects on acetylcholine-induced tone. 2-PEA responses were also potentiated by indomethacin but there were no changes in the response to H2-receptor agonists in the presence of indomethacin. The indomethacin-mediated potentiation of histamine was blocked by 5, 8, 11, 14-eicosatetraynoic acid (10 microM). FPL 55712 had no effect on these responses. 4 Mepacrine (100 microM) inhibited responses to histamine but not those to acetylcholine. No effect was observed with dexamethasone (up to 100 microM). 5 Prostaglandin E2 caused relaxation but arachidonic acid did not. 6 The possibility that histamine H1-agonist-induced contractions are regulated by contractile products of the arachidonic acid lipoxygenase pathway is discussed.

Effect of ETYA and BW 755c on arachidonate-induced contractions in the guinea-pig isolated trachea.

1. Arachidonic acid caused larger contractions to indomethacin-tested guinea-pig trachea than in control tracheae. 2. No change in contractions was obtained in tracheae treated with 5,8, 11, 14, eicosatetraynoic acid (ETYA 10 micro M). 3. ETYA (100 micro M) blocked the effect of indomethacin on arachidonic acid-induced responses. Likewise BW 755c (113 micro M) reversed the effect of indomethacin. 4. The results show that arachidonate lipoxygenase products may be responsible for the contractile responses seen in the presence of indomethacin.

The effect of mixed inhibitors of cyclo-oxygenase and lipoxygenase on the indomethacin-induced hyper-reactivity in the isolated trachea of the pig.

1 Indomethacin enhances muscle contraction to histamine in the pig isolated trachea. The effect of three cyclo-oxygenase/lipoxygenase inhibitors on these potentiated histamine-induced responses was examined. 2 At the higher concentrations BW 755c (113-226 microM), 5, 8, 11, 14-eicosatetraynoic acid (ETYA, 10-100 microM) and phenidone (5-500 microM) abolished the effect of indomethacin on tracheal contractions to histamine (100 microM). 3 Each of the three inhibitors also reduced the slope of the histamine concentration-response curve obtained in the presence of indomethacin. However, the sensitivity (pD2) of the smooth muscle to histamine was not affected by BW 755c, ETYA or phenidone. 4 The results provide further support for a lipoxygenase-dependent component of indomethacin-induced hyper-reactivity in the pig trachea.

Pharmacological studies into cyclo-oxygenase, lipoxygenase and phospholipase in smooth muscle contraction in the isolated trachea.

Indomethacin (1 microM) enhanced histamine-induced contractions in pig and guinea-pig isolated tracheae. Mepacrine (30-50 microM) abolished this effect of indomethacin suggesting that contractile metabolites of arachidonate are involved in the response to indomethacin. Mepacrine (100 microM) in the absence of indomethacin also markedly reduced histamine-induced contractions (81.2% inhibition) in the pig trachea, without affecting responses to acetylcholine. Histamine-induced responses in the guinea-pig trachea were similarly reduced, but with a higher concentration of mepacrine (300 microM). BW755c (226 microM) enhanced histamine-induced contractions in some pig tracheal preparations and caused inhibition in others. These effects of BW755c were negatively correlated to the initial reactivity of the muscle to histamine such that weak contractions were potentiated and strong contractions were inhibited. A similar effect was seen with phenidone (100 microM). The results with BW755c and phenidone suggest that muscle reactivity (to histamine) may be partly determined by the balance between the release of inhibitory and contractile arachidonate metabolites. Mepacrine exerts a different effect indicating that histamine-induced contractions are regulated by a mepacrine-sensitive process which appears to be separate from the metabolism of arachidonate.

Modulation by the epithelium of the extent of bronchial narrowing produced by substances perfused through the lumen.

1 Airway narrowing was determined in vitro as a measure of bronchial reactivity. A bronchial segment from pig lung was perfused with a Krebs solution and the change in flow rate to drugs and small ions perfused intraluminally was compared with that obtained by application to the serosal surface. 2 The sensitivity (EC50) to acetylchloline was 30 times greater on the serosal surface than on the luminal surface. Concentrations of histamine and carbachol which had threshold responses on flow rate when perfused intraluminally virtually stopped flow on the serosal surface. Potassium depolarizing solutions (containing either KCl or K2SO4) and vanadate (VO3-) had little or no effect intraluminally but completely stopped flow through the bronchial segment when applied to the serosal surface, i.e. they closed off the airway. 3 After removal of the epithelium the sensitivity to drugs and K+ perfused intraluminally was increased to equal that on the serosal surface. 4 No evidence for suppression of smooth muscle contraction by a putative epithelium-derived inhibitory factor (EpDIF) could be obtained: no inhibition of smooth muscle contractility was seen when the agents listed above were perfused intraluminally and their perfusion continued while they were applied to outside. 5 It was concluded that the epithelium plays a crucial role as an impermeant barrier in modulating the responsiveness of the airways smooth muscle.

Vagal mechanisms and the effect of indomethacin on bronchoconstrictor stimuli in the guinea-pig.

1. In urethane-anaesthetized guinea-pigs, under spontaneous respiration, indomethacin (1 mg kg-1 i.v., 10-45 min) approximately doubled the bronchoconstrictor effect (increase in airways resistance, R(aw)) of equieffective doses of histamine and 5-hydroxytryptamine (5-HT), but not that of acetylcholine or leukotriene D4 (LTD4). 2. In mechanically ventilated guinea-pigs indomethacin increased R(aw) responses to histamine as well as increasing the fall in dynamic compliance (Cdyn) evoked by this agent. 3. Cooling the cervical vagi, to temperatures shown to block efferent and probably afferent pathways (approximately 9 degrees C), abolished the effect of indomethacin on airways responses. Inhibition of indomethacin-induced hyperreactivity was also observed after vagal section. 4. Electrical stimulation of the peripheral vagus (1-20 Hz, 0.75-5 ms pulses) increased R(aw) and decreased Cdyn but these responses were not markedly altered by indomethacin. 5. It was concluded that the indomethacin-induced hyperreactivity of tracheal smooth muscle, which was demonstrated in vitro, may not account for the airways hyperreactivity observed in the present in vivo experiments. The hyperreactivity to histamine induced by indomethacin in vivo depends on the functional integrity of vagal reflex pathways.

Airways hyperreactivity and bronchoconstriction induced by vanadate in the guinea-pig.

1 The characteristics of vanadate-induced bronchoconstriction and airways hyperreactivity were observed in spontaneously breathing anaesthetized guinea-pigs by measurement of airways resistance (Raw) and dynamic lung compliance (Cdyn). Vanadate (0.3-3 mg kg-1 i.v. over 25 min) increased Raw and decreased Cdyn in a reversible, dose-related manner. This action (1 mg kg-1 vanadate) was not inhibited by atropine (1 mg kg-1 i.v.), propranolol (1 mg kg-1 i.v.) or bilateral vagotomy, suggesting a direct effect on the airways smooth muscle. 2 An aerosol of vanadate (10% w/v in H2O) for 3 min decreased Cdyn by 19.5% (P less than 0.05, n = 6) but caused no change in Raw. 3 Histamine (3 micrograms kg-1 i.v.) caused a bronchoconstriction which was enhanced by vanadate in a dose-related manner. This hyperreactivity (after 1 mg kg-1 i.v. vanadate) was unchanged after propranolol or bilateral vagotomy, but was partly blocked by atropine (enhancement by vanadate of the Cdyn change to histamine was diminished, P less than 0.02, n = 3). 4 Bronchoconstrictor responses to acetylcholine (6 micrograms kg-1 i.v.) and 5-hydroxytryptamine (6 micrograms kg-1 i.v.) were also enhanced by vanadate (1 mg kg-1 i.v.) Hyperreactivity after vanadate to the three bronchoconstrictors tested continued during vanadate infusion and was reversed 45 min after cessation of infusion. 5 Histamine (3 ;Lgkg-' i.v.) caused a transient tachypnoea which was also enhanced by vanadate (0.3-3mgkg-'i.v.), in a dose-related manner, in association with the increased reactivity of the airways (r = 0.66, n = 11). 6 It is concluded that vanadate-induced airways hyperreactivity is non-vagal (efferent) and largely non-cholinergic in origin and appears to involve an action of vanadate within the lung itself.

Transition of functional innervation in the developing porcine airway from nitrergic to catecholaminergic.

1. We determined the distribution and chemical nature of the inhibitory neurotransmitter(s) to the airway smooth muscle (ASM) before and after birth. 2. Relaxation responses to electrical field stimulation (EFS) were studied in isovolumic bronchial segments from foetal (approximately 100/115 days gestation) and adult (25 kg) pigs, and in isovolumic tracheal segments from the foetus, and tracheal smooth muscle strips from the adult pig. Preparations were conditioned in low doses of atropine (10(-7) M) to reduce the effects of excitatory neurotransmission and then exposed to carbachol to produce submaximal muscle tone. Some studies were also carried out on bronchial segments from 4 week old pigs. 3. EFS (65 V, 2 ms, 5-20 Hz for 5 s) produced a TTX-sensitive relaxation in epithelium-intact and epithelium-denuded preparations. In foetal bronchial and tracheal preparations, EFS-induced relaxation was strongly inhibited by N(G)-nitro-L-arginine (L-NOARG, 10(-6) to 10(-4) M; P<0.01-0.001). However, in the adult, only relaxations of the trachea were inhibited by L-NOARG; bronchi were resistant to L-NOARG and also to N-nitro-L-arginine methyl ester (L-NAME, 10(-4) M). The inhibitory actions of L-NOARG (10(-6) to 10(-4) M) were substantially reversed by 10(-2) M L-arginine. Experiments with bronchial segments from 4 week old pigs showed partial inhibition of relaxations by L-NOARG. 4. The L-NOARG-insensitive relaxations recorded in the adult bronchus were blocked by propranolol (10(-6) M). 5. The onset of relaxation to EFS was more prompt and the rate of relaxation more rapid in foetal bronchi than in adult bronchi (P<0.0005). Maximum relaxation and recovery times were the same. 6. Foetal and adult bronchi were relaxed by sodium nitroprusside (SNP) with similar sensitivity and maximum effect. The rate of relaxation to SNP was not different in the two ages. 7. In the absence of atropine and carbachol, excitatory cholinergic responses to EFS (65 V, 2 ms, 5 Hz for 20 s) were not altered by L-NOARG (10(-4) M) or L-NAME (10(-4) M) in the adult bronchus but were modestly increased by L-NOARG in the foetal bronchus (P<0.01). 8. The tracheobronchial tree appears functionally innervated by nitrergic input to the smooth muscle before birth. However, at or after 4 weeks of age the inhibitory neural input to the bronchi is catecholaminergic, but it remains nitrergic in the trachea. There is also a weak nitrergic pre- or postsynaptic inhibition of the effects of cholinergic neurotransmission in the foetal bronchus but not in the adult.

The effect of catecholamines on the in vivo and in vitro responses of the cat lung during anaphylaxis.

1. Anaphylaxis in the lung of cats actively sensitized to Ascaris antigen has been investigated in vivo and in vitro. 2. In vivo there was a 100% increase in airways resistance and a 50% decrease in dynamic lung compliance following intravenous challenge with Ascaris antigen. Prostaglandin F2alpha induced similar changes but with histamine only dynamic lung compliance was affected. (-)-Isoprenaline prevented these prostaglandin F2alpha- and histamine-induced changes and caused a delay of about 2 min in the onset of the mechanical changes following anaphylactic challenge. 3. In vitro the isolated lung strip contracted within seconds of challenge whereas there was a delay of 2 to 3 min in the onset of the tracheal anaphylactic response. (-)-Isoprenaline, (-)-adrenaline and (+/-)-noradrenaline reduced the magnitude of anaphylactic contractions of the isolated trachea but did not significantly affect those of the isolated lung strip. This indicated lack of inhibition of mediator release from the lung parenchyma. 4. Histamine was released from sensitized lung fragments following challenge with the Ascaris extract. This release constituted 6.3% of the total tissue histamine and was not inhibited by (-)-isoprenaline (1 micrometer). 5. (-)-Isoprenaline abolished 5-hydroxytryptamine (5-HT)-induced contractions of the isolated trachea but not those elicited in response to acetylcholine. The isolated lung strip responses to histamine, prostaglandin F2alpha and 5-HT were highly resistant to inhibition by (-)-isoprenaline.

The cat lung strip as an in vitro preparation of peripheral airways: a comparison of beta-adrenoceptor agonists, autacoids and anaphylactic challenge on the lung strip and trachea.

1 A new in vitro preparation, the isolated lung strip of the cat, is described for investigating the direct effect of drugs on the smooth muscle of the peripheral airways of the lung. The preparation comprises a thin strip of lung parenchyma which can be mounted in a conventional organ bath for isometric tension recording. Its pharmacological responses have been characterized and compared with the isolated tracheal preparation of the cat. 2 The lung strip exhibited an intrinsic tone which was relaxed by catecholamines, aminophylline and flufenamate. It was contracted strongly by histamine, prostaglandin F2alpha, acetylcholine, compound 48/80, potassium depolarizing solution and alternating current field stimulation. In contrast, the cat trachea was unresponsive to histamine and prostaglandin F2alpha and did not exhibit an intrinsic tone. 3 (-)-Isoprenaline and (-)-adrenaline were much more potent in relaxing the lung strip than the trachea. The potency order of relaxation responses to isoprenaline, adrenaline and (+/-)-noradrenaline in the lung strip was isoprenaline greater than adrenaline greater than noradrenaline but in the trachea was isoprenaline greater than noradrenaline greater than or equal to adrenaline. 4 beta2-Adrenoceptor selective agonists salbutamol and terbutaline were more potent in the lung strip than the trachea, suggesting beta2-adrenoceptors predominated in the lung strip. Propranolol was equipotent in inhibiting isoprenaline relexations of the lung strip and trachea, whereas practolol was much less effective in inhibiting lung strip than trachea, further supporting a predominance of beta2-adrenoceptors in lung strip and beta1-adrenoceptors in trachea. 5 Strong Schultz-Dale type contractions were elicited in both lung strips and trachea by Ascaris lumbricoides antigen in actively sensitized cats. The initial phase of the contractile response of the lung strip following challenge was shown to be due to histamine release and was absent in the trachea. The delayed phase of the contraction which took several minutes to develop in both the mepyramine-treated lung strip and trachea was not due to prostaglandins E1, F2alpha or bradykinin, the probable mediator being slow reacting substance of anaphylaxis (SRS-A). 6 It is concluded that the isolated lung strip of the cat is useful as an in vitro model for investigating the effect of drugs on the smooth muscle of the peripheral airways of the lungs.

Contraction of smooth muscle of pig airway tissues from before birth to maturity.

Two heavy chains of smooth muscle myosin (MHC1 and MHC2) were identified in pig airways and parenchyma. The ratio of MHC1 to MHC2 was the same along the bronchial tree in animals of the same age, but it changed with age (mature, young, suckling, and fetus), ranging from 0.8 in the mature to 2.2 in the fetus. Stress developed in airway (trachea, bronchus, and bronchiole) and parenchymal preparations in response to carbachol and histamine (mN/mm2) was normalized for myosin content (N/mm2 myosin). Airways from sucklings always developed the greatest stress to carbachol and histamine with the rank order of maximum force (Emax) suckling greater than fetus greater than young greater than mature for carbachol in large airways. Airway ranking to histamine was similar except that Emax of fetal bronchus and bronchiole were least. In parenchymal strips, mature animals gave strong responses to carbachol and histamine compared with other age groups. Sensitivity to carbachol was increased in the suckling trachea; otherwise it did not vary with age. Chemically skinned tracheal fibers exhibited three- to fourfold greater sensitivity to Ca2+ in fetal and suckling airways compared with the older animals. It is concluded that maturation of smooth muscle occurs in the expression of myosin, in the Ca2(+)-force relationships of the contractile machinery, and in the pharmacological responsiveness of the intact smooth muscle, with the latter greatest at or soon after birth.

Perfused bronchial segment and bronchial strip: narrowing vs. isometric force by mediators.

When bronchial segments were perfused with Krebs solution at a constant pressure (5-6 cmH2O), the resistance rose exponentially with increasing concentrations of either carbachol or histamine in the lumen. The pressure-flow relationship was linear. Histamine and carbachol caused 43 and 47% muscle shortening, respectively, and produced the same maximum effect (Emax) because they both stopped perfusion. In bronchial strips the maximum isometric force or isotonic shortening to carbachol was more than twice that of histamine and the responses showed a plateau. There were no significant differences in sensitivities [negative log of the concentration producing half-maximal response (EC50)] to either carbachol or histamine in the strips (isotonic and isometric) and the segments perfused at constant pressure. When airway segments were perfused at a constant flow, however, responses plateaued and the sensitivities to carbachol and histamine were reduced more than tenfold compared with the strips [4.71 +/- 0.20 and 6.22 +/- 0.08 (SE) for carbachol in segments and isometric strips, respectively, and 3.92 +/- 0.13 and 4.94 +/- 0.11 (SE) for histamine]. We conclude that when segments are perfused at a constant pressure, airway closure occurs before maximal pharmacological activation, as seen in airway strips.