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Biomedical subjects

H W Mitchell

Publications and source records attributed to H W Mitchell.

At least 37 records · Page 2Linked to original sources
PubMed1996

Effect of diameter on force generation and responsiveness of bronchial segments and rings.

In this study, isovolumic bronchial segments and bronchial rings were used to investigate the influence of airway diameter on smooth muscle force generation and acetylcholine responsiveness. Segments with internal diameters ranging from 1.0-6.0 mm were obtained from the mainstem bronchus of eight pigs. Responses to increasing acetylcholine concentrations were quantified in segments by intralumenal pressure (cmH2O), and in rings by tension (g.cm-1). The negative log of the concentration producing half the maximal effect (EC50) (i.e. pD2) to acetylcholine was calculated for each segment and ring. Ring tension was used to calculate a theoretical lumen pressure for each ring, and this, along with the pD2, was compared with values obtained from segments of the same diameter. Intermediate-sized segments produced significantly greater intralumenal pressures than did large or small segments. Small segments were 160 times more sensitive to acetylcholine than large segments. In contrast to the segments, bronchial rings showed no effect of size on acetylcholine sensitivity. Theoretical ring lumen pressures matched those measured for large and intermediate segments, but not for small segments. The different behaviour of bronchial segments and rings obtained from the same sized airway suggests that the three-dimensional architecture of the airway is an important factor in determining behaviour, particularly in small airways.

Acetylcholine↗
PubMed1995

The epithelial barrier and airway responsiveness.

Epithelial injury and bronchial hyperresponsiveness are commonly associated with airway disease, and are widely considered to occur as the result of inflammatory changes in the airway wall. Mechanistically, the airway epithelium may influence the sensitivity of the airways to provocative stimuli through its primary function as a cellular barrier between the air and the interstitium, or by liberating a variety of bronchoactive mediators, e.g., lipoxygenase and cyclooxygenase products, nitric oxide, and an epithelium-derived relaxing factor (EpDIF). Much attention has focused on the latter function of the epithelium, particularly the putative EpDIF, which has an action considered to be analogous to that of endothelium-derived relaxing factor in blood vessels. The modulation of airway calibre by the epithelium has recently been investigated in vitro using tubular preparations of bronchi, where removal of, or damage to, the epithelium increases the sensitivity to agonists by several orders of magnitude. This contrasts with the effect of removing the epithelium on strips or rings of airway wall, where the increase in sensitivity is small and rather variable, but this has been the primary observation for proposing a putative EpDIF. This review evaluates the barrier or protective function of the airway epithelium and the major role it plays in the modulation of airway responsiveness. A role of a putative EpDIF seems, at best, to be of minor functional significance.

Acetylcholine↗
PubMed1995

Measurement of flow through perfused bronchial segments: optimization of flow head resistance.

Techniques for measuring flow, for example through perfused bronchial segments, frequently use a flow head as the sensing device. The sensitivity of these instruments to changes in airway resistance is dependent on the flow head resistance. This study aims to quantify the effect of flow head resistance on sensitivity and flow, allowing optimization of flow head resistance. A mathematical model of the perfused bronchial segment preparation was constructed and used to explore the effect of flow head resistance on sensitivity. The model's assumptions were experimentally tested using segments of pig bronchus, perfused through the lumen with Krebs solution. The model suggests a complex relationship between flow head resistance and sensitivity that has a distinct maximum when approximately half of the total resistance is present in the flow head. Furthermore, the maximum sensitivity was found to be proportional to the resting flow. Regression between experimentally derived sensitivities and modelled predictions was linear, with a correlation coefficient of 0.93. We have defined the optimum recording conditions for study of flow through isolated bronchial segments.

Acetylcholine↗
PubMed1994

Epithelial modulation of neonatal and fetal porcine bronchial contractile responses.

We tested the hypothesis that bronchial epithelium of newborn and fetal pigs exerts an inhibitory effect on bronchial contractile function. In vitro studies were performed on 13 isolated bronchial segments from seven neonatal pigs (12 h to 4 d old) and on 9 bronchial segments from five late-term pig fetuses (105 d gestation, term = 115 d). This preparation keeps the lumen of the airway physiologically intact and separate from the serosal aspect. Stem bronchi of the right and left lung were mounted horizontally at 6 cm H2O in a chamber containing oxygenated Kreb's solution. One bronchus was intact while the other had the epithelium removed with a cotton-tipped applicator soaked in Kreb's solution. Removal of epithelium was confirmed histologically. The lumen pressure of the mounted bronchi was measured at constant volume after the luminal application of acetylcholine (ACh) or K2SO4 (80 mM/L in place of NaCl), after serosal applications of these two substances at the peak of the luminal response, and after serosal application of ACh alone (newborn only). ACh (1, 10, and 100 microM) in the lumen of intact airways of neonatal pigs caused contractions of 0.1 +/- 0.07 (SEM), 0.4 +/- 0.14, and 0.9 +/- 0.18 cm H2O, respectively; in airways denuded of epithelium, ACh caused contractions of 3.9 +/- 1.1, 15.5 +/- 2.94, and 2.95 +/- 4.97 cm H2O (p < 0.001 versus intact airways). Luminal contractions were 2.9 +/- 2.2%, 2.6 +/- 0.9%, and 2.8 +/- 0.7% of the luminal+serosal values for intact segments, as compared with 65.5 +/- 4.6%, 58.8 +/- 7.7%, and 75.1 +/- 6.2% for denuded segments (p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Video-imaging of lumen narrowing; muscle shortening and flow responsiveness in isolated bronchial segments of the pig.

The amount of smooth muscle shortening required to cause narrowing of the bronchial lumen and to significantly alter airflow in an airway has not not been measured. Accordingly, in this study, the lumens of isolated fluid-filled bronchial segments from pigs were directly visualized using a fibreoptic endoscope and video-recording. The extent of lumen narrowing was related to smooth muscle contraction, determined morphometrically, and to bronchial lumen flow. Narrowing was produced by electrical field stimulation (EFS), (70 V, 20 Hz, 2-3 ms) and acetylcholine (ACh), (10(-5)-10(-2) M). The diameter of the relaxed bronchial lumen and the reduction in diameter produced by maximum electrical field stimulation were both greater when the transmural pressure was increased (from -8 to 20 cmH2O). The percentage change in diameter produced by electrical field stimulation was not, however, significantly different over this pressure range. Electrical field stimulation caused the bronchial lumen to narrow by 28.5 +/- 3.3% in diameter, and by 45.7 +/- 3.8% in cross-sectional area, at zero transmural pressure. Maximum doses of acetylcholine (10(-2) M) reduced the lumen diameter by 48.3 +/- 3.3%, and the area by 70.3 +/- 4.8%. In separate experiments, maximum doses of ACh stopped the flow of Krebs solution perfused through the lumen of bronchial segments. Morphometric measurements indicated that smooth muscle shortened by a maximum of 32.5 +/- 2.8% to acetylcholine 10(2) M, relative to fully-relaxed dimensions. Simultaneous imaging of the lumen and adventitia of bronchi showed that the luminal margin narrowed more than the adventitia.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine↗
PubMed1994

Increased responsiveness to cholinergic stimulation of small compared to large diameter cartilaginous bronchi.

Model studies incorporating known morphological properties of the airways predict different narrowing characteristics between large and small bronchi. However, the flow-response characteristics of airways from different regions in the bronchial tree are still unknown. We compared the responsiveness of large and small bore bronchi of the pig to luminal or adventitial stimulation by acetylcholine (ACh) during perfusion of the airways at optimal driving pressure. Subsequently, each segment was measured morphometrically. Resting flow through the lumen was directly related to the fourth power of the segment internal diameter (ID). The sensitivity of small bronchi (2.00 mm ID) to ACh applied adventitially was about 870 times greater than large (5.85 mm ID) with a longer latency time of contraction in large bronchi. When perfused luminally the sensitivity difference was 42 fold, and after removing the epithelium the sensitivity difference was reduced to approximately five fold. Maximum concentrations of ACh produced airway closure in small segments (100% reduction in flow) but in large segments a plateau developed at 60-90% flow reduction. Similar findings were obtained with carbachol. Morphometry of the airway wall showed increasing cartilage, mucosal and smooth muscle thickness with increasing diameter, but a decreasing relative wall area. The marked differences in sensitivity to narrowing between large and small bronchi are, in part, related to differences in morphological properties of the mucosa (the epithelium) and adventitia, and to airway location. Airway size plays an important role in the maximum response of airway narrowing, with small airways showing closure and large airways a plateau.

Acetylcholine↗
PubMed1994

Foetal airway motor tone in prenatal lung development of the pig.

The terminal airways from embryonic lung in situ or as explants exhibit rhythmic spontaneous contractions. Our objective was to see whether narrowing responses of the airways occurred throughout the bronchial tree in the first trimester foetus and, if so, to characterize them. The bronchial tree was freed of vasculature and parenchyma from the lungs of 20-35 g pig foetuses (44-48 days gestation). The airway lumen was visualized directly with transmitted light, and narrowing was recorded in real time by video-imaging microscopy. From the main stem bronchi to the terminal regions of late generation branches (20-35 microns i.d.) strong bronchoconstrictor responses to micromolar concentrations of acetylcholine (ACh), histamine, substance P and K+ depolarizing solution were seen, whilst inhibition of narrowing with beta-adrenoceptor agonists was evidence of beta-receptors on the smooth muscle. Moreover, strong narrowing responses to electrical field stimulation, which were blocked by atropine, indicated that functional cholinergic nerves were present. A remarkable display of spontaneous narrowing in the airways of many of the bronchial tree preparations caused the movement of lung liquid to and fro. We speculate that the bronchomotor tone and associated spontaneous activity, which move the lung fluid along the airways, serve to maintain an even positive pressure in localized areas of the bronchial tree which is essential to provide the stimulus for continued growth of the lung.

Acetylcholine↗
PubMed1993

The integrity of the epithelium is a major determinant of the responsiveness of the dog bronchial segment to mucosal provocation.

The intact bronchial segment of the dog was used to investigate the role of the epithelium in modulating bronchial responsiveness: firstly, because it retains many of the properties of the airway wall including the separation of the mucosal and adventitial surfaces; and secondly, because airway tissue from dogs has been principally used in advancing evidence for epithelial relaxant substances. The development of tone by the airway smooth muscle was measured as an increase in luminal pressure of the isovolumic segment. Cholinergic excitatory and sympathetic inhibitory responses were obtained to electrical field stimulation. Responses to acetylcholine (ACh), carbachol, depolarizing K+ solution and vanadate (VO3-) applied to the mucosal surface were greatly attenuated both in sensitivity and reactivity compared with their effect on the adventitial surface. Mechanical removal of the epithelium greatly augmented these responses to agents in the lumen (ACh sensitivity increased 100-fold) to equal those by adventitial stimulation. The latter were unaffected by epithelial removal. No functional evidence for epithelial-derived inhibitory factors could be demonstrated. It is concluded that the impermeable nature of the epithelium acts as a barrier to diffusion of hydrophilic and charged molecules thereby producing large concentration gradients across the airway wall. A breach in the integrity of the epithelium enables molecules to gain access to the smooth muscle.

Acetylcholine↗
PubMed1993

Responsiveness of human isolated bronchial segments and its relationship to epithelial loss.

1. The responsiveness of human and bovine bronchi was examined by comparing the magnitude of responses to agonists applied to the adventitial (outside) and to the luminal (inside) surfaces. The development of smooth muscle tone was measured as both an increase in pressure in isovolumic bronchial segments and narrowing in perfused segments. 2. In both closed and perfused human segments, ACh added to the adventitial surface produced highly homogeneous responses while responses to ACh added to the luminal side were extremely variable. 3. Histological examination of the human segments showed that they possessed variable degrees of pre-existing epithelial loss from the mucosal circumference, ranging from 0 to 82%. 4. Denuded human isovolumic segments (exhibiting > 30% epithelial loss) were 40 fold more sensitive to ACh inside than intact segments (< 30%). Denuded segments were also equally responsive to KCl inside or outside while KCl inside produced a contraction that was 50% of that to KCl outside in intact segments. 5. A strong and highly significant relationship was determined between the proportion of epithelial loss and both the responsiveness and rate of contraction of human segments to ACh and KCl introduced onto the lumen. No relationship was observed between epithelial denudation and responsiveness or contractility to agonists added to the adventitial surface. 6. Mechanical denudation of the epithelium from bovine segments had no effect on responsiveness to ACh or KCl added to the outside while significantly augmenting the sensitivity to ACh (9 fold) and reactivity to KCl introduced into the lumen to the extent that it became the same as outside.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine↗
PubMed1992

Multiple motor pathways to single smooth muscle cells in the ferret trachea.

1. We investigated the distribution and characteristics of motor pathways to individual smooth muscle cells activated by electrical stimulation of either, single nerves which enter the tracheal plexus (inlet nerves), or a longitudinal nerve trunk (LNT) located near the entrance of an inlet nerve into the plexus. Excitatory junction potentials (EJPs) were recorded using intracellular microelectrodes as an index of smooth muscle cell activation. In all experiments EJPs were completely blocked by tetrodotoxin and by atropine. 2. In smooth muscle fields located in the caudal direction from the point of inlet or LNT nerve stimulation, neural input decreased as a function of distance. There was evidence of a demarcated area innervated by neurons entering the plexus in one inlet nerve. In smooth muscle fields located in the rostral or transverse direction from the site of nerve stimulation, no such demarcated area could be identified. 3. Of the smooth muscle cells located within the innervated fields studied, 83-95% were activated following stimulation of a single inlet nerve or LNT. Evoked EJPs were similar in different innervated cells or units of electrically coupled cells located within the same 1 mm2 'field'. 4. There was overlapping cholinergic motor input to single smooth muscle cells originating from neurons present in different inlet nerves or different neurons present in the same inlet nerve or region of the LNT. Multiple small step increases in the voltage used to stimulate a LNT resulted in three or four step increases in EJP amplitudes. This gives a minimal value for the number of motor pathways that can be activated by neurons in a region of LNT leading to a single smooth muscle cell. 5. Motor pathways to smooth muscle cells located in caudal and rostral fields ran initially in the LNT and exited in proximity to the smooth muscle cell studied. 6. Motor pathways used in transmitting signals to smooth muscle cells to different areas of trachealis muscle varied in their sensitivity to hexamethonium or curare. EJPs evoked in fields located in the caudal direction from the stimulating electrode were abolished by these drugs. Muscle cells located in different rostral fields showed EJPs that were either sensitive or resistant to these drugs. 7. The rostral hexamethonium-resistant pathway ran initially in the LNT but it exited from the LNT several millimetres before reaching the level of the smooth muscle field innervated. This pathway followed stimulation frequencies up to 25 Hz. The final neuron in this pathway released acetylcholine and evoked EJPs were entirely inhibited by atropine.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Activation of smooth muscle in the airway wall, force production, and airway narrowing.

Airway narrowing depends on smooth muscle force production and muscle shortening, but the structural and geometric properties exhibited by individual generations of the bronchial tree largely determine the extent and characteristics of airway narrowing. Properties of major importance include the nature and integrity of the epithelium, the structural and mechanical properties of the airway wall, as well as airway diameter. The influence of these properties on airway narrowing measured as flow or flow resistance in large and small diameter segments of airways from pig lung is described using a novel preparation, the perfused bronchial segment.

Animals↗
PubMed1992

Early maturation of force production in pig tracheal smooth muscle during fetal development.

The contractility of airway smooth muscle is fully established at late term at birth but its responsiveness during fetal life has not been defined. In this study, the contractile force of airway smooth muscle to acetylcholine (ACh), K+ depolarizing solution, and electrical field stimulation (EFS) was measured in tracheas from small fetal pigs. Contraction to either agonist and to EFS was detectable in fetuses of as low as 9 g body weight, which corresponds to approximately 36 days of gestation. Isometric force increased progressively with age, reaching 4.1 +/- 0.4 mN for K+ and 5.8 +/- 0.5 mN for ACh (10(-4) M) at 600 g fetal weight (90 days). However, when normalized for cross sectional area of smooth muscle, the stress was essentially the same from 17- to 600-g fetuses. (K+: 17 g = 74.4 +/- 10.6 mN/mm2, 600 g = 89.3 +/- 13.0 mN/mm2; ACh [10(-4) M]: 17 g = 76.3 +/- 16.0 mN/mm2, 600 g = 127.0 +/- 13.0 mN/mm2). The sensitivities of the various groups to ACh were not significantly different (e.g., EC50: 30 g = 4.0 +/- 0.2 x 10(-6) M, 600 g = 3.7 +/- 1.1 x 10(-6) M). EFS produced frequency-dependent contractile responses in all groups. With increasing fetal size, there was a corresponding increase in force. When this force was normalized to a maximum ACh response (10(-4) M), there was no significant difference between groups of fetuses. Histologic examination showed the major tissue components of the trachea were present in fetuses above 7 g. Immunocytochemistry detected myosin, caldesmon, and filamin in the smooth muscle from fetuses of 7 g and above, showing that contractile and actin-binding proteins were present from a very early age. It is concluded that smooth muscle contractile function is well developed very early in fetal life in pigs.

Acetylcholine↗
PubMed1992

Increased narrowing of bronchial segments from immature pigs.

Bronchial narrowing was investigated to determine whether changes in smooth muscle force, described previously in different aged pigs, are associated with differences in airway narrowing. The sensitivity of bronchial segments from suckling and young pigs was compared by perfusion through the lumen with a Krebs solution at a pressure of 5 cmH2O, measuring the reduction in flow in response to carbachol and histamine. Segments of the same internal diameter (i.d. approx. 2.5 mm) from each age were used by selecting proximal segments from suckling and distal segments from young pigs. The sensitivity to carbachol or histamine was the same in smooth muscle strips from proximal and distal bronchi in each age. Furthermore, segments from either age had a similar pressure-volume relationship between -10 and 30 cmH2O. However, concentration-flow curves showed that the airways from sucklings were five times more sensitive than airways from young animals to carbachol (p less than 0.01) and, less consistently, to histamine (p greater than 0.05), when flow was reduced by 50% of maximum. Flow was abolished by maximum concentrations of carbachol at both ages whereas histamine stopped flow in the young segments and reduced it by 80% in the suckling age group. Data indicate a greater sensitivity of bronchial narrowing to carbachol in the intact airway--this is consistent with a greater force production in suckling pig airway smooth muscle. These findings support a postnatal development of airway function, as suggested from clinical observations and provocation studies in humans.

Aging↗
PubMed1991

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.

Acetylcholine↗
PubMed1990

The role of epithelium in the responsiveness of the bronchi to stimuli.

Airway narrowing in response to different modes of stimuli was measured using a perfused bronchial segment. Acetylcholine, carbachol, histamine, high [K+] and vanadate perfused through the lumen gave small responses as shown by the reduction in flow, but fully constricted the airway when applied to the outside. ACh was 29 times more sensitive on the outside. When the epithelium was removed airway narrowing by these stimuli was increased to equal that on the outside. It was concluded that the role of the epithelia as a barrier is all important. Evidence for an EpDIF released by the epithelium could not be obtained.

Acetylcholine↗
PubMed1990

Airway diameter determines flow-resistance and sensitivity to contractile mediators in perfused bronchial segments.

To study airway reactivity, flow-resistance to carbachol and histamine was measured in perfused bronchial segments. Small-bore airways were more sensitive, flow was reduced to zero and resistance rose very steeply due to mucosal folding while in large-bore airways maximum flow reduction was 50% and resistance increased sigmoidly. Thus the internal diameter of an airway is a crucial determinant of narrowing.

Airway Resistance↗
PubMed1990

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.

Aging↗