Effect of hypoxia on airway smooth muscle mechanics and electrophysiology

Interaction between volatile anesthetics and hypoxia in porcine tracheal smooth muscle

We investigated the direct interaction between the volatile anesthetics, isoflurane and sevoflurane, and hypoxia in porcine tracheal smooth muscle in vitro by simultaneously measuring muscle tension and intracellular concentration of free Ca(2+) ([Ca2+]i). Muscle tension was measured by using an isometric transducer, and [Ca2+]i was measured by using fura-2, an indicator of Ca2+. Under the condition of bubbling with 95% O2/5% CO2, [Ca2+]i was increased by 1 microM carbachol with a concomitant contraction. Volatile anesthetics significantly inhibited both carbachol-induced muscle contraction and increase in [Ca2+]i. Hypoxia bubbled with 95% N(2)/5% CO2 inhibited the muscle contraction by 30% with an increase in [Ca2+]i by 20%. Exposure to hypoxia substantially enhanced the inhibitory effects of these anesthetics on carbachol-induced muscle contraction, whereas the decreases in [Ca2+]i were significantly prevented by hypoxia. Under Ca2+-free conditions, hypoxia significantly decreased the muscle contraction by 20%; however, it still increased [Ca2+]i by 15%. Exposure to the anesthetics significantly enhanced the inhibitory effect of hypoxia on the muscle contraction; however, it appeared to have little effect on [Ca2+]i. Hypoxia inhibits airway smooth muscle contraction independently of intracellular Ca2+, and it substantially potentiates the inhibitory effects of volatile anesthetics on airway smooth muscle contraction.

IMPLICATIONS

Hypoxia inhibits agonist-induced tracheal smooth muscle contraction with an increase in free Ca2+ [Ca2+]i, which comes from intracellular Ca2+ stores. Hypoxia also potentiates the inhibitory effect of volatile anesthetics on airway smooth muscle contraction. Conversely, there is a possibility that the treatment of asthmatic patients with oxygen partially attenuates the inhibitory effect of volatile anesthetics on airway smooth muscle contractility.

Effect of Ca(2+)-independent mechanisms on the hypoxic relaxation of guinea-pig tracheal rings

Hypoxia induces bronchodilation in vivo and in vitro, but the mechanisms are still unclear. To evaluate whether an extra- or intracellular free Ca(2+) ion is involved in the mechanisms of hypoxic relaxation, we simultaneously measured cytosolic Ca(2+)levels and tensions in both intact and denuded guinea-pig tracheal strips precontracted with histamine (100 microM), and assessed the effect of hypoxia on guinea-pig tracheal rings precontracted with okadaic acid (10 microM) and calyculin-A (0.1 approximately 10 microM) under an extracellular Ca(2+)-free state. The exposure of tracheal rings to hypoxia induced an immediate decrease of tracheal tension without decrease in intracellular free Ca(2+)levels. In the presence of okadaic acid but not calyculin-A, hypoxic air exposure caused significant transient reductions in tracheal tone. Further, thapsigargin (5 microM or 10 microM) did not affect hypoxic bronchodilation, suggesting that the release of intracellular Ca(2+) does not take a role in hypoxic bronchodilation. Hypoxic dilation decreased ATP content in epithelium-intact rings but not epithelium-denuded rings, indicating a relationship between hypoxic dilation and change of adenine nucleotide in epithelium-intact rings. Our findings indicate that the epithelium dependent mechanisms of hypoxic relaxation of guinea pig tracheal rings preconstricted with histamine may not be related to the mobilization of extra and intra-cellular Ca(2+).

Sleep, respiratory physiology, and nocturnal asthma

The nocturnal worsening of asthma is a common feature of this disease that recently has received extensive investigation. Most recent efforts have focused on the role of circadian biorhythms that could promote a nocturnal increase in airway inflammation, leading to a subsequent increase in airflow obstruction and asthma symptoms. However, definitive studies remain lacking. As discussed in this review, there is also substantial evidence that sleep itself may play a direct role in the nocturnal worsening of asthma. Potential mechanisms for such a sleep-related effect could include the supine posture, alterations in sympathetic and parasympathetic "balance," sleep-associated reductions in lung volume, intrapulmonary pooling of blood, and sleep-associated upper airway narrowing, both with and without snoring and obstructive sleep apnea (OSA). These potential contributors to this troublesome phenomenon deserve further consideration when investigating mechanisms of nocturnal asthma.

Effect of hypoxia on respiratory system impedance in dogs

The effects of hypoxia on lung and airway mechanics remain controversial, possibly because of the confounding effects of competing reflexes caused by systemic hypoxemia. We compared the effects of systemic hypoxemia with those of unilateral alveolar hypoxia (with systemic normoxemia) on unilateral respiratory system impedance (Z) in intact, anesthetized dogs. Independent lung ventilation was obtained with a Kottmeier endobronchial tube. Individual left and right respiratory system Z was measured during sinusoidal forcing with 45 ml of volume at frequencies of 0.2-2.1 Hz during control [100% inspired O2 fraction (FIO2)], systemic hypoxemia (10% FIO2), and unilateral alveolar hypoxia (0% FIO2 to left lung, 100% FIO2 to right lung). During systemic hypoxemia, there was a mean Z magnitude increase of 18%. This change was entirely attributable to a decrease in the imaginary component of Z; there was no change in the real component of Z. Administration of atropine (0.2 mg/kg) did not block the increase in Z with systemic hypoxemia. In contrast, there was no change in Z in the lung subjected to unilateral alveolar hypoxia. We conclude that alveolar hypoxia has no direct effect on lung mechanical properties in intact dogs. In contrast, systemic hypoxemia does increase lung impedance, apparently through a noncholinergic mechanism.

Time course of Ca(2+)-dependent K+ and Cl- currents in single smooth muscle cells of guinea-pig trachea

The time course of two types of Ca(2+)-dependent currents were compared in single smooth muscle cells freshly isolated from guinea-pig trachea. When the pipette solution contained mainly 140 mM KCl, depolarization from -60 mV to 0 mV evoked an initial inward current followed by an outward current which consisted of transient (I(to)) and sustained components. In addition, a long-lasting inward tail current (Itail) was occasionally observed after the repolarization to -60 mV. Although I(to) often occurred repetitively during depolarization, the first I(to) reached the peak of approximately 50 ms after the start of depolarization and had the largest amplitude in most cells examined. The amplitude of Itail increased with the increase in depolarization period up to about 500 ms. Pharmacological analyses indicate that I(to) and Itail are Ca(2+)-dependent K+ and Cl- currents (IK-Ca and ICl-Ca), respectively, and suggest that not only Ca(2+)-influx through Ca2+ channels but also subsequent Ca2+ release from stores contributes to activate these currents. Spontaneous transient outward and inward currents, IK-Ca and ICl-Ca, respectively, were simultaneously recorded at -40 mV. In over 80% of the spontaneous current events, outward and inward currents coupled one to one and always occurred in this order. Puff-application of 10 mM caffeine also induced IK-Ca and ICl-Ca in this order at -40 mV. When caffeine was applied twice with various intervals, the current amplitude in the second application depended upon the period of the interval. The recovery of ICl-Ca during the interval was faster than that of IK-Ca. The results indicate that the activation and decay time courses of ICl-Ca are slower but its recovery is faster than those of IK-Ca.

Obstructive sleep apnea syndrome and bronchial hyperreactivity

This study was designed to investigate the prevalence of bronchial hyperreactivity (BH) in patients with obstructive sleep apnea syndrome (OSAS), heavy snorers, and light snorers; its correlation with OSAS severity; and its response to nasal CPAP therapy. Forty-eight age- and sex-matched subjects were selected on the basis of preentry sleep studies: Group I consisted of 16 patients with OSAS (hypopnea-apnea index (HAI) = 35 +/- 9); group II consisted of 16 cases of heavy snorers without OSAS; and group III, a control group, consisted of 16 subjects with only mild snoring. All 48 patients had normal pulmonary function (simple spirometry) prior to study entry and had no history of asthma or allergies. The prevalence of BH was prospectively assessed by giving each subject a methacholine challenge test (MCT). Patients with a positive MCT were treated with 2-3 months of nasal CPAP treatment, after which they had a second MCT. Four of 16 patients in group I had BH on MCT (PD20 = 88, 103, 109, 162 D.U.), whereas none of the group II or III subjects demonstrated BH. There was no correlation between BH and the severity of the OSAS. The 4 patients with BH in group I showed an increase in PD20M after 2-3 months of nasal CPAP treatment. In conclusion, BH may occur in patients with OSAS. It is unrelated to the severity of the OSAS, and nasal constant positive airway pressure (CPAP) therapy can decrease the hyperreactivity to methacholine in these patients.

Hypoxia modulates mediator responses and neurotransmission in guinea-pig trachea in vitro

To discover whether hypoxia affects mediator responses and neurotransmission in tracheal smooth muscle, we studied in vitro tracheal segments from guinea-pigs under isometric conditions. Hypoxia itself did not alter the basal tone. The maximum response to acetylcholine and histamine under hypoxia was less than that under oxygenated conditions. The logarithm of 50% effective concentration (log EC50) of the response to acetylcholine under hypoxia was not altered, but the log EC50 of the response to histamine decreased significantly. In contrast to the response to exogenous acetylcholine, the maximum contractile response to electrical field stimulation (EFS) under hypoxia was not different from that under oxygenated conditions, but the logarithm of 50% effective frequency of contractions caused by EFS under hypoxia decreased significantly. On the other hand, non-adrenergic-non-cholinergic relaxation caused by EFS was unaffected by hypoxia. These observations suggest that hypoxia can modulate the responses of tracheal smooth muscle to acetylcholine, histamine and nerve stimulation.

Evidence for the absence of a functional role for muscarinic M2 inhibitory receptors in cat trachea in vivo: contrast with in vitro results

1. The effect of the selective muscarinic M2 receptor antagonist, gallamine and the selective M2 receptor agonist, pilocarpine, on airway constriction induced by vagal stimulation was studied in anaesthetized cats. In addition, the effect of gallamine on contraction of cat isolated tracheal and bronchi preparations induced by electrical field stimulation was also investigated. 2. In in vivo experiments, extrathoracic airway constriction was measured with an electromechanical caliper that was attached to the outer surface of tracheal ring 4. Intrathoracic airway constriction was determined by measuring the changes in total lung resistance and dynamic compliance during vagal stimulation. 3. Intravenous gallamine (0.1, 1, and 10 mg kg-1) augmented the rise in total lung resistance induced by vagal stimulation in a dose- and frequency-dependent manner. At stimulation frequencies of 8 and 12 Hz the fall in dynamic compliance provoked by vagal stimulation was also significantly increased by gallamine (10 mg kg-1). Gallamine was without effect on airway constriction induced by acetylcholine. 4. Vagal stimulation at 4 Hz produced significant tracheal constriction, but the amount of constriction did not change following injection of increasing doses of gallamine. Similarly, there was no difference in tracheal constriction at any frequency of stimulation (0.5-16 Hz) when frequency-response curves before and after gallamine injection (10 mg kg-1) were compared. 5. Pilocarpine (0.01-10 micrograms kg-1, i.v.) diminished changes in total lung resistance and dynamic compliance induced by vagal stimulation, an effect that was reversed by gallamine (10 mg kg-1, i.v.). Vagally-induced tracheal constriction was not significantly affected by any dose of pilocarpine, nor was it modified by gallamine (10mg kg- ') given subsequently.6. Atropine (0.5 mgkg-') completely blocked tracheal constriction induced by vagal stimulation, indicating that the changes in tracheal ring diameter provoked by stimulation were mediated by muscarinic receptors and that intravenous drugs could reach the cervical trachealis muscle.7. In vitro tissue bath studies demonstrated a significant leftward shift of the frequency-response curve to electrical field stimulation in both tracheal strips and bronchial rings following gallamine (10-4M) administration.8. Although the functional presence of muscarinic M2 autoreceptors was demonstrated in feline isolated tracheal and bronchial preparations, a corresponding functional role was not detected in cat trachea in vivo. This was despite repeated demonstration of muscarinic M2 receptor-mediated limitation of airway constriction of intrathoracic airways in vivo.

Nasal CPAP in nonapneic nocturnal asthma

Nasal CPAP has been shown to improve nocturnal asthma in those patients with associated sleep apnea. We studied seven nonapneic, nonsnoring asthmatics to determine the effect of CPAP in this patient population. On the CPAP night vs the baseline night, there was a significant worsening of sleep architecture. This included increased awake time and decreased REM sleep. For the group, the overnight decrement in FEV1 was not improved. Of interest, two patients did have a marked improvement in FEV1 associated with improved oxygen saturation on the CPAP night. These individuals were restudied only on supplemental oxygen. This intervention also improved the overnight FEV1 and allowed the patients to have better sleep compared to the CPAP night. We concluded that CPAP is associated with disrupted sleep architecture in nonapneic asthmatics and nocturnal oxygen desaturation may play a role in the development of nocturnal asthma.

Effects of tetraethylammonium and 4-aminopyridine on outward currents and excitability in canine tracheal smooth muscle cells

1. The effects of tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on membrane currents and on single channel K currents in smooth muscle cells isolated from canine trachea were examined by use of tight seal whole cell- and patch-clamp techniques. 2. Depolarizing current applied through a recording pipette did not elicit an action potential under current clamp. A strong outward rectification was observed. 3. In most cells under voltage-clamp, only an outward current was observed upon depolarization from -60 mV when a pipette solution contained mainly KCl. The outward current consisted of three components; a large initial transient, a following sustained component and an additional component of irregular small transients on the sustained one. The two transient components were almost abolished when extracellular and pipette solutions contained 2.2 mM Cd2+ (0 mM Ca2+) and 10 mM EGTA, respectively. The sustained component was well maintained under these conditions. 4. TEA at low concentrations (less than 1 mM) effectively decreased the transient components and made the outward current smooth; it also suppressed the sustained component at higher concentrations. In outside-out patches, external 1 mM TEA reduced the single channel conductance of Ca-activated K channels by about 87% whereas 3 mM 4-AP did not. 4-AP at low concentrations (less than 3 mM) selectively reduced the sustained component of the outward current. 5. A Ca current recorded after the suppression of outward current by internal Cs+ had a peak of approximately 200 pA at +10 mV (holding potential: -60 mV). The half inactivation voltage in the steady-state was approximately -30 mV. 6. Simultaneous application of 1 mM TEA and 4-AP reduced the outward current and unmasked a Ca current. Under these conditions, an action potential with overshoot was easily elicited under current clamp. 7. It is concluded that the low excitability of canine tracheal smooth muscle cell upon depolarization is due to a large outward K current which consists of Ca-dependent and Ca-independent components. The peak amplitude of the Ca current is similar to that in highly excitable smooth muscle cells such as those of the ureter.

Lowering Po2 induces epithelium-dependent relaxation in isolated canine bronchi

This study was designed to investigate whether the respiratory epithelium can modulate the tone of the underlying smooth muscle in response to decreases in partial pressure of O2 (PO2). Canine bronchial segments with or without epithelium (diameter, 4-6 mm; length, 50-60 mm) were mounted in organ chambers and perfused intraluminally with modified Krebs-Ringer bicarbonate solution [temperature, 37 degrees C; PO2 varying from 600 (control) to 40 mmHg; PCO2, 36 mmHg]. Isometric tension was recorded by means of stirrups passed through the wall of the central part of the bronchial segment. During contractions to carbachol, the tissues with epithelium showed epithelium-dependent relaxations when the PO2 was decreased. The level of relaxation was dependent on the PO2. The epithelium-dependent relaxation could not be blocked by the following agents: indomethacin, methylene blue, propranolol, or tetrodotoxin (antagonists or blockers of cyclooxygenase, guanylate cyclase, beta-adrenoceptors, and sodium channels, respectively). The epithelium-dependent relaxation was not accompanied by the release of an assayable relaxing factor in the bronchial lumen. The experiments suggest that 1) lowering the PO2 induces the epithelium to release a relaxing factor(s), which is neither a product of cyclooxygenase nor endothelium-derived relaxing factor; 2) a local reflex mechanism is not involved in the phenomenon; and 3) the relaxing factor(s) either is not released into the bronchial lumen or, if it is, is catalyzed rapidly in the lumen on release.

Mild isocapnic hypoxia enhances the bronchial response to methacholine in asthmatic subjects

We studied the effect of mild isocapnic hypoxia (FIO2 = 15.5%) on lung mechanics, heart rate, circulating plasma catecholamines, and bronchial responsiveness to methacholine in ten asthmatic adults. Hypoxia did not alter lung mechanics (i.e., dynamic pulmonary compliance [CLdyn], pulmonary resistance [RL]) nor did it increase plasma catecholamines, but it significantly increased bronchial responsiveness to aerosolized methacholine, as assessed by the fall in forced expiratory volume in one second (FEV1: 1.2 +/- 0.18 versus 0.9 +/- 0.14 L/s, p less than 0.05), the rise in RL (RL: 19.1 +/- 1.4 versus 8.4 +/- 1 cm H2O/L/s, p less than 0.05), and the steeper slope of the dose-response curve to methacholine. We concluded that the hypoxic characteristic of asthmatic attacks may aggravate airflow obstruction.

Electromechanical effects of tetraethylammonium and K+ on histamine-induced contraction in pig isolated tracheal smooth muscle

The effect of tetraethylammonium (TEA) and K+ on contractions to histamine and acetylcholine have been compared in the pig isolated trachea using organ bath and sucrose-gap techniques. Histamine elicited weak contractions, compared with acetylcholine; however, these contractions were markedly potentiated by pretreatment with TEA (10 mM) or by raising the external KCl concentration to 30-50 mM. Neither TEA nor K+ increased the sustained depolarization evoked by histamine (or acetylcholine) although oscillatory depolarizations were often observed in the presence of TEA. Verapamil and a zero Ca2+ Krebs solution reduced contractions to histamine and reduced or abolished the effect of TEA and K+ on histamine-induced contractions. The results unmask different mechanisms of contraction for histamine and acetylcholine. Histamine-induced tone appears to be linked with mechanisms sensitive to TEA and high K+, possibly involving increased translocation of Ca2+ across the plasma membrane.

Time course of pulmonary vasoconstriction with repeated hypoxia and glucose depletion

To examine the effect of hypoxia on pulmonary vascular smooth muscle, rabbit lobar pulmonary artery was suspended in a glucose free solution and both chronologic changes in tension and ATP content were determined together at 30 min intervals after repeated hypoxic challenge (PO2 = 11 +/- 2 mm Hg). The pulmonary artery contracted and its ATP content decreased with hypoxia. This contraction was not inhibited by nifedipine, Ca++ -free EGTA, procaine, phentolamine, isoproterenol, diphenhydramine, prostaglandin E1, atropine or nitroglycerin. Upon reoxygenation (PO2 = 104 +/- 3 mm Hg), the elevated resting tension decreased in a biphasic fashion and the ATP content of the lobar pulmonary artery increased. When hypoxic challenges were repeated, the rate of constriction on hypoxia increased, while the relaxation rate on reoxygenation, tension developed by 30 min of hypoxia and the total amount of ATP decreased. These results suggest that the ATP content in the lobar pulmonary artery is very sensitive to in vitro acute hypoxia and that the Ca++ transport process is more easily impaired by reduction in ATP levels than is the contractile machinery.

Some effects of nifedipine in guinea-pig isolated trachealis

In trachealis depolarized by a K+-rich medium, nifedipine (0.001-1 mumol 1(-1) caused concentration-dependent antagonism of CaCl2-induced increase in tension, moving the CaCl2 log concentration-effect curve to the right and depressing the maximal response. In trachealis in normal Krebs solution, similar concentrations of nifedipine had marked antispasmogenic activity against the responses to potassium chloride (KCl) and tetraethylammonium (TEA). However, nifedipine had little, if any, antispasmogenic activity against the responses to acetylcholine or histamine. Nifedipine 1 mumol 1(-1) was tested for spasmolytic activity in tissues generating tension in response to the EC50 of acetylcholine, KCl or CaCl2. In producing spasmolysis nifedipine was most effective against CaCl2 and least effective against acetylcholine. Nifedipine (0.01-1 mumol-1) had little or no effect on the tone of trachealis in normal Krebs solution. Intracellular electrophysiological recording showed that nifedipine 1 mumol 1(-1) could abolish spontaneous slow wave activity. This was associated with very minor depolarization and little or no loss of mechanical tone. In tissues treated with TEA (8 mmol 1(-1) nifedipine abolished spike and slow wave discharge and reduced mechanical activity to the pre-TEA level. It is concluded that nifedipine prevents KCl- or TEA-induced spasm by inhibition of Ca2+ influx. Spasm evoked by acetylcholine or histamine and the maintenance of spontaneous tone depend largely on mechanisms for increasing the cytoplasmic concentration of free Ca2+ which are resistant to nifedipine.

Anomalous stabilizing action of Ca on sphincter smooth muscle of the rat iris

This paper concerns the first measurement of spontaneous electrical activity in the smooth muscle cells of the mammalian iris sphincter. Although the membrane potential was quite stable, at - 59 mV, under physiological conditions in vitro, bursts of rhythmic electrical activity occurred when barium was substituted for all of the calcium in the medium (Ba-Krebs' solution). These bursts consisted initially of spike potentials, but these were followed by a long-lasting depolarization. Although the amplitude of the spike potentials did not decrease even when the external Na concentration was lowered to 25 mM, addition of a small amount of Ca (greater than 0.3 mM) to the Ba-Krebs' depressed the rhythmic contractions which followed the cessation of the spike potentials. Strontium also depressed the rhythmicity but to a lesser extent. These results show that the smooth muscle of the rat iris sphincter has an ability to generate spike potentials, and that the properties of the latter differ from those observed in other smooth muscles; i.e. the activity is suppressed by Ca ions at far lower concentrations than those in the physiological environment. This anomalous effect is presumably due to the stabilizing action of Ca.

Studies on an isolated innervated preparation of guinea-pig trachea

An isolated preparation of the guinea-pig trachea with intact parasympathetic and sympathetic innervation has been devised. Responses to nerve stimulation were recorded as increases or decreases in intraluminal pressure from the fluid-filled trachea. The preparation maintained a positive resting intraluminal pressure of 3-4 cmH2O. This was unaffected by atropine, hexamethonium or propranolol. Brief pressor responses, which could be completely blocked by atropine or hexamethonium, were obtained by applying short trains of stimuli to the cervical segment of the right vagus. The amplitude of responses was frequency-dependent up to a maximum at 40 Hz. Depressor responses, more delayed and prolonged than the pressor responses and blocked by propranolol but not by hexamethonium, were obtained by stimulation of the right cervical sympathetic trunk or stellate ganglion in 70% of preparations. Dual pressor-depressor responses were observed in the remaining 30% of preparations. The pressor component was blocked by atropine, the depressor component by propranolol. In the presence of atropine and propranolol, sustained sympathetic stimulation sometimes evoked a small, delayed pressor response which was blocked by phentolamine. Under the same conditions, transmural stimulation produced a depressor response evidently due to non-adrenergic, non-cholinergic nerves. Spontaneous activity was observed in some preparations under normal conditions, but could always be evoked by hypoxia. Responses to sympathetic stimulation were reduced both by hypoxia and during periods of spontaneous activity. 8 The principal advantage of this preparation is that it permits both excitatory and inhibitory responses to be elicited by stimulation of vagal and sympathetic nerves separately in the isolated trachea in the absence of agonist and antagonist drugs.

Electrical slow waves and tone of guinea-pig isolated trachealis muscle: effects of drugs and temperature changes

1 Simultaneous recordings of electrical and mechanical activity have been made from guinea-pig isolated trachealis muscle. Electrical activity was recorded both by extracellular and intracellular techniques.2 Extracellular studies showed that the spontaneous development of tone was accompanied by electrical slow waves which frequently exhibited pronounced waxing and waning. Intracellular recording confirmed the discharge of these slow waves in individual cells. Extracellularly-recorded slow waves were often of greatest amplitude while the tissue was developing rather than maintaining tension. Some tissues became electrically quiescent on reaching peak tone.3 Cooling to 27.5 degrees C caused some relaxation. Slow wave amplitude and frequency fell, slow waves eventually being abolished. Subsequent rapid rewarming initially evoked a more profound relaxation. An intense discharge of slow waves then occurred as the tension rapidly rose again towards the pre-cooling value.4 Sodium nitrite, (-)-isoprenaline, adenosine and adenosine triphosphate (ATP) each evoked relaxation and reduced the frequency and amplitude of slow waves. High concentrations of these agents often abolished slow waves. The actions of these drugs were reversible.5 Treatment with methoxyverapamil (D600) 1 mumol/l for 15 min abolished slow wave activity but only evoked partial relaxation of the tissue.6 Acetylcholine, histamine and tetraethylammonium (TEA) each evoked contraction, but TEA was unique in consistently promoting slow waves and (in high concentration) spike activity. Spasm evoked by acetylcholine and histamine did not usually involve the initiation or promotion of slow waves. Indeed in appropriate concentration these two agents always suppressed slow wave activity. The actions of the spasmogens were reversible.7 It is concluded that the smooth muscle cells of the trachealis are electrically coupled. While co-ordinated slow wave activity is associated with the spontaneous development of tension in trachealis, it may not be necessary for the maintenance of the major part of the spontaneous tension exhibited by the tissue or for the spasm evoked by histamine or acetylcholine. Slow wave promotion by TEA suggests that the tissue may have a high resting potassium conductance which normally attenuates the slow waves. Slow waves may be suppressed by a variety of drugs acting by different mechanisms. Since D600 suppresses slow waves of the trachealis the mechanisms underlying the waves may be similar to those underlying spike activity in other smooth muscles.

Effect of procaine on potassium permeability of canine tracheal smooth muscle

Procaine depolarized the cell membrane, and initiated oscillations and spike-like potentials in canine tracheal smooth muscle, at concentrations between 1 and 5 mM, while higher concentrations of this drug suppressed the spontaneous activities. Inhibitory effects of procaine on 86Rb-efflux were not evident in normal solution. In high K solution, procaine decreased the rate of 86Rb-efflux, in a dose dependent manner, and a Scatchard plot suggested two sites of action for procaine. The site with a higher affinity (KD = 0.26 mM) may directly regulate the K permeability and interact with procaine in a one to one manner. In high concentrations (greater than or equal to 10 mM), procaine interacted at another site and the tracheal smooth muscle which contracted in high K solution relaxed almost completely. Thus, the decrease in 86Rb-efflux by high concentrations of procaine may partly result from a decrease in intracellular Ca concentration. It is proposed that procaine has the dual effect by inhibiting the K conductance: (1) depolarization of cell membrane, (2) increase in membrane excitability. These actions would explain the spontaneous electrical activity induced by procaine in canine tracheal smooth muscle.

Pulmonary mechanics during hypoxia in spontaneously breathing anesthetized rabbits

Changes in lung mechanics were measured during hypoxia (FIO2 = 0.10 during 5 min) in spontaneously breathing anesthetized rabbits. In intact animals, hypoxia induced scattered variations in total lung resistance (RL) (decrease, increase or no alteration) and dynamic lung compliance (CL), whereas in carotid body denervated animals it was accompanied by an almost constant decrease in RL (-12%); but if a subsequent vagotomy was performed, there was no significant variation in RL. In animals which had been only vagotomized, CL decreased significantly during hypoxia (-23%) without any associated change in RL. So, the arterial chemoreceptor mediated bronchoconstrictor effects due to hypoxia could be normally masked by the dilator effects (metabolic and/or sympathetic) on the airways.

Alteration of electrophysiological properties of airway smooth muscle from sensitized guinea-pigs

Glass microelectrodes were used to study electrophysiological properties of guinea-pig airway smooth muscle (m. trachealis transversus). The resting membrane potential (Em) of airway smooth muscle was found to be -40.4 +/- 0.5 mV (307 cells, 28 preparations). Twenty-seven percent of all cells successfully impaled showed regular spontaneous electrical activity (amplitude of 2-20 mV, with maximum rate of depolarization 15.0 +/- 2.2 mV . sec-1). Forty-four percent of cells showed irregular fluctuations in Em and the remaining cells showed no electrical activity. All three groups of cells had a similar distribution of individual Em values. The sensitization of animals (14 days incubation period) caused a slight but significant increase (P less than 0.001) in Em to -43.1 +/- 0.9 mV. Repeated daily exposure of sensitized animals to aerosolized albumin for two weeks caused a significant reduction of Em to -27.8 +/- 0.8 mV (P less than 0.001). Five weeks repeated exposure caused a further reduction in Em of airway smooth muscle cells to -22.6 +/- 0.7 mV (P less than 0.001). The responses to both histamine (10(-4) M) and isoprenaline (5 x 10(-6) M), as gauged by changes in Em, were altered in the trachea of chronically exposed guinea-pigs. The changes in airway smooth muscle electrical properties were related to the number of times the animals were exposed to inhaled antigen. Even after two weeks of daily exposure, the changes were marked. Airway smooth muscle alteration may be important in the pathogenesis of bronchial asthma.

Drug interactions in cat isolated tracheal smooth muscle

1. The interactions between some drugs that contract and relax airways smooth muscle have been investigated in the cat isolated trachea. 2. Isoprenaline and theophylline inhibited serotonin-elicited contractions more than acetylcholine-mediated responses. This was observed both in terms of the degree of inhibition and the concentration of the relaxant drug producing this inhibition. 3. The acetylcholine- and serotonin-induced contractions were inhibited more by theophylline than by isoprenaline displaced acetylcholine and serotonin response curves to the right whereas theophylline caused a flattening of the curves. 4. Isoprenaline was more effective in hibiting serotonin contractions than acetylcholine contractions when the tracheas were bathed in K+ depolarizing solution, suggesting that the difference in the suscepbibility of serotonin and acetylcholine contractions to isoprenaline was not dependent on the electrical membrane potential. 5. Isoprenaline inhibited the tonic component of acetylcholine contractions more than the phasic component. 6. The differences in the pharmacological responses to the contractile and relaxant drugs in cat tracheal preparations provide further examples in smooth muscle of different mechanisms by which acetylcholine and serotonin induce contraction and isoprenaline and theophylline relaxation.

Intracellular pH in hypoxic smooth muscle

Hypoxia impairs contractility in canine tracheal smooth muscle (TSM). This is attributed to intracellular lactacidosis. The present studies were undertaken to confirm this. Lactate was found to be significantly increased in hypoxic TSM (65.36 +/- 7.37 mg/100 g wet tissue), compared to normoxic (29.83 +/- 5.05). Intracellular pH (pHi) was, however, significantly increased in hypoxic active TSM to 7.71 +/- 0.05 as compared to 7.30 +/- 0.03 in normoxic active muscle. pHi of resting normoxic muscle (7.20 +/- 0.04) was statistically not different from that of resting hypoxic muscle. The pHi's of resting normoxic and active hypoxic muscles were significantly different. These results show that under in vitro, hypoxic conditions: 1) an increase in glycolysis in TSM is indicated by the increased lactate production, 2) there is a surprising, concomitant rise in pHi rather than a decrease as previously expected, and 3) it is mechanical activity of the muscle which leads to this paradoxical result, inasmuch as pHi is unaltered in the resting hypoxic muscle.

Effect of antigen challenge on sensitized guinea pig trachea

Isolated tracheal preparations (ITP) were obtained from control guinea pigs and animals with acute and chronic experimental asthma (AA and CHA). It was observed that after in vitro antigen challenge, in addition to the classic Schultz-Dale response (the immediate increase in the tone of sensitized airway smooth muscle), a significant increase of the active state of airway smooth muscle was seen. This active state was observed as the potentiated, phasic, mechanical activities of ITP and was evaluated by both maximum active tension (AT max) and maximum rate of tension development (dT/dt). No apparent functional differences in the tonic response or the active state of airway smooth muscle between AA and CHA animals were found. Tracheas isolated from animals with CHA demonstrated smooth muscle hypertrophy as measured by wet, dry and relative weights of the tracheas. The repeated administration of antigen into the experimental bath was ineffective in inducing any physiological changes. The administration of histamine to ITP from control animals induced changes of mechanical activities of trachea comparable to those seen in both AA and CHA animals.

Excitation and contraction in bovine tracheal smooth muscle

1. The smooth muscle layer of the bovine trachea was studied in vitro with the micro-electrode and sucrose-gap techniques. The membrane potential was stable at--47-6 plus or minus 0-98 (S.E. of mean) mV, and there was no spontaneous electrical or mechanical activity. 2. The cell membrane had strong rectifying properties, making it impossible to elicit action potentials by electrical stimulation in normal Krebs Solution. The rectification was abolished by TEA (30 mmol/l), which depolarized the membrane and produced plateau-type action potentials. 3. The spontaneous repetitive action potentials produced by TEA were associated with rhythmic oscillatory contractions of the muscle strips. 4. Histamine caused an increased tone, with superimposed rhythmic fluctuations in tension. The electrical response consisted of depolarization, with rhythmic slow oscillations in potential (slow waves) which were synchronous with the fluctuations in tension. 5. Acetylcholine produced smooth, tonic contractures of tracheal muscle strips, and caused simple depolarization of the membrane. No action potentials were recorded. 6. In calcium-free solutions containing EGTA, the mechanical response to TEA was completely abolished; the response to histamine was greatly reduced; the response to acetylcholine was reduced to a lesser extent. All responses reverted to normal when normal concentrations of extracellular calcium were restored. 7. Lanthanum added to the bathing solution abolished the contraction due to TEA even though the solution contained calcium. It reduced the histamine-induced contraction to 26% of control, and reduced the acetylcholine-induced contraction to 58% of control; extracellular calcium was present throughout. 8. It is suggested that TEA produces contraction by promoting influx of calcium ions into the cytoplasm. Acetylcholine, and to a smaller extent histamine, are less dependent upon the presence of extracellular calcium, and may be capable of releasing calcium sequestered within the cell; acetylcholine appears to be more effective in releasing sequestered calcium.