Electromechanical coupling in canine trachealis muscle: acetylcholine contractions

β-Citronellol, an alcoholic monoterpene with inhibitory properties on the contractility of rat trachea

β-Citronellol is an alcoholic monoterpene found in essential oils such Cymbopogon citratus (a plant with antihypertensive properties). β-Citronellol can act against pathogenic microorganisms that affect airways and, in virtue of the popular use of β-citronellol-enriched essential oils in aromatherapy, we assessed its pharmacologic effects on the contractility of rat trachea. Contractions of isolated tracheal rings were recorded isometrically through a force transducer connected to a data-acquisition device. β-Citronellol relaxed sustained contractions induced by acetylcholine or high extracellular potassium, but half-maximal inhibitory concentrations (IC50) for K(+)-elicited stimuli were smaller than those for cholinergic contractions. It also inhibited contractions induced by electrical field stimulation or sodium orthovanadate with pharmacologic potency equivalent to that seen against acetylcholine-induced contractions. When contractions were evoked by selective recruitment of Ca2+ from the extracellular medium, β-citronellol preferentially inhibited contractions that involved voltage-operated (but not receptor-operated) pathways. β-Citronellol (but not verapamil) inhibited contractions induced by restoration of external Ca2+ levels after depleting internal Ca2+ stores with the concomitant presence of thapsigargin and recurrent challenge with acetylcholine. Treatment of tracheal rings with L-NAME, indomethacin or tetraethylammonium did not change the relaxing effects of β-citronellol. Inhibition of transient receptor potential vanilloid subtype 1 (TRPV1) or transient receptor potential ankyrin 1 (TRPA1) receptors with selective antagonists caused no change in the effects of β-citronellol. In conclusion, β-citronellol exerted inhibitory effects on rat tracheal rings, with predominant effects on contractions that recruit Ca2+ inflow towards the cytosol by voltage-gated pathways, whereas it appears less active against contractions elicited by receptor-operated Ca2+ channels.

The cellular and molecular basis of bitter tastant-induced bronchodilation

Bitter tastants can activate bitter taste receptors on constricted smooth muscle cells to inhibit L-type calcium channels and induce bronchodilation.

Bronchodilators are a standard medicine for treating airway obstructive diseases, and β2 adrenergic receptor agonists have been the most commonly used bronchodilators since their discovery. Strikingly, activation of G-protein-coupled bitter taste receptors (TAS2Rs) in airway smooth muscle (ASM) causes a stronger bronchodilation in vitro and in vivo than β2 agonists, implying that new and better bronchodilators could be developed. A critical step towards realizing this potential is to understand the mechanisms underlying this bronchodilation, which remain ill-defined. An influential hypothesis argues that bitter tastants generate localized Ca²⁺ signals, as revealed in cultured ASM cells, to activate large-conductance Ca²⁺-activated K⁺ channels, which in turn hyperpolarize the membrane, leading to relaxation. Here we report that in mouse primary ASM cells bitter tastants neither evoke localized Ca²⁺ events nor alter spontaneous local Ca²⁺ transients. Interestingly, they increase global intracellular [Ca²⁺]_i, although to a much lower level than bronchoconstrictors. We show that these Ca²⁺ changes in cells at rest are mediated via activation of the canonical bitter taste signaling cascade (i.e., TAS2R-gustducin-phospholipase Cβ [PLCβ]- inositol 1,4,5-triphosphate receptor [IP3R]), and are not sufficient to impact airway contractility. But activation of TAS2Rs fully reverses the increase in [Ca²⁺]_i induced by bronchoconstrictors, and this lowering of the [Ca²⁺]_i is necessary for bitter tastant-induced ASM cell relaxation. We further show that bitter tastants inhibit L-type voltage-dependent Ca²⁺ channels (VDCCs), resulting in reversal in [Ca²⁺]_i, and this inhibition can be prevented by pertussis toxin and G-protein βγ subunit inhibitors, but not by the blockers of PLCβ and IP3R. Together, we suggest that TAS2R stimulation activates two opposing Ca²⁺ signaling pathways via Gβγ to increase [Ca²⁺]_i at rest while blocking activated L-type VDCCs to induce bronchodilation of contracted ASM. We propose that the large decrease in [Ca²⁺]_i caused by effective tastant bronchodilators provides an efficient cell-based screening method for identifying potent dilators from among the many thousands of available bitter tastants.

Bitter taste receptors (TAS2Rs), a G-protein-coupled receptor family long thought to be solely expressed in taste buds on the tongue, have recently been detected in airways. Bitter substances can activate TAS2Rs in airway smooth muscle to cause greater bronchodilation than β2 adrenergic receptor agonists, the most commonly used bronchodilators. However, the mechanisms underlying this bronchodilation remain elusive. Here we show that, in resting primary airway smooth muscle cells, bitter tastants activate a TAS2R-dependent signaling pathway that results in an increase in intracellular calcium levels, albeit to a level much lower than that produced by bronchoconstrictors. In bronchoconstricted cells, however, bitter tastants reverse the bronchoconstrictor-induced increase in calcium levels, which leads to the relaxation of smooth muscle cells. We find that this reversal is due to inhibition of L-type calcium channels. Our results suggest that under normal conditions, bitter tastants can activate TAS2Rs to modestly increase calcium levels, but that when smooth muscle cells are constricted, they can block L-type calcium channels to induce bronchodilation. We postulate that this novel mechanism could operate in other extraoral cells expressing TAS2Rs.

Airway smooth muscle electrophysiology in a state of flux?

Activation of chloride currents and release of internally sequestered Ca2+ in airway smooth muscle have long been associated with excitation and contraction. Surprisingly, however, two recent publications (Deshpande DA, Wang WC, McIlmoyle EL, Robinett KS, Schillinger RM, An SS, Sham JS, Liggett SB. Nat Med 16: 1299–1304, 2010; Gallos G, Yim P, Chang S, Zhang Y, Xu D, Cook JM, Gerthoffer WT, Emala CW Sr. Am J Physiol Lung Cell Mol Physiol 302: L248–L256, 2012) have linked both events to relaxation. This begs a closer look at our understanding of airway smooth muscle electrophysiology and its contribution to excitation-contraction coupling. This Editorial Focus highlights those two aforementioned studies and several other equally paradoxical findings and proposes some possible reinterpretations of the data and/or new directions of research in which the answers might be found.

Calcium, calcium channel blockade and airways function

This review will highlight recent advances in understanding the physiological role of calcium and effects of calcium channel blockers on pathogenetic factors in asthma, including airway smooth muscle contraction, mast cell degranulation and mucus secretion. A review of clinical studies with calcium channel blockers in asthma will also be presented.

Essential oil of Pterodon polygalaeflorus inhibits electromechanical coupling on rat isolated trachea

The present work studied the effects of the essential oil of Pterodon polygalaeflorus (EOPP), a plant used to treat bronchitis and amigdalytis, on rat airway smooth muscle in vitro. In Ca(2+)-containing medium, EOPP (100-1300 microg/ml) inhibited preferentially high KCl- than 5-HT-induced muscle contractions in a concentration-dependent fashion, but did affect neither basal muscle tension nor ACh-induced contractions. In preparations maintained in either 60 mM K(+) or 10 microM ACh in Ca(2+)-free medium, EOPP (100, 600 and 1300 microg/ml) inhibited maximum contractile response induced by cumulative Ca(2+) addition (0.1-20 mM). Verapamil (10, 30 and 100 microg/ml), a Ca(2+) channel blocker, also inhibited Ca(2+)-induced concentration-effect curve in presence of ACh in Ca(2+)-free medium, whilst it was ineffective to decrease cholinergic contractions in Ca(2+)-containing medium. In presence of 150 mM K(+) in Ca(2+)-containing medium, EOPP (1300 microg/ml) did not reversed ACh-induced contractions. In contrast, under similar conditions, EOPP almost fully relaxed cholinergic contractions of tracheal smooth muscle in Ba(2+)-containing medium. In medium containing 10 mM tetraethylammonium and 2 mM Ba(2+) instead of Ca(2+), both EOPP (1300 microg/ml) and verapamil (approximately 5 microg/ml) significantly decreased ACh-induced contractions. Thus, in rat isolated trachea, EOPP induces inhibitor effects on contractions preferentially triggered by an electromechanical coupling mode.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

Probing excitation-contraction coupling in trachealis smooth muscle with the mycotoxin cyclopiazonic acid

1. Muscarinic stimulation-induced tonic contraction of airway smooth muscle is independent of membrane potential. This contraction is not sensitive to inhibition by voltage-operated Ca2+ channel blockers or by K+ channel openers. 2. Cyclopiazonic acid (CPA) inhibits Ca2+ loading of internal stores but does not affect maximal tonic contraction induced by acetylcholine (ACh) in steady state conditions. 3. After depletion of internal Ca2+ stores with CPA, ACh-induced tonic contraction becomes dependent upon values of membrane potential. The contraction is then sensitive to voltage-operated Ca2+ channel blockers and to K+ channel openers. 4. Treatment of trachealis muscle with CPA potentiates the M2-mediated component of ACh stimulation, but this potentiation is not entirely responsible for the switch in excitation-contraction (E-C) coupling. 5. It is proposed that depletion of internal Ca2+ stores with CPA and promotion of M2-stimulation can lead to a switch in E-C coupling in trachealis smooth muscle from pharmaco- to electromechanical mode, perhaps by targeting a plasma membrane K+ channel.

Effect of phospholipase C inhibitor U-73122 on antigen-induced airway smooth muscle contraction in guinea pigs

The importance of phospholipase C (PLC) in airway smooth muscle contraction was studied, using an inhibitor of PLC, 1-[6-[[17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl] amino]hexyl]-1H-pyrrole-2,5-dione (U-73122). Tracheas from ovalbumin (OA)-sensitized guinea pigs contracted rapidly after exposure to low concentrations of antigen (OA). However, tracheas treated with U-73122 for 10 min prior to the addition of antigen, demonstrated a 3 log rightward shift in the OA dose-response curve with an IC50 of 7 microM. The analogue of U-73122, 1-[6[[17 beta-3-methoxyestra-1,3,5 trien-17-yl]amino]hexyl]-2,5-pyrrolidine-dione (U-73433), was approximately 5-fold less active in inhibiting smooth muscle contraction. In addition to the inhibition of antigen-induced smooth muscle contraction, U-73122 inhibited carbachol- and leukotriene D4-induced smooth muscle contraction. Furthermore, U-73122 inhibited in a dose-dependent manner antigen-induced histamine release from guinea pig tracheal tissue. The inhibition of smooth muscle contraction by U-73122 correlated well with the inhibition of polyphosphoinositide mediates smooth muscle contractile responses to muscarinic agonists and leukotrienes as well as antigenic-induced contraction.

Control of resting membrane potential by delayed rectifier potassium currents in ferret airway smooth muscle cells

1. In order to determine the physiological role of specific potassium currents in airway smooth muscle, potassium currents were measured in freshly dissociated ferret trachealis cells using the nystatin-permeabilized, whole-cell method, at 35 degrees C. 2. The magnitude of the outward currents was markedly increased as bath temperature was increased from 22 to 35 degrees C. This increase was primarily due to the increase in maximum potassium conductance (gK,max), although there was also a small leftward shift in the relationship between gK and voltage at higher temperatures. The maximum conductance and the kinetics of current activation and inactivation were also temperature dependent. At 35 degrees C, gating of the current was steeply voltage dependent between -40 and 0 mV. Current activation was well fitted by fourth-order kinetics; the mean time constants of activation (30 mV clamp step) were 1.09 +/- 0.17 and 1.96 +/- 0.27 ms at 35 and 22 degrees C, respectively. 3. Outward currents using the nystatin method were qualitatively similar to delayed rectifier currents recorded in dialysed cells with high calcium buffering capacity solutions. 4-Aminopyridine (4-AP; 2 mM), a specific blocker of delayed rectifier potassium channels in this tissue, inhibited over 80% of the outward current evoked by voltage-clamp steps to between -10 and +20 mV (n = 6). Less than 5% of the outward current was blocked over the same voltage range by charybdotoxin (100 nM; n = 15), a specific antagonist of large-conductance, calcium-activated potassium channels in this tissue. 4. The degree to which delayed rectifier and calcium-activated potassium conductances control resting membrane potential was examined in current-clamp experiments. The resting membrane potential of current clamped cells was -33.6 +/- 1.0 mV (n = 62). Application of 4-AP (2 mM) resulted in a 14.4 +/- 1.0 mV depolarization (n = 8) and an increase in input resistance. Charybdotoxin (100 nM) had no effect on resting membrane potential (n = 6). 5. Force measurements were made in isolated strips of trachealis muscle to determine the effect of pharmacological blockade of individual potassium conductances on resting tone. In the presence of tetrodotoxin (1 microM) and atropine (1 microM), 4-AP increased baseline tension in a dose-dependent manner, with an EC50 of 1.8 mM (n = 13); application of 5 mM 4-AP increased tone to 86.8 +/- 8.1% of that produced by 1 microM methacholine, and this tone was almost completely inhibited by nifedipine (1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in the contractile responses to carbachol and in the inhibitory effects of verapamil and nitrendipine on isolated smooth muscle preparations from rats subchronically exposed to Co2+ and Ni2+

Male Wistar rats were exposed to subtoxic doses of Co2+ or Ni2+, receiving Co(NO3)2 or NiSO4 with drinking water for 30 days. No significant differences in the body weight and no visible changes in the behaviour of the controls and experimental animals were established. Cumulative concentration-effect curves for carbachol were obtained in ileum and trachea isolated from control and Co(2+)- or Ni(2+)-treated rats. The effect of the Ca2+ antagonists on the carbachol-induced contractions was studied by adding increasing concentrations of verapamil or nitrendipine to the bath solution 20 min prior to carbachol. The results showed that exposure of rats to subtoxic doses of Co(NO3)2 or NiSO4 altered the contractile responses to carbachol. The changes in the pD2 values and the shift to the left of the concentration-effect curves suggest a higher sensitivity to carbachol in preparations from the ileum of Co(2+)- or Ni(2+)-exposed rats. The tracheal strips isolated from control and heavy metal-treated rats showed a less potent sensitiveness to carbachol as compared to the ileal segments. An opposite tendency for decreased cholinergic reactivity was observed in tracheal strips from Co(2+)- and Ni(2+)-treated animals. The inhibitory effect of the Ca(2+)-antagonists on the contractility of ileal preparations from Co(2+)-treated rats increased at all concentrations of verapamil and at the highest concentration of nitrendipine, but decreased at lower concentrations of nitrendipine. The effect of verapamil on the preparations from Ni(2+)-exposed rats was unchanged or even decreased at higher verapamil concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Pertussis toxin augments beta-adrenergic relaxation of muscarinic contraction in canine trachealis

We studied the effect of pertussis toxin (PT) and partial muscarinic antagonism using pirenzepine (PIR) on beta-adrenergic relaxation of muscarinic contraction in 188 tracheal smooth muscle (TSM) preparations from 25 dogs in vitro. Strips of TSM were incubated for 4 h at 37 degrees C in Krebs-Henseleit (K-H) perfusate with or without 10 micrograms/ml of PT. In tissues contracted to target tension (TT; 50% of maximal response to 127 mM potassium-substituted K-H [KCl]) with acetylcholine (ACh), pretreatment with PT decreased the concentration of isoproterenol (ISO) causing 30% relaxation from TT (RC30) from 1.3 +/- 0.8 x 10(-7) M (control) to 2.8 +/- 0.7 x 10(-8) M (p = 0.013). Pretreatment with PT also augmented the maximal relaxation elicited by 10(-5) M ISO. In separate studies, strips of TSM were contracted with ACh; pretreatment with 10(-7) M PIR decreased the concentration of ISO causing 50% relaxation (RC50) from 3.4 +/- 0.6 x 10(-7) to 9.6 +/- 1.5 x 10(-8) M (p = 0.042). Pretreatment with PIR did not affect relaxation elicited by ISO for strips contracted equivalently with KCl. In addition, PIR increased both the potency and efficacy of ISO in relaxing muscarinic contraction in sham-incubated strips of TSM but had no effect after incubation with PT. Neither PT nor PIR affected beta-adrenergic relaxation of TSM contracted with KCl. Our data demonstrate that beta-adrenergic receptor relaxation of muscarinic contraction is augmented by (1) incubation with PT and (2) partial blockade of muscarinic receptors.

Delayed rectifier potassium channels in canine and porcine airway smooth muscle cells

1. In order to define the ion channels underlying the inactivating, calcium-insensitive current in airway smooth muscle cells, unitary potassium currents were recorded from canine and porcine trachealis cells, and compared with macroscopic currents. On-cell and inside-out single-channel currents were compared with whole-cell recordings made in dialysed cells. 2. Depolarizing voltage steps evoked outward unitary currents. In addition to a large conductance, calcium-activated potassium channel (KCa), a lower conductance potassium channel was identified. This channel has a conductance of 12.7 pS (on-cell; 1 mM-K+ in the pipette). 3. The lower conductance channel (Kdr) was not sensitive to cytosolic Ca2+ concentration and unitary current openings occurred following a delay after the voltage step. The time course of activation of the current composed of averaged single-channel events was very similar to that of the whole-cell, delayed rectifier potassium current (IdK), recorded under conditions of low intracellular calcium (Kotlikoff, 1990). 4. Kdr channels also inactivated with kinetics similar to those of the macroscopic current. Averaged single-channel records revealed a current that inactivated with kinetics that could be described by two exponentials (tau 1 = 0.14 s, tau 2 = 1.1 s; at 5 mV). These values corresponded well with previously determined values for time-dependent inactivation of IdK. Inactivation of Kdr channels was markedly voltage dependent, and was well fitted by a Boltzmann equation with V50 = -53 mV; this was similar to measurements of the macroscopic current, although the V50 value was shifted to more positive potentials in whole-cell measurements. When only the inactivating component of the macroscopic current was considered, the voltage dependence of inactivation of the single-channel current and macroscopic current were quite similar. 5. Single-channel kinetics indicated that Kdr channels occupy one open and two closed states. The mean open time was 1.7 ms. Inactivation results in a prominent increase in the long closed time, with little effect on the mean open time or short closed time. 6. The Kdr channel was not blocked by tetraethylammonium (TEA; 1 mM), charybdotoxin (ChTX; 100 nM) or glibenclamide (20 microM), but was blocked by 4-aminopyridine (4-AP; 1 mM). Similarly, 4-AP blocked the inactivating component of the macroscopic current, but a non-inactivating current remained. KCa currents were blocked by TEA (0.5-1 mM) and charybdotoxin (40 nM), but were insensitive to to 4-AP (1 mM) and glibenclamide (20 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Halothane alters the response of isolated airway smooth muscle to carbon dioxide

Rings of canine bronchi were studied in vitro to determine the effects of halothane on the responses of airway smooth muscle to hypercapnia and hypocapnia. Bronchi were first contracted to 50% of maximal active force with acetylcholine (ACh), 5-hydroxytryptamine (5HT), potassium chloride (KCl), or the muscarinic agonist McN-A-343 (McN). The CO2 concentration of the bathing solution was then changed from 6% to either 1% (hypocapnia) or 10% (hypercapnia). In the absence of halothane, changes in CO2 concentration had no significant effect on muscles contracted with ACh. With all other contractile agonists, increasing the CO2 concentration caused bronchial relaxation, while decreasing the CO2 concentration caused contraction. In the presence of 2 MAC halothane, hypocapnia relaxed bronchi contracted with the muscarinic agonists ACh or McN; the responses to hypocapnia of bronchi contracted with KCl and 5HT were not significantly changed by halothane. Halothane had no effect on the responses of the bronchi to hypercapnia. We conclude that airway smooth muscle contracted with cholinergic agonist relaxes in response to hypocapnia when exposed to 2 MAC halothane; this mechanism may contribute to the depression of hypocapnic bronchoconstriction caused by halothane in vivo.

The internal calcium store in airway muscle: emptying, refilling and chloride. Possible new directions for drug development

This review examines the ionic mechanisms underlying acetylcholine (Ach) depolarization of airway smooth muscle and suggests that multiple mechanisms are involved. Increased chloride and nonspecific cation conductance, and decreased or rapidly inactivating potassium conductances seem to be involved. Chloride ions also seem to play an important role in determining whether Ca2+ remains inside or is replenished in the sarcoplasmic reticulum (SR). The physiological role of Ach-induced depolarization is analysed and is suggested to be the promotion of the refilling of Ca2+ stores, partly through a direct refilling of SR-Ca2+ stores by way of an L-type Ca2+ channel. This refilling is promoted by Ca2+ channel agonists and is independent of the transmembrane potential. Ca(2+)-release by a variety of agonists leads to depolarization and stable membrane oscillations which depend on the action of the Ca(2+)-store uptake mechanisms in order to function. These oscillations may play a role in prolonged bronchoconstriction. Better knowledge of the control mechanisms of Cai2+ is likely to reveal new targets for the therapy of asthma and provide a better understanding of the function of airway smooth muscle.

Acetylcholine activates non-selective cation and chloride conductances in canine and guinea-pig tracheal myocytes

1. Membrane currents activated by acetylcholine (ACh) were investigated in isolated canine and guinea-pig tracheal myocytes using the nystatin perforated patch configuration of whole-cell recording. ACh caused depolarization accompanied by a membrane conductance increase. 2. When cells were held under voltage clamp (holding potential, Vh = -60 mV), ACh elicited inward current (IACh) of up to 3900 pA, with a reversal potential (Erev) of approximately -20 mV. 3. Removal of extracellular Na+ (Na+o) reduced but did not eliminate IACh. IACh remaining in the absence of Na+ reversed direction close to the predicted equilibrium potential for Cl-. Erev shifted 32 +/- 4 mV per 10-fold change of [Cl-]i. Increasing external [K+] caused Erev to shift in the positive direction. These results suggest that ACh activated chloride and non-selective cation conductances. 4. In the absence of Na+o, the Cl- channel blockers SITS or niflumic acid reversibly antagonized IACh. 5. Caffeine and ryanodine elicited currents both in the presence and absence of Na+o; these currents had a reversal potential similar to that of IACh. Caffeine applied before ACh occluded the response to ACh. 6. We also observed two types of spontaneous membrane currents. Spontaneous transient outward currents (STOCs) may represent Ca(2+)-activated K+ currents. Spontaneous inward currents were also observed which were reduced in magnitude (but not eliminated) by removal of Na+o and reversed direction at approximately the Cl- equilibrium potential. The spontaneous inward currents and STOCs were coincident and were reversibly suppressed by ACh. 7. ACh elicited contractions of cells under voltage clamp at -60 mV, an effect also observed in the absence of extracellular Ca2+ or when IACh was reduced by omission of Na+o and exposure to Cl- channel blockers. The number of cells which did contract in response to ACh decreased, however, when the concentration of internal Cl- decreased. 8. All effects of ACh on contraction and membrane currents were antagonized by atropine. 9. We conclude that activation of muscarinic receptors in mammalian tracheal myocytes causes release of Ca2+ from intracellular stores and subsequent activation of Cl- and non-selective cation conductances. This is the first direct demonstration of these conductances in tracheal smooth muscle cells. Activation of these conductances does not appear to be required for contraction. However, regulation of cytosolic Cl- levels may be important for release and uptake of Ca2+ from internal stores.

Inhaled verapamil-induced bronchoconstriction in mild asthma

Methacholine challenges were performed in ten subjects with mild asthma at 2 h before and 20 min after placebo or 5, 10, 20, 40, 80, and 160 mg of inhaled verapamil given in a single-blind randomized crossover manner on different days. While verapamil did not have a bronchodilator effect, the 10-mg dose modestly increased the concentration of methacholine required to decrease FEV1 by 20 percent (PC20). The mean (+/- SEM) increase in PC20 from baseline was 2.1 +/- 0.2 times baseline after 10 mg of verapamil, compared to 1.1 +/- 0.1 times baseline after placebo (p less than 0.001). Unexpectedly, bronchoconstriction (greater than 10 percent decrease in FEV1) associated with cough or wheezing was observed in seven of ten subjects at doses of 20 mg or more. This adverse effect was not related to the osmolarity of the nebulized solutions. Thirty minutes before a standardized exercise challenge, 13 subjects inhaled placebo, 10 mg, or the highest dose of verapamil tolerated during the methacholine study (20 to 160 mg) in a double-blind randomized crossover manner. The exercise challenge was aborted in three subjects because of bronchospasm that occurred after administration of the higher dose. The mean (+/- SEM) maximum change in FEV1 after exercise in the ten subjects completing all three regimens of treatment was -17.1 +/- 4.0 percent after placebo, -12.7 +/- 4.3 percent after 10 mg (p less than 0.05), and -6.4 +/- 3.6 percent after the highest dose (p less than 0.05). We conclude that increasing the dose of verapamil above 10 mg did not provide greater benefit but, paradoxically, induced bronchoconstriction in most of the subjects. Because of this potential bronchoconstrictor effect, high doses of oral or intravenous verapamil should be used with caution in asthmatic subjects.

Modulation of equine tracheal smooth muscle contractility by epithelial-derived and cyclooxygenase metabolites

The role of epithelium in the modulation of contractile responses to electrical field stimulation (EFS), acetylcholine (ACh), and KCl were studied in vitro in strips of equine tracheal smooth muscle (TSM). EFS with 0.5 ms pulses of voltage (70 V) resulted in frequency dependent contractions of equine TSM that were sensitive to tetrodotoxin (TTX) and atropine. In TSM without epithelium, preincubation with indomethacin significantly potentiated contractile responses to EFS. The potentiating effect of indomethacin on EFS contractions was abolished by the addition of 3 nM prostaglandin E2 (PGE2). ACh and KCl cumulative concentration-response curves were shifted to the left by removal of epithelium from equine TSM strips with a significant decrease in the 50% effective concentration (EC50) for both ACh and KCl. The mean EC50 (+/- SE) for ACh in TSM without epithelium was 0.51 +/- 0.09 microM vs 4.30 +/- 1.03 microM in TSM with epithelium. Similarly, the mean EC50 (+/- SE) for KCl in TSM without epithelium was 22.20 +/- 2.61 mM vs 32.35 +/- 2.66 mM in TSM with epithelium. The addition of indomethacin (3 microM) had no effect on the ACh concentration-response curves in TSM strips with or without epithelium. Our results suggest that in the equine airway there is (1) an epithelial-derived relaxant factor that modulates tracheal smooth muscle contractility postsynaptically, and (2) a nonepithelial-derived inhibitory factor, possibly PGE2, that modulates ACh release from nerves presynaptically.

Receptor-activated calcium influx in human airway smooth muscle cells

1. Fluorescence measurements of intracellular calcium concentrations ([Ca2+]i) were made on cultured human airway smooth muscle cells using the dye Fura-2. The response to either histamine (100 microM) or bradykinin (1 microM) was biphasic, with a transient increase in [Ca2+]i followed by a sustained [Ca2+]i increase lasting many minutes. The average steady-state (plateau) [Ca2+]i following agonist activation was 267 +/- 5 nM, whereas the average basal [Ca2+]i was 148 +/- 4 nM. 2. The sustained rise in [Ca2+]i required the continued presence of either histamine or bradykinin and was dependent on extracellular Ca2+. The magnitude of the transient rise in [Ca2+]i was not dependent on extracellular Ca2+. Sustained, receptor-activated rises in [Ca2+]i were rapidly abolished by chelation of extracellular Ca2+, or addition of non-permeant polyvalent cations, whereas these agents had minor effects in the absence of agonist. These data indicate that the sustained increase in [Ca2+]i was dependent on receptor-activated Ca2+ influx. 3. Receptor-activated Ca2+ influx was not affected by treatment with organic Ca2+ channel antagonists (nifedipine (10 microM), nisoldipine (10 microM) or diltiazem (10 microM] or agonists (Bay K 8644 (500 nM to 10 microM) or Bay R 5417 (500 nM]. The magnitude of the sustained rise was also not affected by pre-treatment with ouabain (100 microM) indicating little involvement of Na(+)-Ca2+ exchange in the influx mechanism. 4. Receptor-activated Ca2+ influx could be completely inhibited by several polyvalent cations (Co2+, Mn2+, Ni2+, -Cd2+ or La3+). Quantitative estimates of the potency of block were obtained for Ni2+ and La3+. These measurements indicate that the pKi for Ni2+ was 3.6 and for La3+ was 3.5. 5. Both Mn2+ and Co2+ ions caused a time-dependent quench of intracellular Fura-2; however, permeation of neither ion was increased following receptor activation, indicating that the influx pathway is not permeable to these cations. 6. Fura-2 was used to monitor the rate of Ba2+ entry into airway smooth muscle cells by monitoring the Ca(2+)-Fura-2 and Ba(2+)-Fura-2 isosbestic points as well as the 340 and 380 nm signals. Cell activation did not increase the rate of Ba2+ entry indicating that the Ca2+ influx pathway was poorly permeant to Ba2+ ions. Ba2+ (2 mM) was able to inhibit Ca2+ entry as shown by its effects on the Ba(2+)-independent, Ca(2+)-dependent wavelength (371 nm). 7. The voltage dependence of Ca2+ influx was examined before and after agonist-induced activation. The effect of KCl-induced depolarization prior to cell activation was to cause a slight increase in [Ca2+]i.(ABSTRACT TRUNCATED AT 400 WORDS)

Cromakalim and K+ channels in canine trachealis

The effects of cromakalim and glibenclamide on membrane properties and responses to acetylcholine of canine trachea were studied in the double sucrose gap to evaluate the presence and function of ATP-sensitive K+ channels. Cromakalim produced a concentration-dependent hyperpolarization of muscle membrane potential which at maximum brought the membrane potential near the potassium equilibrium potential. Current clamping by hyperpolarizing current to this equilibrium potential abolished the hyperpolarization but not the membrane resistance decrease to cromakalim. Glibenclamide had no effect on resting membrane properties but reduced or abolished effects of cromakalim. Another K+ channel antagonist, tetraethylammonium at 20 mM, also reduced the effects of cromakalim, but 4-aminopyridine (5 mM), Ba2+ (1 mM), and apamin (10(-6) M) had no antagonistic effect. The EJP produced on stimulation of cholinergic nerves sometimes increased just after cromakalim-induced hyperpolarization, but within 5-10 min as membrane resistance dramatically fell it was reduced, as was the depolarization to infused acetylcholine. Initially the reduction in EJP amplitude could be partially overcome by applying hyperpolarizing currents or by applying a second field stimulation; later the EJP was reduced further and was unaffected by these procedures. Even when depolarization to acetylcholine was markedly reduced, the contraction was not. Glibenclamide had no effects alone but antagonized all the effects of cromakalim. These results suggest that ATP-sensitive cromakalim activated K+ channels are present in canine trachea but are usually closed during resting conditions under our experimental conditions. When they are opened by cromakalim, they hyperpolarize to near EK, markedly decrease membrane resistance and reduce the depolarization response to acetylcholine, probably by short circuiting the acetylcholine-induced current.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of membrane currents in isolated smooth muscle cells from guinea-pig trachea

Tracheal smooth muscle cells were enzymatically isolated from guinea-pig trachea. These cells contracted in response to acetylcholine (0.01-10 microM) in a concentration-dependent fashion. Under current-clamp conditions with 140 mM K+ in the pipette solution, the membrane potential oscillated spontaneously at around -30 mV. Under voltage-clamp conditions, there appeared spontaneous but steady oscillations of outward current (IO). On depolarization from a holding potential at -40 mV, three components of outward current were elicited: transient outward current (IT), steady-state outward current (IS) and IO. These three components of outward current reversed around the K+ equilibrium potential and were abolished by Cs+ in the pipette, indicating that K+ was the major charge carrier of these outward currents. All these three components were completely suppressed by extracellular tetraethylammonium (10 mM). Both IT and IO were depressed by quinidine (1 mM), 4-aminopyridine (10 mM) and nifedipine (100 nM), but IS was not affected. IT and IO were suppressed by a Ca2(+)-free perfusate with less than 1 nM Ca2+ in the pipette, while with 10 nM Ca2+ in the pipette, only IO was suppressed. In both conditions, IS was not affected by the Ca2(+)-free perfusate. Therefore, it is suggested that IO, IT and IS are separate types of K+ current. With Cs+ in the pipette, K+ currents were almost completely suppressed and a transient inward current was observed during depolarizing pulses. The inward current was not affected by tetrodotoxin and increased when the concentration of extracellular Ca2+ was raised, indicating that the current is a Ca2+ channel current. Even with a holding potential of -80 mV, the low-threshold inward current could not be observed. The high-threshold Ca2+ current was abolished by nifedipine (100 nM) and was enhanced by Bay K 8644 (100 nM). The order of permeation of divalent cations through the Ca2+ channel was Ba2+ greater than Sr2+ approximately Ca2+. Cd2+ blocked the Ca2+ current more effectively than Ni2+. These results may indicate that the Ca2+ current of tracheal smooth muscle cells is mainly composed of the current through an L-type Ca2+ channel.

Phorbol ester effects on coupling mechanisms during cholinergic contraction of swine tracheal smooth muscle

1. We studied effects of the phorbol ester, phorbol 12,13-dibutyrate (PDB), on carbachol-induced contractions of swine trachealis muscle. PDB (1-10 microM) markedly inhibited 5.5 microM-carbachol-induced inositol phosphate synthesis allowing us to study (a) whether the membrane potential-independent component of force (pharmacomechanical coupling component) developed in carbachol-stimulated trachealis muscle is dependent on activation of inositol phospholipid metabolism, and (b) whether carbachol-induced membrane depolarization and contraction are altered in muscle where second messenger signals generated by inositol phospholipid metabolism are inhibited and activation of protein kinase C (PKC) is already maximal. 2. Application of PDB (10 microM) to unstimulated trachealis muscle resulted in a small slowly developing contraction associated with a 10 m V membrane depolarization. PDB-evoked contractions were not influenced by Na+ or Cl- ion substitutions, or administration of amiloride, all of which inhibited PDB-evoked membrane depolarization. 3. Pre-treatment with PDB had no effect on [K+]-force, or [K+]-membrane potential relationships, over a range of extracellular [K+] from 40 to 70 mM. Pretreatment with PDB had no effect on extracellular [Ca2+]-force relationships during 40 mM-K+. 4. Carbachol-evoked contractions of muscle treated with PDB became similar to K+ contractions in regard to effects of organic Ca2+ antagonist drugs or decrease in bathing solution [Ca2+]. At low carbachol concentrations, verapamil plus PDB completely inhibited force development. With 5.5 microM-carbachol, over 90% of total carbachol-induced force was inhibited by verapamil, or nifedipine, plus PDB. 5. Control carbachol-evoked contractions were associated with 20-25 mV membrane depolarizations. In PDB-treated muscle, carbachol-evoked contraction occurred with a blunted depolarization, i.e. about 5 mV. 6. Force controlled by pharmacomechanical coupling mechanisms operating during maintained carbachol-evoked contractions was inhibited by treatment with PDB. Carbachol-induced force dependent on pharmacomechanical coupling mechanisms could be explained by signals generated via inositol phospholipid metabolism. 7. Electromechanical coupling mechanisms were augmented during carbachol in PDB-treated muscle. This appears to be due primarily to changes in the properties or number of surface membrane voltage-gated Ca2+ channels. 8. Data suggest an important role of PKC-mediated phosphorylations for control of both pharmacomechanical coupling mechanisms mediated by activation of inositol phospholipid metabolism and electromechanical coupling mechanisms mediated by effects on operation of surface membrane ion channels.

The intracellular calcium stores in the rabbit trachealis

We investigated the role of the intracellular Ca2+ stores in the regulation of the rabbit tracheal smooth muscle contraction. Carbachol (10 microM)- and 80K-induced contractions were reduced by preincubating tissues in Ca2+-free (EGTA-PSS) solution. Contractile amplitude plotted as a function of the duration of EGTA-PSS preexposure was described by a biexponential for carbachol and a monoexponential for 80K. In EGTA-PSS, a prior caffeine (50 mM)-induced contraction prevented any subsequent phasic carbachol response; the converse was also true. In contrast, prior exposure to 80K increased the amplitude of a subsequent carbachol or caffeine contraction measured in EGTA-PSS. Repletion of Ca2+ plus either 80K or a low concentration of carbachol (0.3 microM) resulted in delayed tension development. Preincubation in forskolin (10(-5) M) in PSS also delayed tension development. We propose that the internal stores, most likely the sarcoplasmic reticulum in the airway muscle function both to supply and remove Ca2+ from the cytoplasm.

Membrane properties of bovine airway smooth muscle cells: effects of maturation

Airway smooth muscle cells of bovine trachealis muscle from immature (2 weeks old), developing (5 months old) and mature animals (greater than 5 years old) have resting membrane potentials of -63.5 +/- 0.4 mV, -62.2 +/- 0.8 mV and -60.3 +/- 0.7 mV, respectively. These resting potentials were not significantly different. The contribution of the electrogenic sodium pump to the resting membrane potential of airway smooth muscle cells decreases significantly with age being 38.3% +/- 0.9, 30.6% +/- 0.8 and 21.3% +/- 0.9 in tissues from immature, developing and mature cows, respectively. The contribution of the electrogenic sodium pump to resting membrane potential was not influenced by the degree of stretch (applied load) of the airway smooth muscle cells. However, resting membrane potential was reduced with increasing tone especially when loads between 5 to 10 g were applied. The densities of Na+/K+ pump sites, as measured by [3H] ouabain binding, decreased significantly as a consequence of maturation and were 4.12 +/- 5.23, 3.08 +/- 0.26 and 1.94 +/- 0.38 pmoles/mg protein ouabain binding in tissues from immature, developing and mature animals respectively. The affinity of the pump sites for the cardiac glycoside, ouabain, did not change during maturation. We conclude that maturation alters both the number of Na+/K+ pump sites and the contribution of the electrogenic sodium pump to the membrane potential of airway smooth muscle cells. These age related changes may contribute to the reduced airway reactivity to both bronchoconstrictor and bronchodilator agents previously observed both in vitro and in vivo.

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.

Electrophysiological and other aspects of excitation-contraction coupling and uncoupling in mammalian airway smooth muscle

In this article the electrophysiological events which are believed to underly agonist-induced contraction and relaxation of airway smooth muscle are reviewed, with special emphasis on the indispensable role of the Ca ion. The contribution made by Na, K, Ca and Cl to, and the role that the electrogenic Na:K-dependent ATPase plays in, the maintenance of the resting membrane potential in both normal and sensitised airway smooth muscle cells is described together with the permeability changes that occur in the plasmalemma in response to excitatory and inhibitory agonists. In addition, the currently available evidence for the existence of potential-sensitive and receptor-operated Ca channels in respiratory smooth muscle, and how such channels may be involved in the regulation of airway calibre, is critically assessed.

Myosin light chain phosphorylation during phasic contractions of tracheal smooth muscle

Rapid, coordinated contractions of tracheal smooth muscle were elicited by either direct electrical depolarization of muscle cells or treatment with tetraethylammonium which produced spontaneous phasic contractile activity. Both types of contraction were blocked by the calcium channel antagonist verapamil, indicating that these contractions are supported primarily by calcium of extracellular origin. With direct electrical stimulation, force was biphasic and phosphate content of the phosphorylatable light chain (P-light chain) of myosin increased rapidly (approximately 2.5 s) from 0.1 to 0.4 mol phosphate/mol P-light chain, then decreased to levels above resting values. Phosphorylation increased more rapidly than force. Under conditions of spontaneous activity, phasic contractions occurred above a level of basal tone significantly greater than resting force, and minimum values of phosphorylation measured at the base of contraction were significantly greater than those observed in the resting muscle. Phosphorylation oscillated with force (from 0.2 to 0.4 mol phosphate/mol P-light chain) and peak values occurred during the rising phase of contraction. Time courses of phosphorylation and force showed evidence of a prolonged state of activation of myosin following dephosphorylation. These results suggest that phosphorylation and dephosphorylation of myosin P-light chain are sufficiently rapid to participate in regulation of contractility during phasic mechanical activity.

Acetylcholine-induced contraction in human isolated bronchial smooth muscle: role of an intracellular calcium store

The aim of this study was to investigate the role played by an intracellular calcium store in human bronchoconstriction. Human isolated bronchial smooth muscle strips (5-6 mm long, 0.4-0.5 mm wide) were taken from lung specimens during pneumonectomy. Isometric contraction was recorded after stimulation by 10(-4) M acetylcholine in physiological solutions. The amplitude of acetylcholine-induced contractions was measured in the presence and absence of calcium ions in the perfusing medium. When the perfusing medium was switched to a calcium-free solution the amplitude of the acetylcholine-induced contraction was measured with respect to the duration of calcium-free perfusion. The amplitude of the contraction was 82.1 +/- 11.3%, 77.2 +/- 15.4%, 63 +/- 10.6% of the maximum contraction after the strips were perfused in calcium-free solutions for 1, 3 and 5 min respectively. Several successive contractions could be elicited, and even after 20 min of calcium-free perfusion, acetylcholine was still able to elicit contractions. These results suggest that an intracellular calcium store may be involved in human bronchoconstriction. This finding may help further our understanding of the effects of calcium antagonists on human airways.

Biochemical regulation of airway smooth muscle tone: current knowledge and therapeutic implications

Evidence collected during the last decade indicates that the molecular processes responsible for smooth muscle contraction are fundamentally different from those responsible for skeletal muscle contraction. Furthermore, because of the diverse functional roles of various smooth muscles, it would not be surprising if significant differences in regulatory processes also exist among different smooth muscles. Such diversity may already be exemplified by differences in cross-bridge kinetics and sources of activator Ca2+. Additional unique regulatory features of various smooth muscle types will undoubtedly be uncovered by further research. A convincing body of data suggests that activation of the adenylate cyclase/protein kinase cascade is responsible for the bronchodilation produced by beta-adrenoceptor agonists. Although the exact mechanism by which the activation of cAMP-dependent protein kinase induces relaxation is not clear, the phosphorylation of multiple substrates may be involved. Phosphorylation of these substrates can promote relaxation by decreasing the myoplasmic Ca2+ concentration, decreasing the Ca2+ sensitivity of the contractile apparatus, or both. Thus, because beta-adrenoceptor agonists act as physiologic antagonists of broncho-constriction, they should relax airway smooth muscle regardless of the mediator(s) responsible for the bronchospasm. Perhaps this is the major reason that the beta-adrenoceptor agonists have become the premier class of drugs used in the treatment of bronchial asthma. As useful as the sympathomimetic bronchodilators have been, they are not without liabilities. These liabilities include: cardiovascular and skeletal muscle side effects, an inherent subsensitivity of the asthmatic patient population to beta-adrenoceptor agonists, the development of tolerance and a loss of efficacy during severe asthmatic episodes. The fact that these drawbacks are probably shared by all sympathomimetic bronchodilators suggests that little therapeutic advantage will be gained by developing new beta-adrenoceptor agonists. The task of developing novel bronchodilators will be facilitated by an understanding at the molecular level of the diversity among smooth muscles and the processes that regulate smooth muscle tone. Hopefully, such knowledge will lead to a new generation of highly effective, tissue-selective bronchodilators with significant therapeutic advantages over those currently available.

Effects of PCO2, pH and extracellular calcium on contraction of airway smooth muscle from rats

The effect of changes in PCO2 on airway smooth muscle was studied in acetylcholine-induced contractions of isolated rat trachea. Elevation of superfusate PCO2 from control PCO2, 38 mm Hg (pH 7.49), to 168 mm Hg (pH 6.74) decreased tension to 68% of control tension; reduction of PCO2 to 19 mm Hg (pH 7.84) increased tension to 104%. Similar effects on tension occurred when pH was altered by varying superfusate bicarbonate concentration at constant PCO2. Modification of the response to changes in PCO2 by varying extracellular calcium (Ca2+) concentration and also by verapamil indicated that changes in PCO2 and pH may alter Ca2+ uptake by the smooth muscle. Calcium uptake was measured by 45Ca2+ and the lanthanum method. At control pH 7.49, net Ca2+ uptake was 5.34 mmol Ca2+/kg trachea 60 min after the onset of contraction; this decreased to 4.26 at pH 6.88, and increased to 6.58 at pH 7.85. The results suggest that the mechanism whereby changes in PCO2 affect airway smooth muscle contraction is a pH-dependent alteration of Ca2+ uptake.

Effects of verapamil on the response of the guinea-pig tracheal muscle to carbachol

The effects of verapamil on the contraction of the guinea-pig tracheal smooth muscle induced by calcium (Ca2+) or barium (Ba2+) were investigated in three different conditions: (a) in excess K solution, (b) in the presence of carbachol, and (c) in excess K solution containing carbachol. In order to clarify the contractions, the effects of removal and readdition of the divalent cations were also investigated. In Ca2+-loaded tissues, application of carbachol in Ca-free medium produced a transient contraction, the magnitude of which decreased the longer the duration of exposure to Ca2+-free solution. In Ca2+-depleted, Ba2+-loaded tissues, application of carbachol in a Ba2+- and Ca2+-free medium produced a transient contraction the magnitude of which decreased the longer the duration of exposure to the Ba2+- and Ca2+-free solution. After exposure to Ca2+-free solution for 40 min, the sensitivity of the tissue to Ca2+ was greater in the presence of 30 microM carbachol (ED50 = 0.06 mM) than in the presence of 40 mM K+ (ED50 = 0.3 mM). The Ca2+-sensitivity in the presence of 30 microM carbachol plus K+ (40 mM) was not different from that in the presence of 30 microM carbachol alone. In Ca2+-free solution, the sensitivity of the tissue to Ba2+ in the presence of 40 mM K+ (ED50 = 1.4 mM) was not different from that observed in the presence of 30 microM carbachol (ED50 = 1.3 mM). 6 After exposure to Ca2+-free solution, verapamil produced a parallel rightward shift in the concentration-response curves to added Ca2+ and Ba2+ in the presence of either 40 mM K+, 30 microM carbachol or 40mM K+ plus 30 microM carbachol. 7 The pA2 values of verapamil against Ca2+ responses in the presence of 40 mM K+, 30 microM carbachol and 40 mM K+ plus 30 microM carbachol were 7.0, 6.5 and 6.5, respectively. The pA2 values of verapamil against Ba2+ responses under these conditions were 7.1, 7.0 and 7.1, respectively. 8 It is concluded that the sustained contraction produced by carbachol requires the influx of Ca2+ and that Ba2+ can substitute for Ca2+ in this process. Furthermore, the ionic channels which admit Ca2+ may be modified by carbachol to different degrees depending on the presence of Ca2+ or Ba2+. Such changes alter the affinity of the channel to verapamil.

Cooling-induced subsensitivity to carbachol in the tracheal smooth muscle of the guinea-pig

Isolated tracheal strip-chain preparation of the guinea-pig was used to study the effect of temperature on carbachol-induced contraction. The preparation was suspended in the organ bath containing Krebs bicarbonate solution for isometric tension recording. A decrease of bath temperature from 37 degrees C to 20 degrees C (cooling) caused a transient increase in tension and thereafter inhibited the contractile response of the trachea caused by carbachol (30 nmol/l-3 mumol/l). Isosmotic potassium chloride (KCl, 64.7 mmol/l)-induced contraction or calcium chloride (CaCl2, 0.1--3 mmol/l)-induced contraction in K+-depolarized muscle was markedly inhibited by cooling. Verapamil in concentrations of 1 mumol/l or greater, which markedly depressed the CaCl2-induced contraction, caused partial depression of the contractile response to carbachol. On the other hand, carbachol-induced contraction of the trachea which was incubated with K+-rich, verapamil (3 mumol/l) containing Krebs solution and with Ca2+-free, EGTA (0.4 mmol/l) containing Krebs solution were both augmented at 20 degrees C. From these observations, it is concluded that decreased responsiveness of the guinea-pig airway smooth muscle to carbachol with lowered temperature may be due to an inhibition of Ca2+ influx through voltage-dependent Ca2+ channels which involves part of the contraction.

Cooling-induced supersensitivity to acetylcholine in the isolated airway smooth muscle of the rat

Isolated tracheal and bronchial strip-chain preparations of the rat were used to study the effect of temperature on electrically or acetylcholine-induced contraction. The preparations were suspended in the organ bath containing Krebs bicarbonate solution for isometric tension recording. A decrease of bath temperature from 37 degrees C to 20 degrees C (cooling) had no effect on basal tone but augmented the contractile responses of the trachea and bronchus caused by stimulation of intramural cholinergic nerves (0.5-5 Hz) or acetylcholine (3 mumol/l-0.3 mmol/l). Cooling-induced augmentation of the contractile response to acetylcholine was not affected by pretreatment of the tissue with physostigmine (0.1 mumol/l) or tetrodotoxin (0.3 mumol/l). The affinity of acetylcholine for the tracheal muscarinic receptors at 20 degrees C, determined from its dissociation constant (KA), was not significantly different from that at 37 degrees C. On the other hand, acetylcholine-induced contraction of trachea which was incubated with isosmotic K+- rich Krebs solution and with Ca-free, EGTA (0.1 mmol/l) containing Krebs solution were both augmented at 20 degrees C. Caffeine or vanadate, each at a lower concentration than the threshold for causing contraction by itself, augmented the contractile responses of the trachea to acetylcholine (1 mumol/l-0.3 mmol/l). These potentiating effects of caffeine and vanadate were greater at 20 degrees C then 37 degrees C. From these observations, it is concluded that increased responsiveness of the rat airway smooth muscle to acetylcholine with lowered temperature may involve the acceleration of Ca release from intracellular storage sites, inhibition of Ca extrusion from the cell and or the inhibition of Ca reuptake by intracellular storage sites.

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.

Effect of diltiazem on histamine- and carbachol-induced bronchospasm in normal and asthmatic subjects

Recently, several transmembrane calcium-channel blockers have been used in experimental models to investigate the mechanisms through which Ca++ ions contribute to the regulation of the contractile response of airway smooth muscle and to determine the therapeutic use of these drugs in bronchial asthma. Since the data from these studies are inconsistent and inconclusive, we studied the effect of diltiazem, a calcium-channel blocker previously not examined to our knowledge, on histamine- and carbachol-induced bronchoconstriction in healthy and in asymptomatic allergic bronchial asthma. The study was performed in a double-blind, randomized, placebo-controlled fashion, using a single oral dose of 60 mg of diltiazem. Airway reactivity to histamine and carbachol expressed by PD35SGaw was significantly but weakly attenuated by diltiazem in the asthmatic, but not in the normal subjects. Baseline lung function was not significantly influenced by diltiazem. We concluded that the effect of diltiazem on unspecific airway hyperresponsiveness in asthmatic subjects is too weak to justify a recommendation as therapy.

Pharmacomechanical coupling in smooth muscle may involve phosphatidylinositol metabolism

Cholinergic contraction of canine trachealis muscle, a contraction that primarily utilizes membrane potential-independent mechanisms for activating contractile proteins (pharmacomechanical coupling), is associated with a decline in the phosphatidylinositol pool, an increase in the phosphatidic acid and diacylglycerol pools, and an increased incorporation of 32PO4 into phosphatidylinositol. We found that these changes occur during development of the contraction and during maintenance of tension and are independent of membrane depolarization or increases in cytosolic Ca2+ concentration. These findings suggest that phosphatidylinositol turnover may be part of a receptor transduction process controlling receptor-operated Ca2+ channels or other membrane potential-independent mechanisms involved in pharmacomechanical coupling in smooth muscle.

Some features of the spasmogenic actions of acetylcholine and histamine in guinea-pig isolated trachealis

Intracellular electrophysiological recording showed that acetylcholine (1 mumol l-1) and histamine (2 mumol l-1) depolarized trachealis cells and often increased the frequency of slow waves. Higher concentrations of these agents caused greater depolarization and abolition of slow waves. Marked depolarization was often associated with the appearance of electrical 'noise'. These electrical phenomena were accompanied by tonic tension development in a contiguous segment of trachea. Electrical 'noise' and tension evoked by high concentrations of acetylcholine or histamine could be dissipated by washing the agonist from the tissue. Acetylcholine-induced 'noise' was resistant to tetrodotoxin (3 mumol l-1) and to hexamethonium (1 mmol l-1). Neither acetylcholine (10-1,000 mumol l-1) nor histamine (2-200 mumol l-1) increased the lanthanum-resistant calcium fraction of muscle-containing strips of trachea. It is concluded that, while developing tension under the influence of acetylcholine or histamine, trachealis cells depolarize markedly but there is relatively little cellular influx of Ca2+.

Calcium, the control of smooth muscle function and bronchial hyperreactivity

The Ca2+ requirements for excitation-contraction coupling in smooth muscle may be satisfied from both intracellular and extracellular sources, the relative extent of use of which is both tissue- and stimulant-dependent. Extracellular Ca2+ is apparently mobilized through two separate pathways, receptor operated (ROC) and potential dependent (PDC) Ca2+ channels. The latter process is sensitive to the Ca2+-channel antagonists, a heterogeneous group of compounds including verapamil, nifedipine and diltiazem. Ca2+ mobilization in respiratory smooth muscle is reviewed. The available evidence for this multiple stimulant-sensitive system indicates that both intra- and extracellular sources of Ca2+ are used. Data from bovine, canine and guinea pig tracheal muscle indicate, from studies of Ca2+-dependence of response and Ca2+ channel antagonist sensitivity, that the extent of use of extracellular Ca2+ lies in the order K+ greater than histamine greater than or equal to 5-hydroxytryptamine greater than acetylcholine. The bronchodilator activity of the Ca2+ channel antagonists is noted. Bronchial hyperreactivity is characterized by an increased sensitivity to a variety of stimulants including cold air, exercise, histamine and acetylcholine. The possible origins of this defect are noted. It is suggested that a defect in Ca2+ mobilization or in the receptor - Ca2+ mobilization coupling process at the level of the smooth muscle may constitute an important underlying cause of bronchial hyperreactivity. Potential analogies to reactivity changes seen in hypertensive vascular smooth muscle are noted.

Oxygen-induced contraction in the guinea pig neonatal ductus arteriosus

We investigated the mechanism of oxygen-induced contractions in ductus arteriosus isolated from neonatal guinea pig. A preparation equilibrated at low Po2 (less than 40 mm Hg) displayed a steady membrane potential of -54.8 mV. Application of oxygen (Po2 (less than approximately or equal to 300 mm Hg) resulted in: (1) stepwise development of tension coupled to action potentials and (2) sustained membrane depolarization to -32.9 mV associated with tonic contraction. Mechanical sensitivity to oxygen persisted at any[K]o up to 126 mM, and tension was always larger at a given [K]o or a given membrane potential with high Po2 than with low Po2. The change in membrane potential per decade change in [K]o was 35 mV at low Po2 and 16 mV at high Po2. Oxygen contractions occurred when the ductal strips were bathed in K-free media or exposed to ouabain. We conclude that oxygen caused a conductance change in the sarcolemma resulting in depolarization, which is coupled to contraction. There is also evidence of a membrane potential-independent contraction mechanism.