大黄素对豚鼠胆囊平滑肌L型钙电流的调节

目的研究大黄素对豚鼠胆囊肌条收缩及L型钙电流的调节。方法取成年豚鼠,分离胆囊肌条,使用张力传感器记录胆囊肌条的收缩活动。采用酶消化法急性分离豚鼠胆囊平滑肌细胞,使用全细胞膜片钳技术记录L型钙电流。结果预先加入硝苯地平,大黄素对胆囊肌条的收缩效应明显减弱。大黄素增加胆囊平滑肌细胞L型钙电流,呈浓度依赖性。与正常对照组相比,10μmol·L-1大黄素明显增强+10mV时L型钙电流幅度的(45.2±2.26)%(P<0.05)。预先加入PKC抑制剂十字孢碱,大黄素对钙电流的调节基本被抑制。结论大黄素可通过PKC途径增强胆囊平滑肌L型钙电流,从而促进胆囊平滑肌收缩。     

Emodin increases Ca(2+) influx through L-type Ca(2+) channel in guinea pig gallbladder smooth muscle

Emodin is known to be used in the treatment of cholesterol stones and cholecystitis. This study sought to investigate the effects of emodin on the contraction of gallbladder smooth muscle (GBSM), intracellular Ca(2+) concentration and L-type calcium current in GBSM cells. Gallbladder muscle strips were obtained from adult guinea pigs and the resting tension was recorded. Gallbladder smooth muscle cells were isolated by enzymatic digestion. Cells were loaded with fluo-3/AM and [Ca(2+)](i) was determined by a laser confocal microscope. Calcium current was recorded by the whole-cell patch clamp method. Emodin increased the resting tension of GBSM strips in a dose-dependent manner. Emodin elevated [Ca(2+)](i) in GBSM cells, and this effect was attenuated by pretreatment with nifedipine. In addition, Emodin increased L-type calcium current at concentrations of 1 to 30 microM (at +10 mV, 10 microM, 45.1+/-5.2% compared to control, EC(50) =3.11 microM). In the presence of protein kinase C (PKC) inhibitor, Staurosporine, emodin did not significantly affect the calcium current. However, phorbol 12, 13-dibutyrate mimicked emodin in enhancement of the calcium current. These results suggest that emodin promotes gallbladder contraction by increasing Ca(2+) influx through L-type calcium channel via PKC pathway.     

Celikalim (WAY-120491) Increases K+ Outward Flux in Dog and Human Airway Smooth Muscle

Summary: The ability of the potassium channel opener celikalim (WAY-120491) to increase potassium conductance in airway smooth muscle cells was investigated. The rate of ⁸⁶Rb⁺ efflux was measured from dog trachealis muscle strips and human trachealis smooth muscle cells in culture. Whole-cell currents were recorded from dog trachealis smooth muscle cells freshly dissociated using the nystatin-perforated patch technique. Celikalim (1-10 μM) enhanced the rate of ⁸⁶Rb⁺ efflux from dog airway smooth muscle in a concentration-dependent manner. At 1 μM, the rate of ⁸⁶Rb⁺ efflux was enhanced by 25% in human airway smooth muscle cells. In current recordings, celikalim (1 μM) elicited a glyburide-sensitive outward current, increasing the steady-state current from 367±20 pA to 688±172 pA at +20 mV (n=5). At -60 mV, a voltage closer to the resting potential, the holding current was increased by only +26±15 pA (n=5). This smaller increase was sufficient to hyperpolarize the membrane by 8 mV. These results indicate that celikalim is a potent potassium channel opener in dog and human airway smooth muscles. The present data support the hypothesis that an increase in resting K⁺ conductance by potassium channel openers may account for their relaxing effect in airway smooth muscles.     

Bitter tastants induce relaxation of rat thoracic aorta precontracted with high K+

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Inhibitory effect of the tricyclic antidepressant amitriptyline on voltage‐dependent K+ channels in rabbit coronary arterial smooth muscle cells

Amitriptyline, a tricyclic antidepressant (TCA) drug, is widely used in treatment of psychiatric disorders. However, the side effects of amitriptyline on vascular K⁺ channels remain to be determined. Therefore, we investigated the effect of the tricyclic antidepressant and serotonin reuptake inhibitor amitriptyline on voltage‐dependent K⁺ (Kv) channels in freshly isolated rabbit coronary arterial smooth muscle cells, using the whole‐cell patch clamp technique. The Kv current amplitudes were inhibited by amitriptyline in a concentration‐dependent manner, with an apparent IC₅₀ value of 2.2 ± 0.14 μmol/L and a Hill coefficient of 0.87 ± 0.03. Amitriptyline shifted the activation curve to a more positive potential, but had no significant effect on the inactivation curve, suggesting that amitriptyline altered the voltage sensitivity of Kv channels. Pretreatment with Kv1.5 and Kv1.2 channel inhibitors did not alter the inhibitory effect of amitriptyline on Kv channels. Additionally, application of train pulses (1 and 2 Hz) did not affect amitriptyline‐induced inhibition of Kv currents, which suggested that the action of amitriptyline on Kv channels was not use (state)‐dependent. From these results, we concluded that amitriptyline inhibited the channels in a concentration‐dependent, but state‐independent manner.     

The anticholinergic drug oxybutynin inhibits voltage‐dependent K+ channels in coronary arterial smooth muscle cells

This study demonstrates the inhibitory effect of anticholinergic drug oxybutynin on voltage‐dependent K⁺ (Kv) channels in rabbit coronary arterial smooth muscle cells. Oxybutynin inhibited vascular Kv channels in a concentration‐dependent manner, with an IC₅₀ value of 11.51 ± 0.38 μmol/L and a Hill coefficient (n) of 2.25 ± 0.12. Application of oxybutynin shifted the activation curve to the right and the inactivation curve to the left. Pretreatment with the Kv1.5 subtype inhibitor DPO‐1 and the Kv2.1 subtype inhibitor guangxitoxin suppressed the oxybutynin‐induced inhibition of the Kv current. However, application of the Kv7 subtype inhibitor linopirdine did not affect the inhibition by oxybutynin of the Kv current. The anticholinergic drug atropine did not inhibit the Kv current nor influence oxybutynin‐induced inhibition of the Kv current. From these results, we concluded that oxybutynin inhibited the vascular Kv current in a concentration‐dependent manner by influencing the steady‐state activation and inactivation curves independent of its anticholinergic effect.     

Geraniol Blocks Calcium and Potassium Channels in the Mammalian Myocardium: Useful Effects to Treat Arrhythmias

Geraniol is a monoterpene present in several essential oils, and it is known to have a plethora of pharmacological activities. In this study, we explored the contractile and electrophysiological properties of geraniol and its antiarrhythmic effects in the heart. The geraniol effects on atrial contractility, L‐type Ca²⁺ current, K⁺ currents, action potential (AP) parameters, ECG profile and on the arrhythmia induced by ouabain were evaluated. In the atrium, geraniol reduced the contractile force (~98%, EC = 1,510 ± 160 μM) and diminished the positive inotropism of CaCl₂ and BAY K8644. In cardiomyocytes, the I_Ca,L was reduced by 50.7% (n = 5) after perfusion with 300 μM geraniol. Moreover, geraniol prolonged the AP duration (APD) measured at 50% (n = 5) after repolarization, without changing the resting potential. The increased APD could be attributed to the blockade of the transient outward K⁺ current (I_to) (59.7%, n = 4), the non‐inactivation K⁺ current (I_ss) (39.2%, n = 4) and the inward rectifier K⁺ current (I_K1) (33.7%, n = 4). In isolated hearts, geraniol increased PRi and QTi without affecting the QRS complex (n = 6), and it reduced both the left ventricular pressure (83%) and heart rate (16.5%). Geraniol delayed the time to onset of ouabain‐induced arrhythmias by 128%, preventing 30% of the increase in resting tension (n = 6). Geraniol exerts its negative inotropic and chronotropic responses in the heart by decreasing both L‐type Ca²⁺ and voltage‐gated K⁺ currents, ultimately acting against ouabain‐induced arrhythmias.     

大黄素对豚鼠结肠收缩性的影响及其机制的研究

目的研究大黄素对豚鼠离体远端结肠平滑肌收缩活动的影响及其可能机制。方法急性分离豚鼠远端结肠平滑肌,采用生理记录仪记录不同浓度大黄素对远端结肠收缩的影响;木瓜蛋白酶消化分离豚鼠近端结肠平滑肌细胞,采用全细胞膜片钳技术记录钾电流。结果不同浓度的大黄素（1～50μmol.L-1）对豚鼠远端结肠具有双向调节作用,且其效应呈剂量依赖的趋势1～10μmol.L-1大黄素可浓度依赖性地增强豚鼠远端结肠平滑肌的收缩10μmol.L-1浓度时,大黄素的作用达到平台期。当大黄素的浓度大于10μmol.L-1时,逐渐呈现明显的抑制作用,且其抑制作用与浓度正相关10μmol.L-1硝苯地平孵育肌条后,大黄素不能增强肌条的收缩。大黄素（1～30μmol.L-1）浓度依赖性地抑制豚鼠近端结肠平滑肌细胞钾电流。结论大黄素增强豚鼠结肠的收缩作用可能与L-型钙通道及钾通道有关。     

The selective serotonin reuptake inhibitor dapoxetine inhibits voltage‐dependent K+ channels in rabbit coronary arterial smooth muscle cells

We investigated the inhibitory effect of dapoxetine, a selective serotonin reuptake inhibitor (SSRI), on voltage‐dependent K⁺ (Kv) channels using native smooth muscle cells from rabbit coronary arteries. Dapoxetine inhibited Kv channel currents in a concentration‐dependent manner, with an IC₅₀ value of 2.68±0.94 μmol/L and a slope value (Hill coefficient) of 0.63±0.11. Application of 10 μmol/L dapoxetine accelerated the rate of inactivation of Kv currents. Although dapoxetine did not modify current activation kinetics, it caused a significant negative shift in the inactivation curves. Application of train step (1 or 2 Hz) progressively increased the inhibitory effect of dapoxetine on Kv channels. In addition, the recovery time constant was extended in its presence, suggesting that the longer recovery time constant from inactivation underlies a use‐dependent inhibition of the channel. From these results, we conclude that dapoxetine inhibits Kv channels in a dose‐, time‐, use‐, and state (open)‐dependent manner, independent of serotonin reuptake inhibition.     

MECHANISMS FOR REGULATION OF ARTERIAL TONE BY Ca2+- DEPENDENT K+ CHANNELS IN HYPERTENSION

1. The membrane potential and reactivity of arterial smooth muscle cells is regulated by a variety of K⁺ channels, which are highly expressed in vascular smooth muscle meuscle membrances. 2. Of these K⁺ channel types, the high-conductance, Ca²⁺-ependent K⁺ channel appears to be up-regulated in arterial smooth muscle membrances from hypertensive animals. 3. Patch-clamp studies show that whole-cell membrances and membrane patches of arterial smooth muscle obtained from rats with genetic or renal hypertension show an increased macroscopic and single-channel Ca²⁺-activated K⁺ current. Pharmacological block of this K⁺ current profoundly constricts aortic, renal, mesenteric and femoral arteries obtained from the same hypertensive animals, suggesting that Ca²⁺-dependent K⁺ current is a critical determinant of resting membrane potential in arterial muscle exposed to elevated blood pressure. 4. Thus, K⁺ efflux through Ca²⁺-dependent K⁺ channels appears to constitute an important homeostatic mechanism for buffering increases in arterial reactivity in hypertension.     

The Vasodilatory Effects of Ketamine on Isolated Rabbit Portal Veins

Abstract: The vasodilation mechanism induced by ketamine was investigated in isolated smooth muscle strips of rabbit portal veins. Ketamine inhibited both the phasic and tonic components of K⁺-induced contraction at concentrations greater than 500 μM and 100 μM, respectively. This effect was reversible and concentration-dependent with concentrations up to 1 mM. These effects were similar to those producd by verapamil. In the presence of 60 mM K⁺, application of Ca²⁺ (2.5 mM) in the perfusing solution caused tonic contraction of the smooth muscle, and ketamine at concentrations larger than 10 μM strongly inhibited this Ca²⁺-induced contraction. Ketamine (100 μM) also inhibited the K⁺-induced contractions significantly in the absence and presence of guanethidine, tetrodotoxin and propranolol. Ketamine produced similar concentration-dependent relaxations in the tissues with and without endothelium. These results indicate that in rabbit portal vein, vasodilation produced by ketamine is not endothelium-dependent but is likely to be due to blockade of the voltage-gated influx of extracellular Ca²⁺.     

Multiple effects of caffeine on contraction and cytosolic free Ca2+ levels in vascular smooth muscle of rat aorta

Summary 1. Effects of caffeine on cytosolic Ca²⁺ level ([Ca²⁺]_cyt), measured simultaneously with muscle tension using fura-2-Ca²⁺ fluorescence, were examined in isolated smooth muscle of rat aorta. 2. Caffeine (20 mmol/l) induced a large transient increase in [Ca²⁺]_cyt followed by a plateau which was higher than resting level. However, muscle tension showed a transient increase followed by a decrease to or below the resting level. In Ca²⁺-free solution, caffeine induced only a transient increase in both [Ca²⁺]_cyt, and muscle tension. 3. At low temperature (22°C), high K⁺ (72.7 mmol/l) induced sustained increase in both [Ca²⁺]_cyt and muscle tension which were smaller than those observed at 37°C. At 22°C, however, caffeine-induced transient changes were greater than those observed at 37°C. 4. Ryanodine (10 mol/l) inhibited the transient changes due to caffeine but showed little effects on the sustained changes due to high K⁺. 5. During the sustained increase in [Ca²⁺]_cyt induced by noradrenaline (10 gmmol/l) or high K⁺ (140 mmol/l), addition of caffeine transiently increased [Ca²⁺]_cyt followed by a decrease to a level slightly lower than that before the addition of caffeine. In contrast to this, muscle tension transiently increased and then decreased to or below the resting level. 6. These results suggest that caffeine-induced contraction is due to the release of Ca²⁺ from cellular store. Caffeine also has an inhibitory effect which is partly attributable to decrease in [Ca²⁺ _cyt, and partly to the decrease in the sensitivity to Ca²⁺ of the contractile elements.Send offprint requests to H. Ozaki at the above address     

Impairment of α1‐adrenoceptor‐mediated contractile activity in caudal arterial smooth muscle from Type 2 diabetic Goto‐Kakizaki rats

1. In the present study, we compared the responsiveness of de‐endothelialized caudal artery smooth muscle strips, isolated from Type 2 diabetic Goto‐Kakizaki (GK) and normal Wistar rats, to α₁‐adrenoceptor stimulation (cirazoline) and membrane depolarization (K⁺). 2. The contractile and myosin 20 kDa light chain (LC₂₀) phosphorylation responses to 0.3 μmol/L cirazoline of caudal artery strips isolated from 12‐week‐old GK rats were significantly reduced compared with those of age‐matched Wistar rats, whereas the contractile and LC₂₀ phosphorylation responses to 60 mmol/L K⁺ were unaltered. 3. Stimulation of fura 2‐AM‐loaded strips from GK rats with 0.3 μmol/L cirazoline induced a significantly smaller rise in [Ca²⁺]_i (by ～20%) compared with that in strips from Wistar rats, whereas comparable Ca²⁺ transients were evoked by K⁺ in both. 4. Using quantitative polymerase chain reaction, no significant differences were detected in the mRNA expression of α_1A‐, α_1B‐ and α_1D‐adrenoceptor subtypes between GK and Wistar rats. 5. Cirazoline (1 μmol/L)‐ and caffeine (20 mmol/L)‐induced contractions in the absence of extracellular Ca²⁺ were unaltered in GK rats, suggesting that the release of Ca²⁺ from the sarcoplasmic reticulum in response to cirazoline does not differ between GK and Wistar rats. 6. The results of the present study suggest that Ca²⁺ entry from the extracellular space via α₁‐adrenoceptor‐activated, Ca²⁺‐permeable channels, but not via membrane depolarization and voltage‐gated L‐type Ca²⁺ channels, is impaired in caudal artery smooth muscle of GK rats.     

Mechanisms of changes in coronary arterial tone induced by bee venom toxins

Our study elucidates some mechanisms of contractions or relaxations of isolated porcine left anterior descending coronary artery (LAD) induced by two peptides from the honeybee venom, melittin and apamin. Contractions or relaxations were measured on relaxed or precontracted arteries, respectively. Melittin at lower concentrations (0.1-10 μg/ml) induced transient relaxation, and contraction at higher concentrations (≥7 μg/ml). The removing of the endothelium diminished the melittin-induced relaxation but did not affect the maximal contraction. The inhibition of prostaglandin and nitric oxide (NO) synthesis (by indomethacin and by N-omega-Nitro-l-arginine, respectively) and the use of K⁺ channel inhibitors (apamin and charybdotoxin) showed that melittin evoked relaxation via an endothelium-dependent mechanism (NO production), and by activation of charybdotoxin-sensitive K⁺ channels of smooth muscle. Apamin alone did not affect contraction or relaxation, but the inhibition of NO and prostanoid production revealed the involvement of apamin-sensitive K⁺ channels of smooth muscle in melittin-induced relaxation. Our data show that melittin and apamin could affect contractility of porcine LAD at concentrations similar to those encountered in multiple honeybee stings in humans. Melittin could directly affect contractility of porcine LAD, whereas apamin acts as a modulator of the relaxant response to melittin.     

The role of Na+, K+-ATPase in the hypoxic vasoconstriction in isolated rat basilar artery

Hypoxia-induced cerebrovascular dysfunction is a key factor in the occurrence and the development of cerebral ischemia. Na⁺, K⁺-ATPase affects the regulation of intracellular Ca^2 + concentration and plays an important role in vascular smooth muscle function. However, the potential role of Na⁺, K⁺-ATPase in hypoxia-induced cerebrovascular dysfunction is unknown. In this study, we found that the KCl-induced contraction under hypoxia in rat endothelium-intact basilar arteries is similar to that of denuded arteries, suggesting that hypoxia may cause smooth muscle cell (SMC)-dependent vasoconstriction in the basilar artery. The Na⁺, K⁺–ATPase activity of the isolated basilar artery with or without endothelium significantly reduced with prolonged hypoxia. Blocking the Na⁺–Ca^2 + exchanger with Ni^2 + (10^− 3 M) or the L-type Ca^2 + channel with nimodipine (10^− 8 M) dramatically attenuated KCl-induced contraction under hypoxia. Furthermore, prolonged hypoxia significantly reduced Na⁺, K⁺-ATPase activity and increased [Ca^2 +]_i in cultured rat basilar artery SMCs. Hypoxia reduced the protein and mRNA expression of the α₂ isoform of Na⁺, K⁺-ATPase in SMCs in vitro. We used a low concentration of the Na⁺, K⁺-ATPase inhibitor ouabain, which possesses a high affinity for the α₂ isoform. The contractile response in the rat basilar artery under hypoxia was partly inhibited by ouabain pretreatment. The decreased Na⁺, K⁺-ATPase activity in isolated basilar artery and the increased [Ca^2 +]_i in SMCs induced by hypoxia were partly inhibited by pretreatment with a low concentration of ouabain. These results suggest that hypoxia may educe Na⁺, K⁺-ATPase activity in SMCs through the α₂ isoform contributing to vasoconstriction in the rat basilar artery.     

Relaxing Responses to Hydrogen Peroxide and Nitric Oxide in Human Pericardial Resistance Arteries Stimulated with Endothelin‐1

In human pericardial resistance arteries, effects of the endothelium‐dependent vasodilator bradykinin are mediated by NO during contraction induced by K⁺ or the TxA₂ analogue U46619 and by H₂O₂ during contraction by endothelin‐1 (ET‐1), respectively. We tested the hypotheses that ET‐1 reduces relaxing effects of NO and increases those of H₂O₂ in resistance artery smooth muscle of patients with cardiovascular disease. Arterial segments, dissected from the parietal pericardium of 39 cardiothoracic surgery patients, were studied by myography during amplitude‐matched contractions induced by K⁺, the TXA₂ analogue U46619 or ET‐1. Effects of the NO donor Na‐nitroprusside (SNP) and of exogenous H₂O₂ were recorded in the absence and presence of inhibitors of cyclooxygenases, NO synthases and small and intermediate conductance calcium‐activated K⁺ channels. During contractions induced by either of the three stimuli, the potency of SNP did not differ and was not modified by the inhibitors. In vessels contracted with ET‐1, the potency of H₂O₂ was on average and in terms of interindividual variability considerably larger than in K⁺‐contracted vessels. Both differences were not statistically significant in the presence of inhibitors of mechanisms of endothelium‐dependent vasodilatation. In resistance arteries from patients with cardiovascular disease, ET‐1 does not selectively modify smooth muscle relaxing responses to NO or H₂O₂. Furthermore, the candidate endothelium‐derived relaxing factor H₂O₂ also acts as an endothelium‐dependent vasodilator.     

Relaxant effect induced by wogonin from Scutellaria baicalensis on rat isolated uterine smooth muscle

Context: Wogonin is a flavone derivative isolated from Scutellaria baicalensis Georgi (Labiatae) root, which is a traditional Chinese drug used as an anti-inflammatory and for management of dysmenorrhea.

Objective: The effect of wogonin on the uterus has not yet been examined. We investigated the relaxant effects of wogonin on contractile activity of isolated uterine strips of rats.

Materials and methods: The effect of wogonin on spontaneous uterine contraction, and uterine contraction induced by agonists, K⁺-depolarization and oxytocin in Ca²⁺-free solution was observed. To clarify the type of potassium channel, we tested the effects of 4-aminopyridine, tetraethylammonium and glibenclamide.

Results: Wogonin reduced the contractile amplitude of uterine strip smooth muscle of rats in a dose-dependent manner. The concentration of wogonin for reducing the contraction amplitude by 50% (IC₅₀) on spontaneous contractions was 60.5 μM. Wogonin also inhibited the contraction induced by three agonists (oxytocin, prostaglandin F_2α and acetylcholine). For the uterine strips pretreated with oxytocin in Ca²⁺-free solution or K⁺-depolarization, wogonin showed relaxant effect on the induced uterine contractions. In addition, whereas the inhibitive effect of wogonin on the contraction of uterine smooth muscle in rats could be partly blocked by 4-aminopyridine and tetraethylammonium, it was not influenced by glibenclamide.

Discussion and conclusion: Wogonin significantly inhibited the contraction of rat uterine smooth muscle probably through the inhibition of the inflow of extracellular calcium into cells via cell membrane, and intracellular release of calcium ions. In addition, the relaxant effect induced by wogonin might be due in part to the opening of voltage-dependent and large conductance Ca²⁺-activated K⁺ channels.     

SODIUM-DEPENDENT RELAXATION IN ARTERIAL SMOOTH MUSCLE *

1. The effects of isosmotic substitution of choline for sodium on resting tension and on relaxation after noradrenaline-induced contraction was studied in rabbit isolated aortic strips immersed in Hepes-buffered physiologic salt solution (PSS) warmed to 37°C and gassed with 100% O₂. 2. Isosmotic substitution of choline for sodium produced a sustained increase in resting tension which effectively prevented any evaluation of the influence of sodium on relaxation. The increase in resting tension was insensitive to 10^?8 mol/1 atropine but was abolished by 10 min exposure to calcium-free PSS prior to replacement of sodium. 3. Under sodium-calcium free conditions which eliminated the increase in resting tension observed in sodium-free PSS, stimulation with 10^?5 mol/1 noradrenaline initiated contractions that were 55 ± 7.5% of the control response in normal PSS. Washout of noradrenaline with sodium-calcium-free PSS failed to produce any decrement in tension. However, restoration of the normal sodium resulted in gradual relaxation. 4. These results suggest that sodium is required for relaxation after noradrenaline-induced contraction of arterial smooth muscle.     

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, AChinduced 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.